50 files changed, 2773 insertions, 1279 deletions

diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index d443b124c..52d72c3c5 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -1712,15 +1712,17 @@ impl Type {
         resolver: &Resolver,
         ty: Ty,
     ) -> Type {
-        let environment =
-            resolver.generic_def().map_or_else(Default::default, |d| db.trait_environment(d));
+        let environment = resolver
+            .generic_def()
+            .map_or_else(|| Arc::new(TraitEnvironment::empty(krate)), |d| db.trait_environment(d));
         Type { krate, env: environment, ty }
     }
 
     fn new(db: &dyn HirDatabase, krate: CrateId, lexical_env: impl HasResolver, ty: Ty) -> Type {
         let resolver = lexical_env.resolver(db.upcast());
-        let environment =
-            resolver.generic_def().map_or_else(Default::default, |d| db.trait_environment(d));
+        let environment = resolver
+            .generic_def()
+            .map_or_else(|| Arc::new(TraitEnvironment::empty(krate)), |d| db.trait_environment(d));
         Type { krate, env: environment, ty }
     }
 
@@ -2051,11 +2053,7 @@ impl Type {
         name: Option<&Name>,
         mut callback: impl FnMut(&Ty, AssocItem) -> Option<T>,
     ) -> Option<T> {
-        // There should be no inference vars in types passed here
-        // FIXME check that?
-        // FIXME replace Unknown by bound vars here
-        let canonical =
-            Canonical { value: self.ty.clone(), binders: CanonicalVarKinds::empty(&Interner) };
+        let canonical = hir_ty::replace_errors_with_variables(self.ty.clone());
 
         let env = self.env.clone();
         let krate = krate.id;
@@ -2223,8 +2221,9 @@ impl Type {
         walk_type(db, self, &mut cb);
     }
 
-    pub fn could_unify_with(&self, other: &Type) -> bool {
-        could_unify(&self.ty, &other.ty)
+    pub fn could_unify_with(&self, db: &dyn HirDatabase, other: &Type) -> bool {
+        let tys = hir_ty::replace_errors_with_variables((self.ty.clone(), other.ty.clone()));
+        could_unify(db, self.env.clone(), &tys)
     }
 }
 
diff --git a/crates/hir_def/src/attr.rs b/crates/hir_def/src/attr.rs
index aadd4e44a..89a1ea770 100644
--- a/crates/hir_def/src/attr.rs
+++ b/crates/hir_def/src/attr.rs
@@ -2,7 +2,7 @@
 
 use std::{
     convert::{TryFrom, TryInto},
-    ops,
+    fmt, ops,
     sync::Arc,
 };
 
@@ -648,6 +648,15 @@ pub enum AttrInput {
     TokenTree(Subtree),
 }
 
+impl fmt::Display for AttrInput {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            AttrInput::Literal(lit) => write!(f, " = \"{}\"", lit.escape_debug()),
+            AttrInput::TokenTree(subtree) => subtree.fmt(f),
+        }
+    }
+}
+
 impl Attr {
     fn from_src(
         db: &dyn DefDatabase,
diff --git a/crates/hir_def/src/item_tree.rs b/crates/hir_def/src/item_tree.rs
index cad8a7479..528270d49 100644
--- a/crates/hir_def/src/item_tree.rs
+++ b/crates/hir_def/src/item_tree.rs
@@ -1,6 +1,9 @@
 //! A simplified AST that only contains items.
 
 mod lower;
+mod pretty;
+#[cfg(test)]
+mod tests;
 
 use std::{
     any::type_name,
@@ -132,6 +135,7 @@ impl ItemTree {
             let ItemTreeData {
                 imports,
                 extern_crates,
+                extern_blocks,
                 functions,
                 params,
                 structs,
@@ -154,6 +158,7 @@ impl ItemTree {
 
             imports.shrink_to_fit();
             extern_crates.shrink_to_fit();
+            extern_blocks.shrink_to_fit();
             functions.shrink_to_fit();
             params.shrink_to_fit();
             structs.shrink_to_fit();
@@ -203,6 +208,10 @@ impl ItemTree {
         }
     }
 
+    pub fn pretty_print(&self) -> String {
+        pretty::print_item_tree(self)
+    }
+
     fn data(&self) -> &ItemTreeData {
         self.data.as_ref().expect("attempted to access data of empty ItemTree")
     }
@@ -239,6 +248,7 @@ static VIS_PUB_CRATE: RawVisibility = RawVisibility::Module(ModPath::from_kind(P
 struct ItemTreeData {
     imports: Arena<Import>,
     extern_crates: Arena<ExternCrate>,
+    extern_blocks: Arena<ExternBlock>,
     functions: Arena<Function>,
     params: Arena<Param>,
     structs: Arena<Struct>,
@@ -432,6 +442,7 @@ macro_rules! mod_items {
 mod_items! {
     Import in imports -> ast::Use,
     ExternCrate in extern_crates -> ast::ExternCrate,
+    ExternBlock in extern_blocks -> ast::ExternBlock,
     Function in functions -> ast::Fn,
     Struct in structs -> ast::Struct,
     Union in unions -> ast::Union,
@@ -508,6 +519,13 @@ pub struct ExternCrate {
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]
+pub struct ExternBlock {
+    pub abi: Option<Interned<str>>,
+    pub ast_id: FileAstId<ast::ExternBlock>,
+    pub children: Box<[ModItem]>,
+}
+
+#[derive(Debug, Clone, Eq, PartialEq)]
 pub struct Function {
     pub name: Name,
     pub visibility: RawVisibilityId,
@@ -549,17 +567,6 @@ pub struct Struct {
     pub generic_params: Interned<GenericParams>,
     pub fields: Fields,
     pub ast_id: FileAstId<ast::Struct>,
-    pub kind: StructDefKind,
-}
-
-#[derive(Debug, Clone, Eq, PartialEq)]
-pub enum StructDefKind {
-    /// `struct S { ... }` - type namespace only.
-    Record,
-    /// `struct S(...);`
-    Tuple,
-    /// `struct S;`
-    Unit,
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]
@@ -691,6 +698,7 @@ impl ModItem {
         match self {
             ModItem::Import(_)
             | ModItem::ExternCrate(_)
+            | ModItem::ExternBlock(_)
             | ModItem::Struct(_)
             | ModItem::Union(_)
             | ModItem::Enum(_)
@@ -715,6 +723,7 @@ impl ModItem {
         match self {
             ModItem::Import(it) => tree[it.index].ast_id().upcast(),
             ModItem::ExternCrate(it) => tree[it.index].ast_id().upcast(),
+            ModItem::ExternBlock(it) => tree[it.index].ast_id().upcast(),
             ModItem::Function(it) => tree[it.index].ast_id().upcast(),
             ModItem::Struct(it) => tree[it.index].ast_id().upcast(),
             ModItem::Union(it) => tree[it.index].ast_id().upcast(),
@@ -774,6 +783,10 @@ impl<T> IdRange<T> {
     fn new(range: Range<Idx<T>>) -> Self {
         Self { range: range.start.into_raw().into()..range.end.into_raw().into(), _p: PhantomData }
     }
+
+    fn is_empty(&self) -> bool {
+        self.range.is_empty()
+    }
 }
 
 impl<T> Iterator for IdRange<T> {
diff --git a/crates/hir_def/src/item_tree/lower.rs b/crates/hir_def/src/item_tree/lower.rs
index fe348091d..91cf75371 100644
--- a/crates/hir_def/src/item_tree/lower.rs
+++ b/crates/hir_def/src/item_tree/lower.rs
@@ -147,9 +147,7 @@ impl<'a> Ctx<'a> {
             ast::Item::MacroCall(ast) => self.lower_macro_call(ast).map(Into::into),
             ast::Item::MacroRules(ast) => self.lower_macro_rules(ast).map(Into::into),
             ast::Item::MacroDef(ast) => self.lower_macro_def(ast).map(Into::into),
-            ast::Item::ExternBlock(ast) => {
-                Some(ModItems(self.lower_extern_block(ast).into_iter().collect::<SmallVec<_>>()))
-            }
+            ast::Item::ExternBlock(ast) => Some(self.lower_extern_block(ast).into()),
         };
 
         if !attrs.is_empty() {
@@ -230,12 +228,7 @@ impl<'a> Ctx<'a> {
         let generic_params = self.lower_generic_params(GenericsOwner::Struct, strukt);
         let fields = self.lower_fields(&strukt.kind());
         let ast_id = self.source_ast_id_map.ast_id(strukt);
-        let kind = match strukt.kind() {
-            ast::StructKind::Record(_) => StructDefKind::Record,
-            ast::StructKind::Tuple(_) => StructDefKind::Tuple,
-            ast::StructKind::Unit => StructDefKind::Unit,
-        };
-        let res = Struct { name, visibility, generic_params, fields, ast_id, kind };
+        let res = Struct { name, visibility, generic_params, fields, ast_id };
         Some(id(self.data().structs.alloc(res)))
     }
 
@@ -397,19 +390,7 @@ impl<'a> Ctx<'a> {
             ret_type
         };
 
-        let abi = func.abi().map(|abi| {
-            // FIXME: Abi::abi() -> Option<SyntaxToken>?
-            match abi.syntax().last_token() {
-                Some(tok) if tok.kind() == SyntaxKind::STRING => {
-                    // FIXME: Better way to unescape?
-                    Interned::new_str(tok.text().trim_matches('"'))
-                }
-                _ => {
-                    // `extern` default to be `extern "C"`.
-                    Interned::new_str("C")
-                }
-            }
-        });
+        let abi = func.abi().map(lower_abi);
 
         let ast_id = self.source_ast_id_map.ast_id(func);
 
@@ -647,8 +628,10 @@ impl<'a> Ctx<'a> {
         Some(id(self.data().macro_defs.alloc(res)))
     }
 
-    fn lower_extern_block(&mut self, block: &ast::ExternBlock) -> Vec<ModItem> {
-        block.extern_item_list().map_or(Vec::new(), |list| {
+    fn lower_extern_block(&mut self, block: &ast::ExternBlock) -> FileItemTreeId<ExternBlock> {
+        let ast_id = self.source_ast_id_map.ast_id(block);
+        let abi = block.abi().map(lower_abi);
+        let children: Box<[_]> = block.extern_item_list().map_or(Box::new([]), |list| {
             list.extern_items()
                 .filter_map(|item| {
                     self.collect_inner_items(item.syntax());
@@ -673,13 +656,20 @@ impl<'a> Ctx<'a> {
                             self.data().type_aliases[foreign_ty.index].is_extern = true;
                             foreign_ty.into()
                         }
-                        ast::ExternItem::MacroCall(_) => return None,
+                        ast::ExternItem::MacroCall(call) => {
+                            // FIXME: we need some way of tracking that the macro call is in an
+                            // extern block
+                            self.lower_macro_call(&call)?.into()
+                        }
                     };
                     self.add_attrs(id.into(), attrs);
                     Some(id)
                 })
                 .collect()
-        })
+        });
+
+        let res = ExternBlock { abi, ast_id, children };
+        id(self.data().extern_blocks.alloc(res))
     }
 
     /// Lowers generics defined on `node` and collects inner items defined within.
@@ -879,3 +869,17 @@ fn is_intrinsic_fn_unsafe(name: &Name) -> bool {
     ]
     .contains(&name)
 }
+
+fn lower_abi(abi: ast::Abi) -> Interned<str> {
+    // FIXME: Abi::abi() -> Option<SyntaxToken>?
+    match abi.syntax().last_token() {
+        Some(tok) if tok.kind() == SyntaxKind::STRING => {
+            // FIXME: Better way to unescape?
+            Interned::new_str(tok.text().trim_matches('"'))
+        }
+        _ => {
+            // `extern` default to be `extern "C"`.
+            Interned::new_str("C")
+        }
+    }
+}
diff --git a/crates/hir_def/src/item_tree/pretty.rs b/crates/hir_def/src/item_tree/pretty.rs
new file mode 100644
index 000000000..5ec02d1be
--- /dev/null
+++ b/crates/hir_def/src/item_tree/pretty.rs
@@ -0,0 +1,525 @@
+//! `ItemTree` debug printer.
+
+use std::fmt::{self, Write};
+
+use crate::{attr::RawAttrs, visibility::RawVisibility};
+
+use super::*;
+
+pub(super) fn print_item_tree(tree: &ItemTree) -> String {
+    let mut p = Printer { tree, buf: String::new(), indent_level: 0, needs_indent: true };
+
+    if let Some(attrs) = tree.attrs.get(&AttrOwner::TopLevel) {
+        p.print_attrs(attrs, true);
+    }
+    p.blank();
+
+    for item in tree.top_level_items() {
+        p.print_mod_item(*item);
+    }
+
+    let mut s = p.buf.trim_end_matches('\n').to_string();
+    s.push('\n');
+    s
+}
+
+macro_rules! w {
+    ($dst:expr, $($arg:tt)*) => {
+        drop(write!($dst, $($arg)*))
+    };
+}
+
+macro_rules! wln {
+    ($dst:expr) => {
+        drop(writeln!($dst))
+    };
+    ($dst:expr, $($arg:tt)*) => {
+        drop(writeln!($dst, $($arg)*))
+    };
+}
+
+struct Printer<'a> {
+    tree: &'a ItemTree,
+    buf: String,
+    indent_level: usize,
+    needs_indent: bool,
+}
+
+impl<'a> Printer<'a> {
+    fn indented(&mut self, f: impl FnOnce(&mut Self)) {
+        self.indent_level += 1;
+        wln!(self);
+        f(self);
+        self.indent_level -= 1;
+        self.buf = self.buf.trim_end_matches('\n').to_string();
+    }
+
+    /// Ensures that a blank line is output before the next text.
+    fn blank(&mut self) {
+        let mut iter = self.buf.chars().rev().fuse();
+        match (iter.next(), iter.next()) {
+            (Some('\n'), Some('\n')) | (Some('\n'), None) | (None, None) => {}
+            (Some('\n'), Some(_)) => {
+                self.buf.push('\n');
+            }
+            (Some(_), _) => {
+                self.buf.push('\n');
+                self.buf.push('\n');
+            }
+            (None, Some(_)) => unreachable!(),
+        }
+    }
+
+    fn print_attrs(&mut self, attrs: &RawAttrs, inner: bool) {
+        let inner = if inner { "!" } else { "" };
+        for attr in &**attrs {
+            wln!(
+                self,
+                "#{}[{}{}]  // {:?}",
+                inner,
+                attr.path,
+                attr.input.as_ref().map(|it| it.to_string()).unwrap_or_default(),
+                attr.id,
+            );
+        }
+    }
+
+    fn print_attrs_of(&mut self, of: impl Into<AttrOwner>) {
+        if let Some(attrs) = self.tree.attrs.get(&of.into()) {
+            self.print_attrs(attrs, false);
+        }
+    }
+
+    fn print_visibility(&mut self, vis: RawVisibilityId) {
+        match &self.tree[vis] {
+            RawVisibility::Module(path) => w!(self, "pub({}) ", path),
+            RawVisibility::Public => w!(self, "pub "),
+        };
+    }
+
+    fn print_fields(&mut self, fields: &Fields) {
+        match fields {
+            Fields::Record(fields) => {
+                w!(self, " {{");
+                self.indented(|this| {
+                    for field in fields.clone() {
+                        let Field { visibility, name, type_ref } = &this.tree[field];
+                        this.print_attrs_of(field);
+                        this.print_visibility(*visibility);
+                        w!(this, "{}: ", name);
+                        this.print_type_ref(type_ref);
+                        wln!(this, ",");
+                    }
+                });
+                w!(self, "}}");
+            }
+            Fields::Tuple(fields) => {
+                w!(self, "(");
+                self.indented(|this| {
+                    for field in fields.clone() {
+                        let Field { visibility, name, type_ref } = &this.tree[field];
+                        this.print_attrs_of(field);
+                        this.print_visibility(*visibility);
+                        w!(this, "{}: ", name);
+                        this.print_type_ref(type_ref);
+                        wln!(this, ",");
+                    }
+                });
+                w!(self, ")");
+            }
+            Fields::Unit => {}
+        }
+    }
+
+    fn print_mod_item(&mut self, item: ModItem) {
+        self.print_attrs_of(item);
+
+        match item {
+            ModItem::Import(it) => {
+                let Import { visibility, path, is_glob, alias, ast_id: _, index } = &self.tree[it];
+                self.print_visibility(*visibility);
+                w!(self, "use {}", path);
+                if *is_glob {
+                    w!(self, "::*");
+                }
+                if let Some(alias) = alias {
+                    w!(self, " as {}", alias);
+                }
+                wln!(self, ";  // {}", index);
+            }
+            ModItem::ExternCrate(it) => {
+                let ExternCrate { name, alias, visibility, ast_id: _ } = &self.tree[it];
+                self.print_visibility(*visibility);
+                w!(self, "extern crate {}", name);
+                if let Some(alias) = alias {
+                    w!(self, " as {}", alias);
+                }
+                wln!(self, ";");
+            }
+            ModItem::ExternBlock(it) => {
+                let ExternBlock { abi, ast_id: _, children } = &self.tree[it];
+                w!(self, "extern ");
+                if let Some(abi) = abi {
+                    w!(self, "\"{}\" ", abi);
+                }
+                w!(self, "{{");
+                self.indented(|this| {
+                    for child in &**children {
+                        this.print_mod_item(*child);
+                    }
+                });
+                wln!(self, "}}");
+            }
+            ModItem::Function(it) => {
+                let Function {
+                    name,
+                    visibility,
+                    generic_params: _, // FIXME print these somehow
+                    abi,
+                    params,
+                    ret_type,
+                    ast_id: _,
+                    flags,
+                } = &self.tree[it];
+                if flags.bits != 0 {
+                    wln!(self, "// flags = 0x{:X}", flags.bits);
+                }
+                self.print_visibility(*visibility);
+                if let Some(abi) = abi {
+                    w!(self, "extern \"{}\" ", abi);
+                }
+                w!(self, "fn {}(", name);
+                if !params.is_empty() {
+                    self.indented(|this| {
+                        for param in params.clone() {
+                            this.print_attrs_of(param);
+                            match &this.tree[param] {
+                                Param::Normal(ty) => {
+                                    w!(this, "_: ");
+                                    this.print_type_ref(ty);
+                                    wln!(this, ",");
+                                }
+                                Param::Varargs => {
+                                    wln!(this, "...");
+                                }
+                            };
+                        }
+                    });
+                }
+                w!(self, ") -> ");
+                self.print_type_ref(ret_type);
+                wln!(self, ";");
+            }
+            ModItem::Struct(it) => {
+                let Struct { visibility, name, fields, generic_params: _, ast_id: _ } =
+                    &self.tree[it];
+                self.print_visibility(*visibility);
+                w!(self, "struct {}", name);
+                self.print_fields(fields);
+                if matches!(fields, Fields::Record(_)) {
+                    wln!(self);
+                } else {
+                    wln!(self, ";");
+                }
+            }
+            ModItem::Union(it) => {
+                let Union { name, visibility, fields, generic_params: _, ast_id: _ } =
+                    &self.tree[it];
+                self.print_visibility(*visibility);
+                w!(self, "union {}", name);
+                self.print_fields(fields);
+                if matches!(fields, Fields::Record(_)) {
+                    wln!(self);
+                } else {
+                    wln!(self, ";");
+                }
+            }
+            ModItem::Enum(it) => {
+                let Enum { name, visibility, variants, generic_params: _, ast_id: _ } =
+                    &self.tree[it];
+                self.print_visibility(*visibility);
+                w!(self, "enum {} {{", name);
+                self.indented(|this| {
+                    for variant in variants.clone() {
+                        let Variant { name, fields } = &this.tree[variant];
+                        this.print_attrs_of(variant);
+                        w!(this, "{}", name);
+                        this.print_fields(fields);
+                        wln!(this, ",");
+                    }
+                });
+                wln!(self, "}}");
+            }
+            ModItem::Const(it) => {
+                let Const { name, visibility, type_ref, ast_id: _ } = &self.tree[it];
+                self.print_visibility(*visibility);
+                w!(self, "const ");
+                match name {
+                    Some(name) => w!(self, "{}", name),
+                    None => w!(self, "_"),
+                }
+                w!(self, ": ");
+                self.print_type_ref(type_ref);
+                wln!(self, " = _;");
+            }
+            ModItem::Static(it) => {
+                let Static { name, visibility, mutable, is_extern, type_ref, ast_id: _ } =
+                    &self.tree[it];
+                self.print_visibility(*visibility);
+                w!(self, "static ");
+                if *mutable {
+                    w!(self, "mut ");
+                }
+                w!(self, "{}: ", name);
+                self.print_type_ref(type_ref);
+                w!(self, " = _;");
+                if *is_extern {
+                    w!(self, "  // extern");
+                }
+                wln!(self);
+            }
+            ModItem::Trait(it) => {
+                let Trait {
+                    name,
+                    visibility,
+                    is_auto,
+                    is_unsafe,
+                    bounds,
+                    items,
+                    generic_params: _,
+                    ast_id: _,
+                } = &self.tree[it];
+                self.print_visibility(*visibility);
+                if *is_unsafe {
+                    w!(self, "unsafe ");
+                }
+                if *is_auto {
+                    w!(self, "auto ");
+                }
+                w!(self, "trait {}", name);
+                if !bounds.is_empty() {
+                    w!(self, ": ");
+                    self.print_type_bounds(bounds);
+                }
+                w!(self, " {{");
+                self.indented(|this| {
+                    for item in &**items {
+                        this.print_mod_item((*item).into());
+                    }
+                });
+                wln!(self, "}}");
+            }
+            ModItem::Impl(it) => {
+                let Impl {
+                    target_trait,
+                    self_ty,
+                    is_negative,
+                    items,
+                    generic_params: _,
+                    ast_id: _,
+                } = &self.tree[it];
+                w!(self, "impl ");
+                if *is_negative {
+                    w!(self, "!");
+                }
+                if let Some(tr) = target_trait {
+                    self.print_path(&tr.path);
+                    w!(self, " for ");
+                }
+                self.print_type_ref(self_ty);
+                w!(self, " {{");
+                self.indented(|this| {
+                    for item in &**items {
+                        this.print_mod_item((*item).into());
+                    }
+                });
+                wln!(self, "}}");
+            }
+            ModItem::TypeAlias(it) => {
+                let TypeAlias {
+                    name,
+                    visibility,
+                    bounds,
+                    type_ref,
+                    is_extern,
+                    generic_params: _,
+                    ast_id: _,
+                } = &self.tree[it];
+                self.print_visibility(*visibility);
+                w!(self, "type {}", name);
+                if !bounds.is_empty() {
+                    w!(self, ": ");
+                    self.print_type_bounds(bounds);
+                }
+                if let Some(ty) = type_ref {
+                    w!(self, " = ");
+                    self.print_type_ref(ty);
+                }
+                w!(self, ";");
+                if *is_extern {
+                    w!(self, "  // extern");
+                }
+                wln!(self);
+            }
+            ModItem::Mod(it) => {
+                let Mod { name, visibility, kind, ast_id: _ } = &self.tree[it];
+                self.print_visibility(*visibility);
+                w!(self, "mod {}", name);
+                match kind {
+                    ModKind::Inline { items } => {
+                        w!(self, " {{");
+                        self.indented(|this| {
+                            for item in &**items {
+                                this.print_mod_item((*item).into());
+                            }
+                        });
+                        wln!(self, "}}");
+                    }
+                    ModKind::Outline {} => {
+                        wln!(self, ";");
+                    }
+                }
+            }
+            ModItem::MacroCall(it) => {
+                let MacroCall { path, ast_id: _, fragment: _ } = &self.tree[it];
+                wln!(self, "{}!(...);", path);
+            }
+            ModItem::MacroRules(it) => {
+                let MacroRules { name, ast_id: _ } = &self.tree[it];
+                wln!(self, "macro_rules! {} {{ ... }}", name);
+            }
+            ModItem::MacroDef(it) => {
+                let MacroDef { name, visibility, ast_id: _ } = &self.tree[it];
+                self.print_visibility(*visibility);
+                wln!(self, "macro {} {{ ... }}", name);
+            }
+        }
+
+        self.blank();
+    }
+
+    fn print_type_ref(&mut self, type_ref: &TypeRef) {
+        // FIXME: deduplicate with `HirDisplay` impl
+        match type_ref {
+            TypeRef::Never => w!(self, "!"),
+            TypeRef::Placeholder => w!(self, "_"),
+            TypeRef::Tuple(fields) => {
+                w!(self, "(");
+                for (i, field) in fields.iter().enumerate() {
+                    if i != 0 {
+                        w!(self, ", ");
+                    }
+                    self.print_type_ref(field);
+                }
+                w!(self, ")");
+            }
+            TypeRef::Path(path) => self.print_path(path),
+            TypeRef::RawPtr(pointee, mtbl) => {
+                let mtbl = match mtbl {
+                    Mutability::Shared => "*const",
+                    Mutability::Mut => "*mut",
+                };
+                w!(self, "{} ", mtbl);
+                self.print_type_ref(pointee);
+            }
+            TypeRef::Reference(pointee, lt, mtbl) => {
+                let mtbl = match mtbl {
+                    Mutability::Shared => "",
+                    Mutability::Mut => "mut ",
+                };
+                w!(self, "&");
+                if let Some(lt) = lt {
+                    w!(self, "{} ", lt.name);
+                }
+                w!(self, "{}", mtbl);
+                self.print_type_ref(pointee);
+            }
+            TypeRef::Array(elem, len) => {
+                w!(self, "[");
+                self.print_type_ref(elem);
+                w!(self, "; {}]", len);
+            }
+            TypeRef::Slice(elem) => {
+                w!(self, "[");
+                self.print_type_ref(elem);
+                w!(self, "]");
+            }
+            TypeRef::Fn(args_and_ret, varargs) => {
+                let (ret, args) =
+                    args_and_ret.split_last().expect("TypeRef::Fn is missing return type");
+                w!(self, "fn(");
+                for (i, arg) in args.iter().enumerate() {
+                    if i != 0 {
+                        w!(self, ", ");
+                    }
+                    self.print_type_ref(arg);
+                }
+                if *varargs {
+                    if !args.is_empty() {
+                        w!(self, ", ");
+                    }
+                    w!(self, "...");
+                }
+                w!(self, ") -> ");
+                self.print_type_ref(ret);
+            }
+            TypeRef::Macro(_ast_id) => {
+                w!(self, "<macro>");
+            }
+            TypeRef::Error => drop(write!(self, "{{unknown}}")),
+            TypeRef::ImplTrait(bounds) => {
+                w!(self, "impl ");
+                self.print_type_bounds(bounds);
+            }
+            TypeRef::DynTrait(bounds) => {
+                w!(self, "dyn ");
+                self.print_type_bounds(bounds);
+            }
+        }
+    }
+
+    fn print_type_bounds(&mut self, bounds: &[TypeBound]) {
+        for (i, bound) in bounds.iter().enumerate() {
+            if i != 0 {
+                w!(self, " + ");
+            }
+
+            match bound {
+                TypeBound::Path(path) => self.print_path(path),
+                TypeBound::Lifetime(lt) => w!(self, "{}", lt.name),
+                TypeBound::Error => w!(self, "{{unknown}}"),
+            }
+        }
+    }
+
+    fn print_path(&mut self, path: &Path) {
+        if path.type_anchor().is_none()
+            && path.segments().iter().all(|seg| seg.args_and_bindings.is_none())
+        {
+            w!(self, "{}", path.mod_path());
+        } else {
+            // too complicated, just use `Debug`
+            w!(self, "{:?}", path);
+        }
+    }
+}
+
+impl<'a> Write for Printer<'a> {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        for line in s.split_inclusive('\n') {
+            if self.needs_indent {
+                match self.buf.chars().last() {
+                    Some('\n') | None => {}
+                    _ => self.buf.push('\n'),
+                }
+                self.buf.push_str(&"    ".repeat(self.indent_level));
+                self.needs_indent = false;
+            }
+
+            self.buf.push_str(line);
+            self.needs_indent = line.ends_with('\n');
+        }
+
+        Ok(())
+    }
+}
diff --git a/crates/hir_def/src/item_tree/tests.rs b/crates/hir_def/src/item_tree/tests.rs
new file mode 100644
index 000000000..100ae9b97
--- /dev/null
+++ b/crates/hir_def/src/item_tree/tests.rs
@@ -0,0 +1,244 @@
+use base_db::fixture::WithFixture;
+use expect_test::{expect, Expect};
+
+use crate::{db::DefDatabase, test_db::TestDB};
+
+fn check(ra_fixture: &str, expect: Expect) {
+    let (db, file_id) = TestDB::with_single_file(ra_fixture);
+    let item_tree = db.file_item_tree(file_id.into());
+    let pretty = item_tree.pretty_print();
+    expect.assert_eq(&pretty);
+}
+
+#[test]
+fn imports() {
+    check(
+        r#"
+//! file comment
+#![no_std]
+//! another file comment
+
+extern crate self as renamed;
+pub(super) extern crate bli;
+
+pub use crate::path::{nested, items as renamed, Trait as _};
+use globs::*;
+
+/// docs on import
+use crate::{A, B};
+        "#,
+        expect![[r##"
+            #![doc = " file comment"]  // AttrId { is_doc_comment: true, ast_index: 0 }
+            #![no_std]  // AttrId { is_doc_comment: false, ast_index: 0 }
+            #![doc = " another file comment"]  // AttrId { is_doc_comment: true, ast_index: 1 }
+
+            pub(self) extern crate self as renamed;
+
+            pub(super) extern crate bli;
+
+            pub use crate::path::nested;  // 0
+
+            pub use crate::path::items as renamed;  // 1
+
+            pub use crate::path::Trait as _;  // 2
+
+            pub(self) use globs::*;  // 0
+
+            #[doc = " docs on import"]  // AttrId { is_doc_comment: true, ast_index: 0 }
+            pub(self) use crate::A;  // 0
+
+            #[doc = " docs on import"]  // AttrId { is_doc_comment: true, ast_index: 0 }
+            pub(self) use crate::B;  // 1
+        "##]],
+    );
+}
+
+#[test]
+fn extern_blocks() {
+    check(
+        r#"
+#[on_extern_block]
+extern "C" {
+    #[on_extern_type]
+    type ExType;
+
+    #[on_extern_static]
+    static EX_STATIC: u8;
+
+    #[on_extern_fn]
+    fn ex_fn();
+}
+        "#,
+        expect![[r##"
+            #[on_extern_block]  // AttrId { is_doc_comment: false, ast_index: 0 }
+            extern "C" {
+                #[on_extern_type]  // AttrId { is_doc_comment: false, ast_index: 0 }
+                pub(self) type ExType;  // extern
+
+                #[on_extern_static]  // AttrId { is_doc_comment: false, ast_index: 0 }
+                pub(self) static EX_STATIC: u8 = _;  // extern
+
+                #[on_extern_fn]  // AttrId { is_doc_comment: false, ast_index: 0 }
+                // flags = 0x60
+                pub(self) fn ex_fn() -> ();
+            }
+        "##]],
+    );
+}
+
+#[test]
+fn adts() {
+    check(
+        r#"
+struct Unit;
+
+#[derive(Debug)]
+struct Struct {
+    /// fld docs
+    fld: (),
+}
+
+struct Tuple(#[attr] u8);
+
+union Ize {
+    a: (),
+    b: (),
+}
+
+enum E {
+    /// comment on Unit
+    Unit,
+    /// comment on Tuple
+    Tuple(u8),
+    Struct {
+        /// comment on a: u8
+        a: u8,
+    }
+}
+        "#,
+        expect![[r##"
+            pub(self) struct Unit;
+
+            #[derive(Debug)]  // AttrId { is_doc_comment: false, ast_index: 0 }
+            pub(self) struct Struct {
+                #[doc = " fld docs"]  // AttrId { is_doc_comment: true, ast_index: 0 }
+                pub(self) fld: (),
+            }
+
+            pub(self) struct Tuple(
+                #[attr]  // AttrId { is_doc_comment: false, ast_index: 0 }
+                pub(self) 0: u8,
+            );
+
+            pub(self) union Ize {
+                pub(self) a: (),
+                pub(self) b: (),
+            }
+
+            pub(self) enum E {
+                #[doc = " comment on Unit"]  // AttrId { is_doc_comment: true, ast_index: 0 }
+                Unit,
+                #[doc = " comment on Tuple"]  // AttrId { is_doc_comment: true, ast_index: 0 }
+                Tuple(
+                    pub(self) 0: u8,
+                ),
+                Struct {
+                    #[doc = " comment on a: u8"]  // AttrId { is_doc_comment: true, ast_index: 0 }
+                    pub(self) a: u8,
+                },
+            }
+        "##]],
+    );
+}
+
+#[test]
+fn misc() {
+    check(
+        r#"
+pub static mut ST: () = ();
+
+const _: Anon = ();
+
+#[attr]
+fn f(#[attr] arg: u8, _: ()) {
+    #![inner_attr_in_fn]
+}
+
+trait Tr: SuperTrait + 'lifetime {
+    type Assoc: AssocBound = Default;
+    fn method(&self);
+}
+        "#,
+        expect![[r##"
+            pub static mut ST: () = _;
+
+            pub(self) const _: Anon = _;
+
+            #[attr]  // AttrId { is_doc_comment: false, ast_index: 0 }
+            #[inner_attr_in_fn]  // AttrId { is_doc_comment: false, ast_index: 1 }
+            // flags = 0x2
+            pub(self) fn f(
+                #[attr]  // AttrId { is_doc_comment: false, ast_index: 0 }
+                _: u8,
+                _: (),
+            ) -> ();
+
+            pub(self) trait Tr: SuperTrait + 'lifetime {
+                pub(self) type Assoc: AssocBound = Default;
+
+                // flags = 0x1
+                pub(self) fn method(
+                    _: &Self,
+                ) -> ();
+            }
+        "##]],
+    );
+}
+
+#[test]
+fn modules() {
+    check(
+        r#"
+/// outer
+mod inline {
+    //! inner
+
+    use super::*;
+
+    fn fn_in_module() {}
+}
+        "#,
+        expect![[r##"
+            #[doc = " outer"]  // AttrId { is_doc_comment: true, ast_index: 0 }
+            #[doc = " inner"]  // AttrId { is_doc_comment: true, ast_index: 1 }
+            pub(self) mod inline {
+                pub(self) use super::*;  // 0
+
+                // flags = 0x2
+                pub(self) fn fn_in_module() -> ();
+            }
+        "##]],
+    );
+}
+
+#[test]
+fn macros() {
+    check(
+        r#"
+macro_rules! m {
+    () => {};
+}
+
+pub macro m2() {}
+
+m!();
+        "#,
+        expect![[r#"
+            macro_rules! m { ... }
+
+            pub macro m2 { ... }
+
+            m!(...);
+        "#]],
+    );
+}
diff --git a/crates/hir_def/src/nameres/collector.rs b/crates/hir_def/src/nameres/collector.rs
index 3896be25d..014ea4de4 100644
--- a/crates/hir_def/src/nameres/collector.rs
+++ b/crates/hir_def/src/nameres/collector.rs
@@ -16,19 +16,20 @@ use hir_expand::{
     FragmentKind, HirFileId, MacroCallId, MacroCallKind, MacroDefId, MacroDefKind,
 };
 use hir_expand::{InFile, MacroCallLoc};
+use itertools::Itertools;
 use rustc_hash::{FxHashMap, FxHashSet};
 use syntax::ast;
 
 use crate::{
-    attr::{AttrId, Attrs},
+    attr::{Attr, AttrId, AttrInput, Attrs},
     builtin_attr,
     db::DefDatabase,
     derive_macro_as_call_id,
     intern::Interned,
     item_scope::{ImportType, PerNsGlobImports},
     item_tree::{
-        self, FileItemTreeId, ItemTree, ItemTreeId, MacroCall, MacroDef, MacroRules, Mod, ModItem,
-        ModKind, StructDefKind,
+        self, Fields, FileItemTreeId, ItemTree, ItemTreeId, MacroCall, MacroDef, MacroRules, Mod,
+        ModItem, ModKind,
     },
     macro_call_as_call_id,
     nameres::{
@@ -94,14 +95,16 @@ pub(super) fn collect_defs(
         unresolved_imports: Vec::new(),
         resolved_imports: Vec::new(),
 
-        unexpanded_macros: Vec::new(),
+        unresolved_macros: Vec::new(),
         mod_dirs: FxHashMap::default(),
         cfg_options,
         proc_macros,
         exports_proc_macros: false,
         from_glob_import: Default::default(),
-        ignore_attrs_on: FxHashSet::default(),
-        derive_helpers_in_scope: FxHashMap::default(),
+        skip_attrs: Default::default(),
+        derive_helpers_in_scope: Default::default(),
+        registered_attrs: Default::default(),
+        registered_tools: Default::default(),
     };
     match block {
         Some(block) => {
@@ -237,7 +240,7 @@ struct DefCollector<'a> {
     glob_imports: FxHashMap<LocalModuleId, Vec<(LocalModuleId, Visibility)>>,
     unresolved_imports: Vec<ImportDirective>,
     resolved_imports: Vec<ImportDirective>,
-    unexpanded_macros: Vec<MacroDirective>,
+    unresolved_macros: Vec<MacroDirective>,
     mod_dirs: FxHashMap<LocalModuleId, ModDir>,
     cfg_options: &'a CfgOptions,
     /// List of procedural macros defined by this crate. This is read from the dynamic library
@@ -247,10 +250,20 @@ struct DefCollector<'a> {
     proc_macros: Vec<(Name, ProcMacroExpander)>,
     exports_proc_macros: bool,
     from_glob_import: PerNsGlobImports,
-    ignore_attrs_on: FxHashSet<InFile<ModItem>>,
+    /// If we fail to resolve an attribute on a `ModItem`, we fall back to ignoring the attribute.
+    /// This map is used to skip all attributes up to and including the one that failed to resolve,
+    /// in order to not expand them twice.
+    ///
+    /// This also stores the attributes to skip when we resolve derive helpers and non-macro
+    /// non-builtin attributes in general.
+    skip_attrs: FxHashMap<InFile<ModItem>, AttrId>,
     /// Tracks which custom derives are in scope for an item, to allow resolution of derive helper
     /// attributes.
     derive_helpers_in_scope: FxHashMap<AstId<ast::Item>, Vec<Name>>,
+    /// Custom attributes registered with `#![register_attr]`.
+    registered_attrs: Vec<String>,
+    /// Custom tool modules registered with `#![register_tool]`.
+    registered_tools: Vec<String>,
 }
 
 impl DefCollector<'_> {
@@ -259,11 +272,39 @@ impl DefCollector<'_> {
         let item_tree = self.db.file_item_tree(file_id.into());
         let module_id = self.def_map.root;
         self.def_map.modules[module_id].origin = ModuleOrigin::CrateRoot { definition: file_id };
-        if item_tree
-            .top_level_attrs(self.db, self.def_map.krate)
-            .cfg()
-            .map_or(true, |cfg| self.cfg_options.check(&cfg) != Some(false))
-        {
+
+        let attrs = item_tree.top_level_attrs(self.db, self.def_map.krate);
+        if attrs.cfg().map_or(true, |cfg| self.cfg_options.check(&cfg) != Some(false)) {
+            // Process other crate-level attributes.
+            for attr in &*attrs {
+                let attr_name = match attr.path.as_ident() {
+                    Some(name) => name,
+                    None => continue,
+                };
+
+                let registered_name = if *attr_name == hir_expand::name![register_attr]
+                    || *attr_name == hir_expand::name![register_tool]
+                {
+                    match &attr.input {
+                        Some(AttrInput::TokenTree(subtree)) => match &*subtree.token_trees {
+                            [tt::TokenTree::Leaf(tt::Leaf::Ident(name))] => name.as_name(),
+                            _ => continue,
+                        },
+                        _ => continue,
+                    }
+                } else {
+                    continue;
+                };
+
+                if *attr_name == hir_expand::name![register_attr] {
+                    self.registered_attrs.push(registered_name.to_string());
+                    cov_mark::hit!(register_attr);
+                } else {
+                    self.registered_tools.push(registered_name.to_string());
+                    cov_mark::hit!(register_tool);
+                }
+            }
+
             ModCollector {
                 def_collector: &mut *self,
                 macro_depth: 0,
@@ -319,7 +360,7 @@ impl DefCollector<'_> {
                 }
             }
 
-            if self.reseed_with_unresolved_attributes() == ReachedFixedPoint::Yes {
+            if self.reseed_with_unresolved_attribute() == ReachedFixedPoint::Yes {
                 break;
             }
         }
@@ -362,27 +403,23 @@ impl DefCollector<'_> {
     }
 
     /// When the fixed-point loop reaches a stable state, we might still have some unresolved
-    /// attributes (or unexpanded attribute proc macros) left over. This takes them, and feeds the
-    /// item they're applied to back into name resolution.
+    /// attributes (or unexpanded attribute proc macros) left over. This takes one of them, and
+    /// feeds the item it's applied to back into name resolution.
     ///
     /// This effectively ignores the fact that the macro is there and just treats the items as
     /// normal code.
     ///
     /// This improves UX when proc macros are turned off or don't work, and replicates the behavior
     /// before we supported proc. attribute macros.
-    fn reseed_with_unresolved_attributes(&mut self) -> ReachedFixedPoint {
+    fn reseed_with_unresolved_attribute(&mut self) -> ReachedFixedPoint {
         cov_mark::hit!(unresolved_attribute_fallback);
 
-        let mut added_items = false;
-        let unexpanded_macros = std::mem::replace(&mut self.unexpanded_macros, Vec::new());
-        for directive in &unexpanded_macros {
-            if let MacroDirectiveKind::Attr { ast_id, mod_item, .. } = &directive.kind {
-                // Make sure to only add such items once.
-                if !self.ignore_attrs_on.insert(ast_id.ast_id.with_value(*mod_item)) {
-                    continue;
-                }
+        let mut unresolved_macros = std::mem::replace(&mut self.unresolved_macros, Vec::new());
+        let pos = unresolved_macros.iter().position(|directive| {
+            if let MacroDirectiveKind::Attr { ast_id, mod_item, attr } = &directive.kind {
+                self.skip_attrs.insert(ast_id.ast_id.with_value(*mod_item), *attr);
 
-                let file_id = self.def_map[directive.module_id].definition_source(self.db).file_id;
+                let file_id = ast_id.ast_id.file_id;
                 let item_tree = self.db.file_item_tree(file_id);
                 let mod_dir = self.mod_dirs[&directive.module_id].clone();
                 ModCollector {
@@ -394,14 +431,20 @@ impl DefCollector<'_> {
                     mod_dir,
                 }
                 .collect(&[*mod_item]);
-                added_items = true;
+                true
+            } else {
+                false
             }
+        });
+
+        if let Some(pos) = pos {
+            unresolved_macros.remove(pos);
         }
 
         // The collection above might add new unresolved macros (eg. derives), so merge the lists.
-        self.unexpanded_macros.extend(unexpanded_macros);
+        self.unresolved_macros.extend(unresolved_macros);
 
-        if added_items {
+        if pos.is_some() {
             // Continue name resolution with the new data.
             ReachedFixedPoint::No
         } else {
@@ -873,7 +916,7 @@ impl DefCollector<'_> {
     }
 
     fn resolve_macros(&mut self) -> ReachedFixedPoint {
-        let mut macros = std::mem::replace(&mut self.unexpanded_macros, Vec::new());
+        let mut macros = std::mem::replace(&mut self.unresolved_macros, Vec::new());
         let mut resolved = Vec::new();
         let mut res = ReachedFixedPoint::Yes;
         macros.retain(|directive| {
@@ -922,14 +965,43 @@ impl DefCollector<'_> {
                         Err(UnresolvedMacro { .. }) => (),
                     }
                 }
-                MacroDirectiveKind::Attr { .. } => {
-                    // not yet :)
+                MacroDirectiveKind::Attr { ast_id, mod_item, attr } => {
+                    if let Some(ident) = ast_id.path.as_ident() {
+                        if let Some(helpers) = self.derive_helpers_in_scope.get(&ast_id.ast_id) {
+                            if helpers.contains(ident) {
+                                cov_mark::hit!(resolved_derive_helper);
+
+                                // Resolved to derive helper. Collect the item's attributes again,
+                                // starting after the derive helper.
+                                let file_id = ast_id.ast_id.file_id;
+                                let item_tree = self.db.file_item_tree(file_id);
+                                let mod_dir = self.mod_dirs[&directive.module_id].clone();
+                                self.skip_attrs.insert(InFile::new(file_id, *mod_item), *attr);
+                                ModCollector {
+                                    def_collector: &mut *self,
+                                    macro_depth: directive.depth,
+                                    module_id: directive.module_id,
+                                    file_id,
+                                    item_tree: &item_tree,
+                                    mod_dir,
+                                }
+                                .collect(&[*mod_item]);
+
+                                // Remove the original directive since we resolved it.
+                                return false;
+                            }
+                        }
+                    }
+
+                    // Not resolved to a derive helper, so try to resolve as a macro.
+                    // FIXME: not yet :)
                 }
             }
 
             true
         });
-        self.unexpanded_macros = macros;
+        // Attribute resolution can add unresolved macro invocations, so concatenate the lists.
+        self.unresolved_macros.extend(macros);
 
         for (module_id, macro_call_id, depth) in resolved {
             self.collect_macro_expansion(module_id, macro_call_id, depth);
@@ -1000,7 +1072,7 @@ impl DefCollector<'_> {
     fn finish(mut self) -> DefMap {
         // Emit diagnostics for all remaining unexpanded macros.
 
-        for directive in &self.unexpanded_macros {
+        for directive in &self.unresolved_macros {
             match &directive.kind {
                 MacroDirectiveKind::FnLike { ast_id, fragment } => match macro_call_as_call_id(
                     ast_id,
@@ -1102,7 +1174,7 @@ impl ModCollector<'_, '_> {
 
         // Prelude module is always considered to be `#[macro_use]`.
         if let Some(prelude_module) = self.def_collector.def_map.prelude {
-            if prelude_module.krate != self.def_collector.def_map.krate {
+            if prelude_module.krate != krate {
                 cov_mark::hit!(prelude_is_macro_use);
                 self.def_collector.import_all_macros_exported(self.module_id, prelude_module.krate);
             }
@@ -1172,6 +1244,7 @@ impl ModCollector<'_, '_> {
                         status: PartialResolvedImport::Unresolved,
                     })
                 }
+                ModItem::ExternBlock(block) => self.collect(&self.item_tree[block].children),
                 ModItem::MacroCall(mac) => self.collect_macro_call(&self.item_tree[mac]),
                 ModItem::MacroRules(id) => self.collect_macro_rules(id),
                 ModItem::MacroDef(id) => self.collect_macro_def(id),
@@ -1203,28 +1276,18 @@ impl ModCollector<'_, '_> {
                 ModItem::Struct(id) => {
                     let it = &self.item_tree[id];
 
-                    // FIXME: check attrs to see if this is an attribute macro invocation;
-                    // in which case we don't add the invocation, just a single attribute
-                    // macro invocation
-                    self.collect_derives(&attrs, it.ast_id.upcast());
-
                     def = Some(DefData {
                         id: StructLoc { container: module, id: ItemTreeId::new(self.file_id, id) }
                             .intern(self.def_collector.db)
                             .into(),
                         name: &it.name,
                         visibility: &self.item_tree[it.visibility],
-                        has_constructor: it.kind != StructDefKind::Record,
+                        has_constructor: !matches!(it.fields, Fields::Record(_)),
                     });
                 }
                 ModItem::Union(id) => {
                     let it = &self.item_tree[id];
 
-                    // FIXME: check attrs to see if this is an attribute macro invocation;
-                    // in which case we don't add the invocation, just a single attribute
-                    // macro invocation
-                    self.collect_derives(&attrs, it.ast_id.upcast());
-
                     def = Some(DefData {
                         id: UnionLoc { container: module, id: ItemTreeId::new(self.file_id, id) }
                             .intern(self.def_collector.db)
@@ -1237,11 +1300,6 @@ impl ModCollector<'_, '_> {
                 ModItem::Enum(id) => {
                     let it = &self.item_tree[id];
 
-                    // FIXME: check attrs to see if this is an attribute macro invocation;
-                    // in which case we don't add the invocation, just a single attribute
-                    // macro invocation
-                    self.collect_derives(&attrs, it.ast_id.upcast());
-
                     def = Some(DefData {
                         id: EnumLoc { container: module, id: ItemTreeId::new(self.file_id, id) }
                             .intern(self.def_collector.db)
@@ -1453,76 +1511,116 @@ impl ModCollector<'_, '_> {
     ///
     /// Returns `Err` when some attributes could not be resolved to builtins and have been
     /// registered as unresolved.
+    ///
+    /// If `ignore_up_to` is `Some`, attributes precending and including that attribute will be
+    /// assumed to be resolved already.
     fn resolve_attributes(&mut self, attrs: &Attrs, mod_item: ModItem) -> Result<(), ()> {
-        fn is_builtin_attr(path: &ModPath) -> bool {
-            if path.kind == PathKind::Plain {
-                if let Some(tool_module) = path.segments().first() {
-                    let tool_module = tool_module.to_string();
-                    if builtin_attr::TOOL_MODULES.iter().any(|m| tool_module == *m) {
-                        return true;
-                    }
-                }
-
-                if let Some(name) = path.as_ident() {
-                    let name = name.to_string();
-                    if builtin_attr::INERT_ATTRIBUTES
-                        .iter()
-                        .chain(builtin_attr::EXTRA_ATTRIBUTES)
-                        .any(|attr| name == *attr)
-                    {
-                        return true;
-                    }
+        let mut ignore_up_to =
+            self.def_collector.skip_attrs.get(&InFile::new(self.file_id, mod_item)).copied();
+        let iter = attrs
+            .iter()
+            .dedup_by(|a, b| {
+                // FIXME: this should not be required, all attributes on an item should have a
+                // unique ID!
+                // Still, this occurs because `#[cfg_attr]` can "expand" to multiple attributes:
+                //     #[cfg_attr(not(off), unresolved, unresolved)]
+                //     struct S;
+                // We should come up with a different way to ID attributes.
+                a.id == b.id
+            })
+            .skip_while(|attr| match ignore_up_to {
+                Some(id) if attr.id == id => {
+                    ignore_up_to = None;
+                    true
                 }
-            }
-
-            false
-        }
-
-        // We failed to resolve an attribute on this item earlier, and are falling back to treating
-        // the item as-is.
-        if self.def_collector.ignore_attrs_on.contains(&InFile::new(self.file_id, mod_item)) {
-            return Ok(());
-        }
-
-        match attrs.iter().find(|attr| !is_builtin_attr(&attr.path)) {
-            Some(non_builtin_attr) => {
-                log::debug!("non-builtin attribute {}", non_builtin_attr.path);
+                Some(_) => true,
+                None => false,
+            });
+
+        for attr in iter {
+            if attr.path.as_ident() == Some(&hir_expand::name![derive]) {
+                self.collect_derive(attr, mod_item);
+            } else if self.is_builtin_or_registered_attr(&attr.path) {
+                continue;
+            } else {
+                log::debug!("non-builtin attribute {}", attr.path);
 
                 let ast_id = AstIdWithPath::new(
                     self.file_id,
                     mod_item.ast_id(self.item_tree),
-                    non_builtin_attr.path.as_ref().clone(),
+                    attr.path.as_ref().clone(),
                 );
-                self.def_collector.unexpanded_macros.push(MacroDirective {
+                self.def_collector.unresolved_macros.push(MacroDirective {
                     module_id: self.module_id,
                     depth: self.macro_depth + 1,
-                    kind: MacroDirectiveKind::Attr { ast_id, attr: non_builtin_attr.id, mod_item },
+                    kind: MacroDirectiveKind::Attr { ast_id, attr: attr.id, mod_item },
                 });
 
-                Err(())
+                return Err(());
             }
-            None => Ok(()),
         }
+
+        Ok(())
     }
 
-    fn collect_derives(&mut self, attrs: &Attrs, ast_id: FileAstId<ast::Item>) {
-        for derive in attrs.by_key("derive").attrs() {
-            match derive.parse_derive() {
-                Some(derive_macros) => {
-                    for path in derive_macros {
-                        let ast_id = AstIdWithPath::new(self.file_id, ast_id, path);
-                        self.def_collector.unexpanded_macros.push(MacroDirective {
-                            module_id: self.module_id,
-                            depth: self.macro_depth + 1,
-                            kind: MacroDirectiveKind::Derive { ast_id, derive_attr: derive.id },
-                        });
-                    }
+    fn is_builtin_or_registered_attr(&self, path: &ModPath) -> bool {
+        if path.kind == PathKind::Plain {
+            if let Some(tool_module) = path.segments().first() {
+                let tool_module = tool_module.to_string();
+                if builtin_attr::TOOL_MODULES
+                    .iter()
+                    .copied()
+                    .chain(self.def_collector.registered_tools.iter().map(|s| &**s))
+                    .any(|m| tool_module == *m)
+                {
+                    return true;
                 }
-                None => {
-                    // FIXME: diagnose
-                    log::debug!("malformed derive: {:?}", derive);
+            }
+
+            if let Some(name) = path.as_ident() {
+                let name = name.to_string();
+                if builtin_attr::INERT_ATTRIBUTES
+                    .iter()
+                    .chain(builtin_attr::EXTRA_ATTRIBUTES)
+                    .copied()
+                    .chain(self.def_collector.registered_attrs.iter().map(|s| &**s))
+                    .any(|attr| name == *attr)
+                {
+                    return true;
+                }
+            }
+        }
+
+        false
+    }
+
+    fn collect_derive(&mut self, attr: &Attr, mod_item: ModItem) {
+        let ast_id: FileAstId<ast::Item> = match mod_item {
+            ModItem::Struct(it) => self.item_tree[it].ast_id.upcast(),
+            ModItem::Union(it) => self.item_tree[it].ast_id.upcast(),
+            ModItem::Enum(it) => self.item_tree[it].ast_id.upcast(),
+            _ => {
+                // Cannot use derive on this item.
+                // FIXME: diagnose
+                return;
+            }
+        };
+
+        match attr.parse_derive() {
+            Some(derive_macros) => {
+                for path in derive_macros {
+                    let ast_id = AstIdWithPath::new(self.file_id, ast_id, path);
+                    self.def_collector.unresolved_macros.push(MacroDirective {
+                        module_id: self.module_id,
+                        depth: self.macro_depth + 1,
+                        kind: MacroDirectiveKind::Derive { ast_id, derive_attr: attr.id },
+                    });
                 }
             }
+            None => {
+                // FIXME: diagnose
+                log::debug!("malformed derive: {:?}", attr);
+            }
         }
     }
 
@@ -1686,7 +1784,7 @@ impl ModCollector<'_, '_> {
             ast_id.path.kind = PathKind::Super(0);
         }
 
-        self.def_collector.unexpanded_macros.push(MacroDirective {
+        self.def_collector.unresolved_macros.push(MacroDirective {
             module_id: self.module_id,
             depth: self.macro_depth + 1,
             kind: MacroDirectiveKind::FnLike { ast_id, fragment: mac.fragment },
@@ -1731,14 +1829,16 @@ mod tests {
             glob_imports: FxHashMap::default(),
             unresolved_imports: Vec::new(),
             resolved_imports: Vec::new(),
-            unexpanded_macros: Vec::new(),
+            unresolved_macros: Vec::new(),
             mod_dirs: FxHashMap::default(),
             cfg_options: &CfgOptions::default(),
             proc_macros: Default::default(),
             exports_proc_macros: false,
             from_glob_import: Default::default(),
-            ignore_attrs_on: FxHashSet::default(),
-            derive_helpers_in_scope: FxHashMap::default(),
+            skip_attrs: Default::default(),
+            derive_helpers_in_scope: Default::default(),
+            registered_attrs: Default::default(),
+            registered_tools: Default::default(),
         };
         collector.seed_with_top_level();
         collector.collect();
diff --git a/crates/hir_def/src/nameres/tests/diagnostics.rs b/crates/hir_def/src/nameres/tests/diagnostics.rs
index 543975e07..75147d973 100644
--- a/crates/hir_def/src/nameres/tests/diagnostics.rs
+++ b/crates/hir_def/src/nameres/tests/diagnostics.rs
@@ -237,3 +237,20 @@ fn good_out_dir_diagnostic() {
         "#,
     );
 }
+
+#[test]
+fn register_attr_and_tool() {
+    cov_mark::check!(register_attr);
+    cov_mark::check!(register_tool);
+    check_no_diagnostics(
+        r#"
+#![register_tool(tool)]
+#![register_attr(attr)]
+
+#[tool::path]
+#[attr]
+struct S;
+        "#,
+    );
+    // NB: we don't currently emit diagnostics here
+}
diff --git a/crates/hir_def/src/nameres/tests/macros.rs b/crates/hir_def/src/nameres/tests/macros.rs
index 6eb5f97be..3065efd65 100644
--- a/crates/hir_def/src/nameres/tests/macros.rs
+++ b/crates/hir_def/src/nameres/tests/macros.rs
@@ -736,6 +736,80 @@ fn unresolved_attributes_fall_back_track_per_file_moditems() {
 }
 
 #[test]
+fn unresolved_attrs_extern_block_hang() {
+    // Regression test for https://github.com/rust-analyzer/rust-analyzer/issues/8905
+    check(
+        r#"
+#[unresolved]
+extern "C" {
+    #[unresolved]
+    fn f();
+}
+    "#,
+        expect![[r#"
+        crate
+        f: v
+    "#]],
+    );
+}
+
+#[test]
+fn macros_in_extern_block() {
+    check(
+        r#"
+macro_rules! m {
+    () => { static S: u8; };
+}
+
+extern {
+    m!();
+}
+    "#,
+        expect![[r#"
+            crate
+            S: v
+        "#]],
+    );
+}
+
+#[test]
+fn resolves_derive_helper() {
+    cov_mark::check!(resolved_derive_helper);
+    check(
+        r#"
+//- /main.rs crate:main deps:proc
+#[derive(proc::Derive)]
+#[helper]
+#[unresolved]
+struct S;
+
+//- /proc.rs crate:proc
+#[proc_macro_derive(Derive, attributes(helper))]
+fn derive() {}
+        "#,
+        expect![[r#"
+            crate
+            S: t v
+        "#]],
+    );
+}
+
+#[test]
+fn unresolved_attr_with_cfg_attr_hang() {
+    // Another regression test for https://github.com/rust-analyzer/rust-analyzer/issues/8905
+    check(
+        r#"
+#[cfg_attr(not(off), unresolved, unresolved)]
+struct S;
+        "#,
+        expect![[r#"
+            crate
+            S: t v
+        "#]],
+    );
+}
+
+#[test]
 fn macro_expansion_overflow() {
     cov_mark::check!(macro_expansion_overflow);
     check(
diff --git a/crates/hir_def/src/path.rs b/crates/hir_def/src/path.rs
index a43441b1c..9b8873fd2 100644
--- a/crates/hir_def/src/path.rs
+++ b/crates/hir_def/src/path.rs
@@ -46,6 +46,15 @@ pub enum ImportAlias {
     Alias(Name),
 }
 
+impl Display for ImportAlias {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            ImportAlias::Underscore => f.write_str("_"),
+            ImportAlias::Alias(name) => f.write_str(&name.to_string()),
+        }
+    }
+}
+
 impl ModPath {
     pub fn from_src(db: &dyn DefDatabase, path: ast::Path, hygiene: &Hygiene) -> Option<ModPath> {
         let ctx = LowerCtx::with_hygiene(db, hygiene);
diff --git a/crates/hir_expand/src/name.rs b/crates/hir_expand/src/name.rs
index 5a5dc9afd..ef67ea2e9 100644
--- a/crates/hir_expand/src/name.rs
+++ b/crates/hir_expand/src/name.rs
@@ -164,6 +164,8 @@ pub mod known {
         doc,
         cfg,
         cfg_attr,
+        register_attr,
+        register_tool,
         // Components of known path (value or mod name)
         std,
         core,
diff --git a/crates/hir_ty/src/builder.rs b/crates/hir_ty/src/builder.rs
index e25ef866d..893e727c2 100644
--- a/crates/hir_ty/src/builder.rs
+++ b/crates/hir_ty/src/builder.rs
@@ -6,15 +6,15 @@ use chalk_ir::{
     cast::{Cast, CastTo, Caster},
     fold::Fold,
     interner::HasInterner,
-    AdtId, BoundVar, DebruijnIndex, Safety, Scalar,
+    AdtId, BoundVar, DebruijnIndex, Scalar,
 };
 use hir_def::{builtin_type::BuiltinType, GenericDefId, TraitId, TypeAliasId};
 use smallvec::SmallVec;
 
 use crate::{
     db::HirDatabase, primitive, to_assoc_type_id, to_chalk_trait_id, utils::generics, Binders,
-    CallableSig, FnPointer, FnSig, FnSubst, GenericArg, Interner, ProjectionTy, Substitution,
-    TraitRef, Ty, TyDefId, TyExt, TyKind, ValueTyDefId,
+    CallableSig, GenericArg, Interner, ProjectionTy, Substitution, TraitRef, Ty, TyDefId, TyExt,
+    TyKind, ValueTyDefId,
 };
 
 /// This is a builder for `Ty` or anything that needs a `Substitution`.
@@ -77,15 +77,7 @@ impl TyBuilder<()> {
     }
 
     pub fn fn_ptr(sig: CallableSig) -> Ty {
-        TyKind::Function(FnPointer {
-            num_binders: 0,
-            sig: FnSig { abi: (), safety: Safety::Safe, variadic: sig.is_varargs },
-            substitution: FnSubst(Substitution::from_iter(
-                &Interner,
-                sig.params_and_return.iter().cloned(),
-            )),
-        })
-        .intern(&Interner)
+        TyKind::Function(sig.to_fn_ptr()).intern(&Interner)
     }
 
     pub fn builtin(builtin: BuiltinType) -> Ty {
diff --git a/crates/hir_ty/src/chalk_db.rs b/crates/hir_ty/src/chalk_db.rs
index 8f054d06b..b108fd559 100644
--- a/crates/hir_ty/src/chalk_db.rs
+++ b/crates/hir_ty/src/chalk_db.rs
@@ -344,20 +344,20 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
     }
 
     fn unification_database(&self) -> &dyn chalk_ir::UnificationDatabase<Interner> {
-        self
+        &self.db
     }
 }
 
-impl<'a> chalk_ir::UnificationDatabase<Interner> for ChalkContext<'a> {
+impl<'a> chalk_ir::UnificationDatabase<Interner> for &'a dyn HirDatabase {
     fn fn_def_variance(
         &self,
         fn_def_id: chalk_ir::FnDefId<Interner>,
     ) -> chalk_ir::Variances<Interner> {
-        self.db.fn_def_variance(self.krate, fn_def_id)
+        HirDatabase::fn_def_variance(*self, fn_def_id)
     }
 
     fn adt_variance(&self, adt_id: chalk_ir::AdtId<Interner>) -> chalk_ir::Variances<Interner> {
-        self.db.adt_variance(self.krate, adt_id)
+        HirDatabase::adt_variance(*self, adt_id)
     }
 }
 
@@ -651,11 +651,7 @@ pub(crate) fn fn_def_datum_query(
     Arc::new(datum)
 }
 
-pub(crate) fn fn_def_variance_query(
-    db: &dyn HirDatabase,
-    _krate: CrateId,
-    fn_def_id: FnDefId,
-) -> Variances {
+pub(crate) fn fn_def_variance_query(db: &dyn HirDatabase, fn_def_id: FnDefId) -> Variances {
     let callable_def: CallableDefId = from_chalk(db, fn_def_id);
     let generic_params = generics(db.upcast(), callable_def.into());
     Variances::from_iter(
@@ -666,7 +662,6 @@ pub(crate) fn fn_def_variance_query(
 
 pub(crate) fn adt_variance_query(
     db: &dyn HirDatabase,
-    _krate: CrateId,
     chalk_ir::AdtId(adt_id): AdtId,
 ) -> Variances {
     let generic_params = generics(db.upcast(), adt_id.into());
diff --git a/crates/hir_ty/src/chalk_ext.rs b/crates/hir_ty/src/chalk_ext.rs
index 5232a7d80..df340a6ca 100644
--- a/crates/hir_ty/src/chalk_ext.rs
+++ b/crates/hir_ty/src/chalk_ext.rs
@@ -18,6 +18,7 @@ pub trait TyExt {
     fn is_unit(&self) -> bool;
     fn is_never(&self) -> bool;
     fn is_unknown(&self) -> bool;
+    fn is_ty_var(&self) -> bool;
 
     fn as_adt(&self) -> Option<(hir_def::AdtId, &Substitution)>;
     fn as_builtin(&self) -> Option<BuiltinType>;
@@ -55,6 +56,10 @@ impl TyExt for Ty {
         matches!(self.kind(&Interner), TyKind::Error)
     }
 
+    fn is_ty_var(&self) -> bool {
+        matches!(self.kind(&Interner), TyKind::InferenceVar(_, _))
+    }
+
     fn as_adt(&self) -> Option<(hir_def::AdtId, &Substitution)> {
         match self.kind(&Interner) {
             TyKind::Adt(AdtId(adt), parameters) => Some((*adt, parameters)),
diff --git a/crates/hir_ty/src/db.rs b/crates/hir_ty/src/db.rs
index 9da0a02e3..be5b9110e 100644
--- a/crates/hir_ty/src/db.rs
+++ b/crates/hir_ty/src/db.rs
@@ -117,10 +117,10 @@ pub trait HirDatabase: DefDatabase + Upcast<dyn DefDatabase> {
     fn fn_def_datum(&self, krate: CrateId, fn_def_id: FnDefId) -> Arc<chalk_db::FnDefDatum>;
 
     #[salsa::invoke(chalk_db::fn_def_variance_query)]
-    fn fn_def_variance(&self, krate: CrateId, fn_def_id: FnDefId) -> chalk_db::Variances;
+    fn fn_def_variance(&self, fn_def_id: FnDefId) -> chalk_db::Variances;
 
     #[salsa::invoke(chalk_db::adt_variance_query)]
-    fn adt_variance(&self, krate: CrateId, adt_id: chalk_db::AdtId) -> chalk_db::Variances;
+    fn adt_variance(&self, adt_id: chalk_db::AdtId) -> chalk_db::Variances;
 
     #[salsa::invoke(chalk_db::associated_ty_value_query)]
     fn associated_ty_value(
@@ -134,14 +134,14 @@ pub trait HirDatabase: DefDatabase + Upcast<dyn DefDatabase> {
     fn trait_solve(
         &self,
         krate: CrateId,
-        goal: crate::Canonical<crate::InEnvironment<crate::DomainGoal>>,
+        goal: crate::Canonical<crate::InEnvironment<crate::Goal>>,
     ) -> Option<crate::Solution>;
 
     #[salsa::invoke(crate::traits::trait_solve_query)]
     fn trait_solve_query(
         &self,
         krate: CrateId,
-        goal: crate::Canonical<crate::InEnvironment<crate::DomainGoal>>,
+        goal: crate::Canonical<crate::InEnvironment<crate::Goal>>,
     ) -> Option<crate::Solution>;
 
     #[salsa::invoke(chalk_db::program_clauses_for_chalk_env_query)]
@@ -168,7 +168,7 @@ fn infer_wait(db: &dyn HirDatabase, def: DefWithBodyId) -> Arc<InferenceResult>
 fn trait_solve_wait(
     db: &dyn HirDatabase,
     krate: CrateId,
-    goal: crate::Canonical<crate::InEnvironment<crate::DomainGoal>>,
+    goal: crate::Canonical<crate::InEnvironment<crate::Goal>>,
 ) -> Option<crate::Solution> {
     let _p = profile::span("trait_solve::wait");
     db.trait_solve_query(krate, goal)
diff --git a/crates/hir_ty/src/infer.rs b/crates/hir_ty/src/infer.rs
index 0ee851a74..f1cebbdb9 100644
--- a/crates/hir_ty/src/infer.rs
+++ b/crates/hir_ty/src/infer.rs
@@ -13,8 +13,6 @@
 //! to certain types. To record this, we use the union-find implementation from
 //! the `ena` crate, which is extracted from rustc.
 
-use std::borrow::Cow;
-use std::mem;
 use std::ops::Index;
 use std::sync::Arc;
 
@@ -27,8 +25,8 @@ use hir_def::{
     path::{path, Path},
     resolver::{HasResolver, Resolver, TypeNs},
     type_ref::TypeRef,
-    AdtId, AssocItemId, DefWithBodyId, EnumVariantId, FieldId, FunctionId, Lookup, TraitId,
-    TypeAliasId, VariantId,
+    AdtId, AssocItemId, DefWithBodyId, EnumVariantId, FieldId, FunctionId, HasModule, Lookup,
+    TraitId, TypeAliasId, VariantId,
 };
 use hir_expand::{diagnostics::DiagnosticSink, name::name};
 use la_arena::ArenaMap;
@@ -36,13 +34,11 @@ use rustc_hash::FxHashMap;
 use stdx::impl_from;
 use syntax::SmolStr;
 
-use super::{
-    DomainGoal, Guidance, InEnvironment, ProjectionTy, Solution, TraitEnvironment, TraitRef, Ty,
-};
+use super::{DomainGoal, InEnvironment, ProjectionTy, TraitEnvironment, TraitRef, Ty};
 use crate::{
     db::HirDatabase, fold_tys, infer::diagnostics::InferenceDiagnostic,
-    lower::ImplTraitLoweringMode, to_assoc_type_id, AliasEq, AliasTy, Canonical, Interner,
-    TyBuilder, TyExt, TyKind,
+    lower::ImplTraitLoweringMode, to_assoc_type_id, AliasEq, AliasTy, Goal, Interner, TyBuilder,
+    TyExt, TyKind,
 };
 
 // This lint has a false positive here. See the link below for details.
@@ -106,6 +102,14 @@ impl Default for BindingMode {
     }
 }
 
+#[derive(Debug)]
+pub(crate) struct InferOk {
+    goals: Vec<InEnvironment<Goal>>,
+}
+#[derive(Debug)]
+pub(crate) struct TypeError;
+pub(crate) type InferResult = Result<InferOk, TypeError>;
+
 /// A mismatch between an expected and an inferred type.
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
 pub struct TypeMismatch {
@@ -217,10 +221,8 @@ struct InferenceContext<'a> {
     owner: DefWithBodyId,
     body: Arc<Body>,
     resolver: Resolver,
-    table: unify::InferenceTable,
+    table: unify::InferenceTable<'a>,
     trait_env: Arc<TraitEnvironment>,
-    obligations: Vec<DomainGoal>,
-    last_obligations_check: Option<u32>,
     result: InferenceResult,
     /// The return type of the function being inferred, or the closure if we're
     /// currently within one.
@@ -252,15 +254,15 @@ fn find_breakable<'c>(
 
 impl<'a> InferenceContext<'a> {
     fn new(db: &'a dyn HirDatabase, owner: DefWithBodyId, resolver: Resolver) -> Self {
+        let krate = owner.module(db.upcast()).krate();
+        let trait_env = owner
+            .as_generic_def_id()
+            .map_or_else(|| Arc::new(TraitEnvironment::empty(krate)), |d| db.trait_environment(d));
         InferenceContext {
             result: InferenceResult::default(),
-            table: unify::InferenceTable::new(),
-            obligations: Vec::default(),
-            last_obligations_check: None,
+            table: unify::InferenceTable::new(db, trait_env.clone()),
+            trait_env,
             return_ty: TyKind::Error.intern(&Interner), // set in collect_fn_signature
-            trait_env: owner
-                .as_generic_def_id()
-                .map_or_else(Default::default, |d| db.trait_environment(d)),
             db,
             owner,
             body: db.body(owner),
@@ -271,19 +273,25 @@ impl<'a> InferenceContext<'a> {
     }
 
     fn err_ty(&self) -> Ty {
-        TyKind::Error.intern(&Interner)
+        self.result.standard_types.unknown.clone()
     }
 
     fn resolve_all(mut self) -> InferenceResult {
         // FIXME resolve obligations as well (use Guidance if necessary)
+        self.table.resolve_obligations_as_possible();
+
+        // make sure diverging type variables are marked as such
+        self.table.propagate_diverging_flag();
         let mut result = std::mem::take(&mut self.result);
         for ty in result.type_of_expr.values_mut() {
-            let resolved = self.table.resolve_ty_completely(ty.clone());
-            *ty = resolved;
+            *ty = self.table.resolve_ty_completely(ty.clone());
         }
         for ty in result.type_of_pat.values_mut() {
-            let resolved = self.table.resolve_ty_completely(ty.clone());
-            *ty = resolved;
+            *ty = self.table.resolve_ty_completely(ty.clone());
+        }
+        for mismatch in result.type_mismatches.values_mut() {
+            mismatch.expected = self.table.resolve_ty_completely(mismatch.expected.clone());
+            mismatch.actual = self.table.resolve_ty_completely(mismatch.actual.clone());
         }
         result
     }
@@ -337,6 +345,14 @@ impl<'a> InferenceContext<'a> {
     fn insert_type_vars_shallow(&mut self, ty: Ty) -> Ty {
         match ty.kind(&Interner) {
             TyKind::Error => self.table.new_type_var(),
+            TyKind::InferenceVar(..) => {
+                let ty_resolved = self.resolve_ty_shallow(&ty);
+                if ty_resolved.is_unknown() {
+                    self.table.new_type_var()
+                } else {
+                    ty
+                }
+            }
             _ => ty,
         }
     }
@@ -346,66 +362,19 @@ impl<'a> InferenceContext<'a> {
     }
 
     fn resolve_obligations_as_possible(&mut self) {
-        if self.last_obligations_check == Some(self.table.revision) {
-            // no change
-            return;
-        }
-        let _span = profile::span("resolve_obligations_as_possible");
-
-        self.last_obligations_check = Some(self.table.revision);
-        let obligations = mem::replace(&mut self.obligations, Vec::new());
-        for obligation in obligations {
-            let in_env = InEnvironment::new(&self.trait_env.env, obligation.clone());
-            let canonicalized = self.canonicalizer().canonicalize_obligation(in_env);
-            let solution =
-                self.db.trait_solve(self.resolver.krate().unwrap(), canonicalized.value.clone());
-
-            match solution {
-                Some(Solution::Unique(canonical_subst)) => {
-                    canonicalized.apply_solution(
-                        self,
-                        Canonical {
-                            binders: canonical_subst.binders,
-                            // FIXME: handle constraints
-                            value: canonical_subst.value.subst,
-                        },
-                    );
-                }
-                Some(Solution::Ambig(Guidance::Definite(substs))) => {
-                    canonicalized.apply_solution(self, substs);
-                    self.obligations.push(obligation);
-                }
-                Some(_) => {
-                    // FIXME use this when trying to resolve everything at the end
-                    self.obligations.push(obligation);
-                }
-                None => {
-                    // FIXME obligation cannot be fulfilled => diagnostic
-                }
-            };
-        }
+        self.table.resolve_obligations_as_possible();
     }
 
     fn push_obligation(&mut self, o: DomainGoal) {
-        self.obligations.push(o);
-        self.last_obligations_check = None;
+        self.table.register_obligation(o.cast(&Interner));
     }
 
     fn unify(&mut self, ty1: &Ty, ty2: &Ty) -> bool {
         self.table.unify(ty1, ty2)
     }
 
-    /// Resolves the type as far as currently possible, replacing type variables
-    /// by their known types. All types returned by the infer_* functions should
-    /// be resolved as far as possible, i.e. contain no type variables with
-    /// known type.
-    fn resolve_ty_as_possible(&mut self, ty: Ty) -> Ty {
+    fn resolve_ty_shallow(&mut self, ty: &Ty) -> Ty {
         self.resolve_obligations_as_possible();
-
-        self.table.resolve_ty_as_possible(ty)
-    }
-
-    fn resolve_ty_shallow<'b>(&mut self, ty: &'b Ty) -> Cow<'b, Ty> {
         self.table.resolve_ty_shallow(ty)
     }
 
@@ -439,7 +408,7 @@ impl<'a> InferenceContext<'a> {
                 };
                 self.push_obligation(trait_ref.cast(&Interner));
                 self.push_obligation(alias_eq.cast(&Interner));
-                self.resolve_ty_as_possible(ty)
+                ty
             }
             None => self.err_ty(),
         }
@@ -452,25 +421,7 @@ impl<'a> InferenceContext<'a> {
     /// call). `make_ty` handles this already, but e.g. for field types we need
     /// to do it as well.
     fn normalize_associated_types_in(&mut self, ty: Ty) -> Ty {
-        let ty = self.resolve_ty_as_possible(ty);
-        fold_tys(
-            ty,
-            |ty, _| match ty.kind(&Interner) {
-                TyKind::Alias(AliasTy::Projection(proj_ty)) => {
-                    self.normalize_projection_ty(proj_ty.clone())
-                }
-                _ => ty,
-            },
-            DebruijnIndex::INNERMOST,
-        )
-    }
-
-    fn normalize_projection_ty(&mut self, proj_ty: ProjectionTy) -> Ty {
-        let var = self.table.new_type_var();
-        let alias_eq = AliasEq { alias: AliasTy::Projection(proj_ty), ty: var.clone() };
-        let obligation = alias_eq.cast(&Interner);
-        self.push_obligation(obligation);
-        var
+        self.table.normalize_associated_types_in(ty)
     }
 
     fn resolve_variant(&mut self, path: Option<&Path>) -> (Ty, Option<VariantId>) {
@@ -720,17 +671,23 @@ impl<'a> InferenceContext<'a> {
 /// When inferring an expression, we propagate downward whatever type hint we
 /// are able in the form of an `Expectation`.
 #[derive(Clone, PartialEq, Eq, Debug)]
-struct Expectation {
-    ty: Ty,
-    /// See the `rvalue_hint` method.
-    rvalue_hint: bool,
+enum Expectation {
+    None,
+    HasType(Ty),
+    // Castable(Ty), // rustc has this, we currently just don't propagate an expectation for casts
+    RValueLikeUnsized(Ty),
 }
 
 impl Expectation {
     /// The expectation that the type of the expression needs to equal the given
     /// type.
     fn has_type(ty: Ty) -> Self {
-        Expectation { ty, rvalue_hint: false }
+        if ty.is_unknown() {
+            // FIXME: get rid of this?
+            Expectation::None
+        } else {
+            Expectation::HasType(ty)
+        }
     }
 
     /// The following explanation is copied straight from rustc:
@@ -754,24 +711,41 @@ impl Expectation {
     /// See the test case `test/ui/coerce-expect-unsized.rs` and #20169
     /// for examples of where this comes up,.
     fn rvalue_hint(ty: Ty) -> Self {
-        Expectation { ty, rvalue_hint: true }
+        match ty.strip_references().kind(&Interner) {
+            TyKind::Slice(_) | TyKind::Str | TyKind::Dyn(_) => Expectation::RValueLikeUnsized(ty),
+            _ => Expectation::has_type(ty),
+        }
     }
 
     /// This expresses no expectation on the type.
     fn none() -> Self {
-        Expectation {
-            // FIXME
-            ty: TyKind::Error.intern(&Interner),
-            rvalue_hint: false,
+        Expectation::None
+    }
+
+    fn resolve(&self, table: &mut unify::InferenceTable) -> Expectation {
+        match self {
+            Expectation::None => Expectation::None,
+            Expectation::HasType(t) => Expectation::HasType(table.resolve_ty_shallow(t)),
+            Expectation::RValueLikeUnsized(t) => {
+                Expectation::RValueLikeUnsized(table.resolve_ty_shallow(t))
+            }
         }
     }
 
-    fn coercion_target(&self) -> Ty {
-        if self.rvalue_hint {
-            // FIXME
-            TyKind::Error.intern(&Interner)
-        } else {
-            self.ty.clone()
+    fn to_option(&self, table: &mut unify::InferenceTable) -> Option<Ty> {
+        match self.resolve(table) {
+            Expectation::None => None,
+            Expectation::HasType(t) |
+            // Expectation::Castable(t) |
+            Expectation::RValueLikeUnsized(t) => Some(t),
+        }
+    }
+
+    fn only_has_type(&self, table: &mut unify::InferenceTable) -> Option<Ty> {
+        match self {
+            Expectation::HasType(t) => Some(table.resolve_ty_shallow(t)),
+            // Expectation::Castable(_) |
+            Expectation::RValueLikeUnsized(_) | Expectation::None => None,
         }
     }
 }
diff --git a/crates/hir_ty/src/infer/coerce.rs b/crates/hir_ty/src/infer/coerce.rs
index 1f463a425..765a02b1c 100644
--- a/crates/hir_ty/src/infer/coerce.rs
+++ b/crates/hir_ty/src/infer/coerce.rs
@@ -2,156 +2,414 @@
 //! happen in certain places, e.g. weakening `&mut` to `&` or deref coercions
 //! like going from `&Vec<T>` to `&[T]`.
 //!
-//! See: https://doc.rust-lang.org/nomicon/coercions.html
+//! See https://doc.rust-lang.org/nomicon/coercions.html and
+//! librustc_typeck/check/coercion.rs.
 
 use chalk_ir::{cast::Cast, Mutability, TyVariableKind};
-use hir_def::lang_item::LangItemTarget;
+use hir_def::{expr::ExprId, lang_item::LangItemTarget};
 
-use crate::{autoderef, Canonical, Interner, Solution, Ty, TyBuilder, TyExt, TyKind};
+use crate::{
+    autoderef, infer::TypeMismatch, static_lifetime, Canonical, DomainGoal, FnPointer, FnSig,
+    Interner, Solution, Substitution, Ty, TyBuilder, TyExt, TyKind,
+};
 
-use super::{InEnvironment, InferenceContext};
+use super::{InEnvironment, InferOk, InferResult, InferenceContext, TypeError};
 
 impl<'a> InferenceContext<'a> {
     /// Unify two types, but may coerce the first one to the second one
     /// using "implicit coercion rules" if needed.
     pub(super) fn coerce(&mut self, from_ty: &Ty, to_ty: &Ty) -> bool {
-        let from_ty = self.resolve_ty_shallow(from_ty).into_owned();
+        let from_ty = self.resolve_ty_shallow(from_ty);
         let to_ty = self.resolve_ty_shallow(to_ty);
-        self.coerce_inner(from_ty, &to_ty)
+        match self.coerce_inner(from_ty, &to_ty) {
+            Ok(result) => {
+                self.table.register_infer_ok(result);
+                true
+            }
+            Err(_) => {
+                // FIXME deal with error
+                false
+            }
+        }
     }
 
     /// Merge two types from different branches, with possible coercion.
     ///
     /// Mostly this means trying to coerce one to the other, but
-    ///  - if we have two function types for different functions, we need to
+    ///  - if we have two function types for different functions or closures, we need to
     ///    coerce both to function pointers;
     ///  - if we were concerned with lifetime subtyping, we'd need to look for a
     ///    least upper bound.
-    pub(super) fn coerce_merge_branch(&mut self, ty1: &Ty, ty2: &Ty) -> Ty {
-        if self.coerce(ty1, ty2) {
-            ty2.clone()
-        } else if self.coerce(ty2, ty1) {
+    pub(super) fn coerce_merge_branch(&mut self, id: Option<ExprId>, ty1: &Ty, ty2: &Ty) -> Ty {
+        let ty1 = self.resolve_ty_shallow(ty1);
+        let ty2 = self.resolve_ty_shallow(ty2);
+        // Special case: two function types. Try to coerce both to
+        // pointers to have a chance at getting a match. See
+        // https://github.com/rust-lang/rust/blob/7b805396bf46dce972692a6846ce2ad8481c5f85/src/librustc_typeck/check/coercion.rs#L877-L916
+        let sig = match (ty1.kind(&Interner), ty2.kind(&Interner)) {
+            (TyKind::FnDef(..), TyKind::FnDef(..))
+            | (TyKind::Closure(..), TyKind::FnDef(..))
+            | (TyKind::FnDef(..), TyKind::Closure(..))
+            | (TyKind::Closure(..), TyKind::Closure(..)) => {
+                // FIXME: we're ignoring safety here. To be more correct, if we have one FnDef and one Closure,
+                // we should be coercing the closure to a fn pointer of the safety of the FnDef
+                cov_mark::hit!(coerce_fn_reification);
+                let sig = ty1.callable_sig(self.db).expect("FnDef without callable sig");
+                Some(sig)
+            }
+            _ => None,
+        };
+        if let Some(sig) = sig {
+            let target_ty = TyKind::Function(sig.to_fn_ptr()).intern(&Interner);
+            let result1 = self.coerce_inner(ty1.clone(), &target_ty);
+            let result2 = self.coerce_inner(ty2.clone(), &target_ty);
+            if let (Ok(result1), Ok(result2)) = (result1, result2) {
+                self.table.register_infer_ok(result1);
+                self.table.register_infer_ok(result2);
+                return target_ty;
+            }
+        }
+
+        // It might not seem like it, but order is important here: ty1 is our
+        // "previous" type, ty2 is the "new" one being added. If the previous
+        // type is a type variable and the new one is `!`, trying it the other
+        // way around first would mean we make the type variable `!`, instead of
+        // just marking it as possibly diverging.
+        if self.coerce(&ty2, &ty1) {
             ty1.clone()
+        } else if self.coerce(&ty1, &ty2) {
+            ty2.clone()
         } else {
-            if let (TyKind::FnDef(..), TyKind::FnDef(..)) =
-                (ty1.kind(&Interner), ty2.kind(&Interner))
-            {
-                cov_mark::hit!(coerce_fn_reification);
-                // Special case: two function types. Try to coerce both to
-                // pointers to have a chance at getting a match. See
-                // https://github.com/rust-lang/rust/blob/7b805396bf46dce972692a6846ce2ad8481c5f85/src/librustc_typeck/check/coercion.rs#L877-L916
-                let sig1 = ty1.callable_sig(self.db).expect("FnDef without callable sig");
-                let sig2 = ty2.callable_sig(self.db).expect("FnDef without callable sig");
-                let ptr_ty1 = TyBuilder::fn_ptr(sig1);
-                let ptr_ty2 = TyBuilder::fn_ptr(sig2);
-                self.coerce_merge_branch(&ptr_ty1, &ptr_ty2)
-            } else {
-                cov_mark::hit!(coerce_merge_fail_fallback);
-                ty1.clone()
+            if let Some(id) = id {
+                self.result
+                    .type_mismatches
+                    .insert(id.into(), TypeMismatch { expected: ty1.clone(), actual: ty2.clone() });
             }
+            cov_mark::hit!(coerce_merge_fail_fallback);
+            ty1.clone()
         }
     }
 
-    fn coerce_inner(&mut self, mut from_ty: Ty, to_ty: &Ty) -> bool {
-        match (from_ty.kind(&Interner), to_ty.kind(&Interner)) {
-            // Never type will make type variable to fallback to Never Type instead of Unknown.
-            (TyKind::Never, TyKind::InferenceVar(tv, TyVariableKind::General)) => {
-                self.table.type_variable_table.set_diverging(*tv, true);
-                return true;
+    fn coerce_inner(&mut self, from_ty: Ty, to_ty: &Ty) -> InferResult {
+        if from_ty.is_never() {
+            // Subtle: If we are coercing from `!` to `?T`, where `?T` is an unbound
+            // type variable, we want `?T` to fallback to `!` if not
+            // otherwise constrained. An example where this arises:
+            //
+            //     let _: Option<?T> = Some({ return; });
+            //
+            // here, we would coerce from `!` to `?T`.
+            match to_ty.kind(&Interner) {
+                TyKind::InferenceVar(tv, TyVariableKind::General) => {
+                    self.table.set_diverging(*tv, true);
+                }
+                _ => {}
             }
-            (TyKind::Never, _) => return true,
+            return Ok(InferOk { goals: Vec::new() });
+        }
 
-            // Trivial cases, this should go after `never` check to
-            // avoid infer result type to be never
-            _ => {
-                if self.table.unify_inner_trivial(&from_ty, &to_ty, 0) {
-                    return true;
-                }
+        // Consider coercing the subtype to a DST
+        if let Ok(ret) = self.try_coerce_unsized(&from_ty, &to_ty) {
+            return Ok(ret);
+        }
+
+        // Examine the supertype and consider auto-borrowing.
+        match to_ty.kind(&Interner) {
+            TyKind::Raw(mt, _) => {
+                return self.coerce_ptr(from_ty, to_ty, *mt);
             }
+            TyKind::Ref(mt, _, _) => {
+                return self.coerce_ref(from_ty, to_ty, *mt);
+            }
+            _ => {}
         }
 
-        // Pointer weakening and function to pointer
-        match (from_ty.kind(&Interner), to_ty.kind(&Interner)) {
-            // `*mut T` -> `*const T`
-            (TyKind::Raw(_, inner), TyKind::Raw(m2 @ Mutability::Not, ..)) => {
-                from_ty = TyKind::Raw(*m2, inner.clone()).intern(&Interner);
+        match from_ty.kind(&Interner) {
+            TyKind::FnDef(..) => {
+                // Function items are coercible to any closure
+                // type; function pointers are not (that would
+                // require double indirection).
+                // Additionally, we permit coercion of function
+                // items to drop the unsafe qualifier.
+                self.coerce_from_fn_item(from_ty, to_ty)
             }
-            // `&mut T` -> `&T`
-            (TyKind::Ref(_, lt, inner), TyKind::Ref(m2 @ Mutability::Not, ..)) => {
-                from_ty = TyKind::Ref(*m2, lt.clone(), inner.clone()).intern(&Interner);
+            TyKind::Function(from_fn_ptr) => {
+                // We permit coercion of fn pointers to drop the
+                // unsafe qualifier.
+                self.coerce_from_fn_pointer(from_ty.clone(), from_fn_ptr, to_ty)
             }
-            // `&T` -> `*const T`
-            // `&mut T` -> `*mut T`/`*const T`
-            (TyKind::Ref(.., substs), &TyKind::Raw(m2 @ Mutability::Not, ..))
-            | (TyKind::Ref(Mutability::Mut, _, substs), &TyKind::Raw(m2, ..)) => {
-                from_ty = TyKind::Raw(m2, substs.clone()).intern(&Interner);
+            TyKind::Closure(_, from_substs) => {
+                // Non-capturing closures are coercible to
+                // function pointers or unsafe function pointers.
+                // It cannot convert closures that require unsafe.
+                self.coerce_closure_to_fn(from_ty.clone(), from_substs, to_ty)
             }
+            _ => {
+                // Otherwise, just use unification rules.
+                self.table.try_unify(&from_ty, to_ty)
+            }
+        }
+    }
 
-            // Illegal mutability conversion
-            (TyKind::Raw(Mutability::Not, ..), TyKind::Raw(Mutability::Mut, ..))
-            | (TyKind::Ref(Mutability::Not, ..), TyKind::Ref(Mutability::Mut, ..)) => return false,
+    fn coerce_ptr(&mut self, from_ty: Ty, to_ty: &Ty, to_mt: Mutability) -> InferResult {
+        let (_is_ref, from_mt, from_inner) = match from_ty.kind(&Interner) {
+            TyKind::Ref(mt, _, ty) => (true, mt, ty),
+            TyKind::Raw(mt, ty) => (false, mt, ty),
+            _ => return self.table.try_unify(&from_ty, to_ty),
+        };
 
-            // `{function_type}` -> `fn()`
-            (TyKind::FnDef(..), TyKind::Function { .. }) => match from_ty.callable_sig(self.db) {
-                None => return false,
-                Some(sig) => {
-                    from_ty = TyBuilder::fn_ptr(sig);
-                }
+        coerce_mutabilities(*from_mt, to_mt)?;
+
+        // Check that the types which they point at are compatible.
+        let from_raw = TyKind::Raw(to_mt, from_inner.clone()).intern(&Interner);
+        // FIXME: behavior differs based on is_ref once we're computing adjustments
+        self.table.try_unify(&from_raw, to_ty)
+    }
+
+    /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
+    /// To match `A` with `B`, autoderef will be performed,
+    /// calling `deref`/`deref_mut` where necessary.
+    fn coerce_ref(&mut self, from_ty: Ty, to_ty: &Ty, to_mt: Mutability) -> InferResult {
+        match from_ty.kind(&Interner) {
+            TyKind::Ref(mt, _, _) => {
+                coerce_mutabilities(*mt, to_mt)?;
+            }
+            _ => return self.table.try_unify(&from_ty, to_ty),
+        };
+
+        // NOTE: this code is mostly copied and adapted from rustc, and
+        // currently more complicated than necessary, carrying errors around
+        // etc.. This complication will become necessary when we actually track
+        // details of coercion errors though, so I think it's useful to leave
+        // the structure like it is.
+
+        let canonicalized = self.canonicalize(from_ty.clone());
+        let autoderef = autoderef::autoderef(
+            self.db,
+            self.resolver.krate(),
+            InEnvironment {
+                goal: canonicalized.value.clone(),
+                environment: self.trait_env.env.clone(),
             },
+        );
+        let mut first_error = None;
+        let mut found = None;
 
-            (TyKind::Closure(.., substs), TyKind::Function { .. }) => {
-                from_ty = substs.at(&Interner, 0).assert_ty_ref(&Interner).clone();
+        for (autoderefs, referent_ty) in autoderef.enumerate() {
+            if autoderefs == 0 {
+                // Don't let this pass, otherwise it would cause
+                // &T to autoref to &&T.
+                continue;
             }
 
-            _ => {}
+            let referent_ty = canonicalized.decanonicalize_ty(referent_ty.value);
+
+            // At this point, we have deref'd `a` to `referent_ty`.  So
+            // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
+            // In the autoderef loop for `&'a mut Vec<T>`, we would get
+            // three callbacks:
+            //
+            // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
+            // - `Vec<T>` -- 1 deref
+            // - `[T]` -- 2 deref
+            //
+            // At each point after the first callback, we want to
+            // check to see whether this would match out target type
+            // (`&'b mut [T]`) if we autoref'd it. We can't just
+            // compare the referent types, though, because we still
+            // have to consider the mutability. E.g., in the case
+            // we've been considering, we have an `&mut` reference, so
+            // the `T` in `[T]` needs to be unified with equality.
+            //
+            // Therefore, we construct reference types reflecting what
+            // the types will be after we do the final auto-ref and
+            // compare those. Note that this means we use the target
+            // mutability [1], since it may be that we are coercing
+            // from `&mut T` to `&U`.
+            let lt = static_lifetime(); // FIXME: handle lifetimes correctly, see rustc
+            let derefd_from_ty = TyKind::Ref(to_mt, lt, referent_ty).intern(&Interner);
+            match self.table.try_unify(&derefd_from_ty, to_ty) {
+                Ok(result) => {
+                    found = Some(result);
+                    break;
+                }
+                Err(err) => {
+                    if first_error.is_none() {
+                        first_error = Some(err);
+                    }
+                }
+            }
         }
 
-        if let Some(ret) = self.try_coerce_unsized(&from_ty, &to_ty) {
-            return ret;
+        // Extract type or return an error. We return the first error
+        // we got, which should be from relating the "base" type
+        // (e.g., in example above, the failure from relating `Vec<T>`
+        // to the target type), since that should be the least
+        // confusing.
+        let result = match found {
+            Some(d) => d,
+            None => {
+                let err = first_error.expect("coerce_borrowed_pointer had no error");
+                return Err(err);
+            }
+        };
+
+        Ok(result)
+    }
+
+    /// Attempts to coerce from the type of a Rust function item into a function pointer.
+    fn coerce_from_fn_item(&mut self, from_ty: Ty, to_ty: &Ty) -> InferResult {
+        match to_ty.kind(&Interner) {
+            TyKind::Function(_) => {
+                let from_sig = from_ty.callable_sig(self.db).expect("FnDef had no sig");
+
+                // FIXME check ABI: Intrinsics are not coercible to function pointers
+                // FIXME Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396)
+
+                // FIXME rustc normalizes assoc types in the sig here, not sure if necessary
+
+                let from_sig = from_sig.to_fn_ptr();
+                let from_fn_pointer = TyKind::Function(from_sig.clone()).intern(&Interner);
+                let ok = self.coerce_from_safe_fn(from_fn_pointer, &from_sig, to_ty)?;
+
+                Ok(ok)
+            }
+            _ => self.table.try_unify(&from_ty, to_ty),
         }
+    }
+
+    fn coerce_from_fn_pointer(
+        &mut self,
+        from_ty: Ty,
+        from_f: &FnPointer,
+        to_ty: &Ty,
+    ) -> InferResult {
+        self.coerce_from_safe_fn(from_ty, from_f, to_ty)
+    }
 
-        // Auto Deref if cannot coerce
-        match (from_ty.kind(&Interner), to_ty.kind(&Interner)) {
-            // FIXME: DerefMut
-            (TyKind::Ref(.., st1), TyKind::Ref(.., st2)) => {
-                self.unify_autoderef_behind_ref(st1, st2)
+    fn coerce_from_safe_fn(
+        &mut self,
+        from_ty: Ty,
+        from_fn_ptr: &FnPointer,
+        to_ty: &Ty,
+    ) -> InferResult {
+        if let TyKind::Function(to_fn_ptr) = to_ty.kind(&Interner) {
+            if let (chalk_ir::Safety::Safe, chalk_ir::Safety::Unsafe) =
+                (from_fn_ptr.sig.safety, to_fn_ptr.sig.safety)
+            {
+                let from_unsafe =
+                    TyKind::Function(safe_to_unsafe_fn_ty(from_fn_ptr.clone())).intern(&Interner);
+                return self.table.try_unify(&from_unsafe, to_ty);
             }
+        }
+        self.table.try_unify(&from_ty, to_ty)
+    }
 
-            // Otherwise, normal unify
-            _ => self.unify(&from_ty, to_ty),
+    /// Attempts to coerce from the type of a non-capturing closure into a
+    /// function pointer.
+    fn coerce_closure_to_fn(
+        &mut self,
+        from_ty: Ty,
+        from_substs: &Substitution,
+        to_ty: &Ty,
+    ) -> InferResult {
+        match to_ty.kind(&Interner) {
+            TyKind::Function(fn_ty) /* if from_substs is non-capturing (FIXME) */ => {
+                // We coerce the closure, which has fn type
+                //     `extern "rust-call" fn((arg0,arg1,...)) -> _`
+                // to
+                //     `fn(arg0,arg1,...) -> _`
+                // or
+                //     `unsafe fn(arg0,arg1,...) -> _`
+                let safety = fn_ty.sig.safety;
+                let pointer_ty = coerce_closure_fn_ty(from_substs, safety);
+                self.table.try_unify(&pointer_ty, to_ty)
+            }
+            _ => self.table.try_unify(&from_ty, to_ty),
         }
     }
 
     /// Coerce a type using `from_ty: CoerceUnsized<ty_ty>`
     ///
     /// See: https://doc.rust-lang.org/nightly/std/marker/trait.CoerceUnsized.html
-    fn try_coerce_unsized(&mut self, from_ty: &Ty, to_ty: &Ty) -> Option<bool> {
+    fn try_coerce_unsized(&mut self, from_ty: &Ty, to_ty: &Ty) -> InferResult {
+        // These 'if' statements require some explanation.
+        // The `CoerceUnsized` trait is special - it is only
+        // possible to write `impl CoerceUnsized<B> for A` where
+        // A and B have 'matching' fields. This rules out the following
+        // two types of blanket impls:
+        //
+        // `impl<T> CoerceUnsized<T> for SomeType`
+        // `impl<T> CoerceUnsized<SomeType> for T`
+        //
+        // Both of these trigger a special `CoerceUnsized`-related error (E0376)
+        //
+        // We can take advantage of this fact to avoid performing unecessary work.
+        // If either `source` or `target` is a type variable, then any applicable impl
+        // would need to be generic over the self-type (`impl<T> CoerceUnsized<SomeType> for T`)
+        // or generic over the `CoerceUnsized` type parameter (`impl<T> CoerceUnsized<T> for
+        // SomeType`).
+        //
+        // However, these are exactly the kinds of impls which are forbidden by
+        // the compiler! Therefore, we can be sure that coercion will always fail
+        // when either the source or target type is a type variable. This allows us
+        // to skip performing any trait selection, and immediately bail out.
+        if from_ty.is_ty_var() {
+            return Err(TypeError);
+        }
+        if to_ty.is_ty_var() {
+            return Err(TypeError);
+        }
+
+        // Handle reborrows before trying to solve `Source: CoerceUnsized<Target>`.
+        let coerce_from = match (from_ty.kind(&Interner), to_ty.kind(&Interner)) {
+            (TyKind::Ref(from_mt, _, from_inner), TyKind::Ref(to_mt, _, _)) => {
+                coerce_mutabilities(*from_mt, *to_mt)?;
+
+                let lt = static_lifetime();
+                TyKind::Ref(*to_mt, lt, from_inner.clone()).intern(&Interner)
+            }
+            (TyKind::Ref(from_mt, _, from_inner), TyKind::Raw(to_mt, _)) => {
+                coerce_mutabilities(*from_mt, *to_mt)?;
+
+                TyKind::Raw(*to_mt, from_inner.clone()).intern(&Interner)
+            }
+            _ => from_ty.clone(),
+        };
+
         let krate = self.resolver.krate().unwrap();
         let coerce_unsized_trait = match self.db.lang_item(krate, "coerce_unsized".into()) {
             Some(LangItemTarget::TraitId(trait_)) => trait_,
-            _ => return None,
+            _ => return Err(TypeError),
         };
 
         let trait_ref = {
             let b = TyBuilder::trait_ref(self.db, coerce_unsized_trait);
             if b.remaining() != 2 {
                 // The CoerceUnsized trait should have two generic params: Self and T.
-                return None;
+                return Err(TypeError);
             }
-            b.push(from_ty.clone()).push(to_ty.clone()).build()
+            b.push(coerce_from.clone()).push(to_ty.clone()).build()
         };
 
-        let goal = InEnvironment::new(&self.trait_env.env, trait_ref.cast(&Interner));
+        let goal: InEnvironment<DomainGoal> =
+            InEnvironment::new(&self.trait_env.env, trait_ref.cast(&Interner));
 
-        let canonicalizer = self.canonicalizer();
-        let canonicalized = canonicalizer.canonicalize_obligation(goal);
+        let canonicalized = self.canonicalize(goal);
 
-        let solution = self.db.trait_solve(krate, canonicalized.value.clone())?;
+        // FIXME: rustc's coerce_unsized is more specialized -- it only tries to
+        // solve `CoerceUnsized` and `Unsize` goals at this point and leaves the
+        // rest for later. Also, there's some logic about sized type variables.
+        // Need to find out in what cases this is necessary
+        let solution = self
+            .db
+            .trait_solve(krate, canonicalized.value.clone().cast(&Interner))
+            .ok_or(TypeError)?;
 
         match solution {
             Solution::Unique(v) => {
                 canonicalized.apply_solution(
-                    self,
+                    &mut self.table,
                     Canonical {
                         binders: v.binders,
                         // FIXME handle constraints
@@ -159,38 +417,40 @@ impl<'a> InferenceContext<'a> {
                     },
                 );
             }
-            _ => return None,
+            // FIXME: should we accept ambiguous results here?
+            _ => return Err(TypeError),
         };
 
-        Some(true)
+        Ok(InferOk { goals: Vec::new() })
     }
+}
 
-    /// Unify `from_ty` to `to_ty` with optional auto Deref
-    ///
-    /// Note that the parameters are already stripped the outer reference.
-    fn unify_autoderef_behind_ref(&mut self, from_ty: &Ty, to_ty: &Ty) -> bool {
-        let canonicalized = self.canonicalizer().canonicalize_ty(from_ty.clone());
-        let to_ty = self.resolve_ty_shallow(&to_ty);
-        // FIXME: Auto DerefMut
-        for derefed_ty in autoderef::autoderef(
-            self.db,
-            self.resolver.krate(),
-            InEnvironment {
-                goal: canonicalized.value.clone(),
-                environment: self.trait_env.env.clone(),
-            },
-        ) {
-            let derefed_ty = canonicalized.decanonicalize_ty(derefed_ty.value);
-            let from_ty = self.resolve_ty_shallow(&derefed_ty);
-            // Stop when constructor matches.
-            if from_ty.equals_ctor(&to_ty) {
-                // It will not recurse to `coerce`.
-                return self.table.unify(&from_ty, &to_ty);
-            } else if self.table.unify_inner_trivial(&derefed_ty, &to_ty, 0) {
-                return true;
-            }
-        }
+fn coerce_closure_fn_ty(closure_substs: &Substitution, safety: chalk_ir::Safety) -> Ty {
+    let closure_sig = closure_substs.at(&Interner, 0).assert_ty_ref(&Interner).clone();
+    match closure_sig.kind(&Interner) {
+        TyKind::Function(fn_ty) => TyKind::Function(FnPointer {
+            num_binders: fn_ty.num_binders,
+            sig: FnSig { safety, ..fn_ty.sig },
+            substitution: fn_ty.substitution.clone(),
+        })
+        .intern(&Interner),
+        _ => TyKind::Error.intern(&Interner),
+    }
+}
+
+fn safe_to_unsafe_fn_ty(fn_ty: FnPointer) -> FnPointer {
+    FnPointer {
+        num_binders: fn_ty.num_binders,
+        sig: FnSig { safety: chalk_ir::Safety::Unsafe, ..fn_ty.sig },
+        substitution: fn_ty.substitution,
+    }
+}
 
-        false
+fn coerce_mutabilities(from: Mutability, to: Mutability) -> Result<(), TypeError> {
+    match (from, to) {
+        (Mutability::Mut, Mutability::Mut)
+        | (Mutability::Mut, Mutability::Not)
+        | (Mutability::Not, Mutability::Not) => Ok(()),
+        (Mutability::Not, Mutability::Mut) => Err(TypeError),
     }
 }
diff --git a/crates/hir_ty/src/infer/expr.rs b/crates/hir_ty/src/infer/expr.rs
index 7278faeec..08c05c67c 100644
--- a/crates/hir_ty/src/infer/expr.rs
+++ b/crates/hir_ty/src/infer/expr.rs
@@ -35,39 +35,43 @@ use super::{
 impl<'a> InferenceContext<'a> {
     pub(super) fn infer_expr(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
         let ty = self.infer_expr_inner(tgt_expr, expected);
-        if ty.is_never() {
+        if self.resolve_ty_shallow(&ty).is_never() {
             // Any expression that produces a value of type `!` must have diverged
             self.diverges = Diverges::Always;
         }
-        let could_unify = self.unify(&ty, &expected.ty);
-        if !could_unify {
-            self.result.type_mismatches.insert(
-                tgt_expr.into(),
-                TypeMismatch { expected: expected.ty.clone(), actual: ty.clone() },
-            );
+        if let Some(expected_ty) = expected.only_has_type(&mut self.table) {
+            let could_unify = self.unify(&ty, &expected_ty);
+            if !could_unify {
+                self.result.type_mismatches.insert(
+                    tgt_expr.into(),
+                    TypeMismatch { expected: expected_ty.clone(), actual: ty.clone() },
+                );
+            }
         }
-        self.resolve_ty_as_possible(ty)
+        ty
     }
 
     /// Infer type of expression with possibly implicit coerce to the expected type.
     /// Return the type after possible coercion.
     pub(super) fn infer_expr_coerce(&mut self, expr: ExprId, expected: &Expectation) -> Ty {
         let ty = self.infer_expr_inner(expr, &expected);
-        let ty = if !self.coerce(&ty, &expected.coercion_target()) {
-            self.result.type_mismatches.insert(
-                expr.into(),
-                TypeMismatch { expected: expected.ty.clone(), actual: ty.clone() },
-            );
-            // Return actual type when type mismatch.
-            // This is needed for diagnostic when return type mismatch.
-            ty
-        } else if expected.coercion_target().is_unknown() {
-            ty
+        let ty = if let Some(target) = expected.only_has_type(&mut self.table) {
+            if !self.coerce(&ty, &target) {
+                self.result.type_mismatches.insert(
+                    expr.into(),
+                    TypeMismatch { expected: target.clone(), actual: ty.clone() },
+                );
+                // Return actual type when type mismatch.
+                // This is needed for diagnostic when return type mismatch.
+                ty
+            } else {
+                target.clone()
+            }
         } else {
-            expected.ty.clone()
+            ty
         };
 
-        self.resolve_ty_as_possible(ty)
+        ty
     }
 
     fn callable_sig_from_fn_trait(&mut self, ty: &Ty, num_args: usize) -> Option<(Vec<Ty>, Ty)> {
@@ -98,10 +102,10 @@ impl<'a> InferenceContext<'a> {
             goal: projection.trait_ref(self.db).cast(&Interner),
             environment: trait_env,
         };
-        let canonical = self.canonicalizer().canonicalize_obligation(obligation.clone());
-        if self.db.trait_solve(krate, canonical.value).is_some() {
+        let canonical = self.canonicalize(obligation.clone());
+        if self.db.trait_solve(krate, canonical.value.cast(&Interner)).is_some() {
             self.push_obligation(obligation.goal);
-            let return_ty = self.normalize_projection_ty(projection);
+            let return_ty = self.table.normalize_projection_ty(projection);
             Some((arg_tys, return_ty))
         } else {
             None
@@ -131,17 +135,21 @@ impl<'a> InferenceContext<'a> {
                 let condition_diverges = mem::replace(&mut self.diverges, Diverges::Maybe);
                 let mut both_arms_diverge = Diverges::Always;
 
+                let mut result_ty = self.table.new_type_var();
                 let then_ty = self.infer_expr_inner(*then_branch, &expected);
                 both_arms_diverge &= mem::replace(&mut self.diverges, Diverges::Maybe);
+                result_ty = self.coerce_merge_branch(Some(*then_branch), &result_ty, &then_ty);
                 let else_ty = match else_branch {
                     Some(else_branch) => self.infer_expr_inner(*else_branch, &expected),
                     None => TyBuilder::unit(),
                 };
                 both_arms_diverge &= self.diverges;
+                // FIXME: create a synthetic `else {}` so we have something to refer to here instead of None?
+                result_ty = self.coerce_merge_branch(*else_branch, &result_ty, &else_ty);
 
                 self.diverges = condition_diverges | both_arms_diverge;
 
-                self.coerce_merge_branch(&then_ty, &else_ty)
+                result_ty
             }
             Expr::Block { statements, tail, label, id: _ } => {
                 let old_resolver = mem::replace(
@@ -277,12 +285,13 @@ impl<'a> InferenceContext<'a> {
                 // Eagerly try to relate the closure type with the expected
                 // type, otherwise we often won't have enough information to
                 // infer the body.
-                self.coerce(&closure_ty, &expected.ty);
+                if let Some(t) = expected.only_has_type(&mut self.table) {
+                    self.coerce(&closure_ty, &t);
+                }
 
                 // Now go through the argument patterns
                 for (arg_pat, arg_ty) in args.iter().zip(sig_tys) {
-                    let resolved = self.resolve_ty_as_possible(arg_ty);
-                    self.infer_pat(*arg_pat, &resolved, BindingMode::default());
+                    self.infer_pat(*arg_pat, &arg_ty, BindingMode::default());
                 }
 
                 let prev_diverges = mem::replace(&mut self.diverges, Diverges::Maybe);
@@ -297,13 +306,13 @@ impl<'a> InferenceContext<'a> {
             }
             Expr::Call { callee, args } => {
                 let callee_ty = self.infer_expr(*callee, &Expectation::none());
-                let canonicalized = self.canonicalizer().canonicalize_ty(callee_ty.clone());
+                let canonicalized = self.canonicalize(callee_ty.clone());
                 let mut derefs = autoderef(
                     self.db,
                     self.resolver.krate(),
                     InEnvironment {
                         goal: canonicalized.value.clone(),
-                        environment: self.trait_env.env.clone(),
+                        environment: self.table.trait_env.env.clone(),
                     },
                 );
                 let (param_tys, ret_ty): (Vec<Ty>, Ty) = derefs
@@ -350,7 +359,7 @@ impl<'a> InferenceContext<'a> {
 
                     let arm_ty = self.infer_expr_inner(arm.expr, &expected);
                     all_arms_diverge &= self.diverges;
-                    result_ty = self.coerce_merge_branch(&result_ty, &arm_ty);
+                    result_ty = self.coerce_merge_branch(Some(arm.expr), &result_ty, &arm_ty);
                 }
 
                 self.diverges = matchee_diverges | all_arms_diverge;
@@ -364,12 +373,6 @@ impl<'a> InferenceContext<'a> {
             }
             Expr::Continue { .. } => TyKind::Never.intern(&Interner),
             Expr::Break { expr, label } => {
-                let val_ty = if let Some(expr) = expr {
-                    self.infer_expr(*expr, &Expectation::none())
-                } else {
-                    TyBuilder::unit()
-                };
-
                 let last_ty =
                     if let Some(ctxt) = find_breakable(&mut self.breakables, label.as_ref()) {
                         ctxt.break_ty.clone()
@@ -377,7 +380,14 @@ impl<'a> InferenceContext<'a> {
                         self.err_ty()
                     };
 
-                let merged_type = self.coerce_merge_branch(&last_ty, &val_ty);
+                let val_ty = if let Some(expr) = expr {
+                    self.infer_expr(*expr, &Expectation::none())
+                } else {
+                    TyBuilder::unit()
+                };
+
+                // FIXME: create a synthetic `()` during lowering so we have something to refer to here?
+                let merged_type = self.coerce_merge_branch(*expr, &last_ty, &val_ty);
 
                 if let Some(ctxt) = find_breakable(&mut self.breakables, label.as_ref()) {
                     ctxt.break_ty = merged_type;
@@ -411,7 +421,9 @@ impl<'a> InferenceContext<'a> {
                     self.write_variant_resolution(tgt_expr.into(), variant);
                 }
 
-                self.unify(&ty, &expected.ty);
+                if let Some(t) = expected.only_has_type(&mut self.table) {
+                    self.unify(&ty, &t);
+                }
 
                 let substs = ty
                     .as_adt()
@@ -442,7 +454,7 @@ impl<'a> InferenceContext<'a> {
             }
             Expr::Field { expr, name } => {
                 let receiver_ty = self.infer_expr_inner(*expr, &Expectation::none());
-                let canonicalized = self.canonicalizer().canonicalize_ty(receiver_ty);
+                let canonicalized = self.canonicalize(receiver_ty);
                 let ty = autoderef::autoderef(
                     self.db,
                     self.resolver.krate(),
@@ -514,6 +526,7 @@ impl<'a> InferenceContext<'a> {
                 self.resolve_associated_type(inner_ty, self.resolve_ops_try_ok())
             }
             Expr::Cast { expr, type_ref } => {
+                // FIXME: propagate the "castable to" expectation (and find a test case that shows this is necessary)
                 let _inner_ty = self.infer_expr_inner(*expr, &Expectation::none());
                 let cast_ty = self.make_ty(type_ref);
                 // FIXME check the cast...
@@ -521,15 +534,17 @@ impl<'a> InferenceContext<'a> {
             }
             Expr::Ref { expr, rawness, mutability } => {
                 let mutability = lower_to_chalk_mutability(*mutability);
-                let expectation = if let Some((exp_inner, exp_rawness, exp_mutability)) =
-                    &expected.ty.as_reference_or_ptr()
+                let expectation = if let Some((exp_inner, exp_rawness, exp_mutability)) = expected
+                    .only_has_type(&mut self.table)
+                    .as_ref()
+                    .and_then(|t| t.as_reference_or_ptr())
                 {
-                    if *exp_mutability == Mutability::Mut && mutability == Mutability::Not {
-                        // FIXME: throw type error - expected mut reference but found shared ref,
+                    if exp_mutability == Mutability::Mut && mutability == Mutability::Not {
+                        // FIXME: record type error - expected mut reference but found shared ref,
                         // which cannot be coerced
                     }
-                    if *exp_rawness == Rawness::Ref && *rawness == Rawness::RawPtr {
-                        // FIXME: throw type error - expected reference but found ptr,
+                    if exp_rawness == Rawness::Ref && *rawness == Rawness::RawPtr {
+                        // FIXME: record type error - expected reference but found ptr,
                         // which cannot be coerced
                     }
                     Expectation::rvalue_hint(Ty::clone(exp_inner))
@@ -556,10 +571,11 @@ impl<'a> InferenceContext<'a> {
             }
             Expr::UnaryOp { expr, op } => {
                 let inner_ty = self.infer_expr_inner(*expr, &Expectation::none());
+                let inner_ty = self.resolve_ty_shallow(&inner_ty);
                 match op {
                     UnaryOp::Deref => match self.resolver.krate() {
                         Some(krate) => {
-                            let canonicalized = self.canonicalizer().canonicalize_ty(inner_ty);
+                            let canonicalized = self.canonicalize(inner_ty);
                             match autoderef::deref(
                                 self.db,
                                 krate,
@@ -612,8 +628,10 @@ impl<'a> InferenceContext<'a> {
                         _ => Expectation::none(),
                     };
                     let lhs_ty = self.infer_expr(*lhs, &lhs_expectation);
+                    let lhs_ty = self.resolve_ty_shallow(&lhs_ty);
                     let rhs_expectation = op::binary_op_rhs_expectation(*op, lhs_ty.clone());
                     let rhs_ty = self.infer_expr(*rhs, &Expectation::has_type(rhs_expectation));
+                    let rhs_ty = self.resolve_ty_shallow(&rhs_ty);
 
                     let ret = op::binary_op_return_ty(*op, lhs_ty.clone(), rhs_ty.clone());
 
@@ -676,7 +694,7 @@ impl<'a> InferenceContext<'a> {
                 if let (Some(index_trait), Some(krate)) =
                     (self.resolve_ops_index(), self.resolver.krate())
                 {
-                    let canonicalized = self.canonicalizer().canonicalize_ty(base_ty);
+                    let canonicalized = self.canonicalize(base_ty);
                     let self_ty = method_resolution::resolve_indexing_op(
                         self.db,
                         &canonicalized.value,
@@ -696,8 +714,12 @@ impl<'a> InferenceContext<'a> {
                 }
             }
             Expr::Tuple { exprs } => {
-                let mut tys = match expected.ty.kind(&Interner) {
-                    TyKind::Tuple(_, substs) => substs
+                let mut tys = match expected
+                    .only_has_type(&mut self.table)
+                    .as_ref()
+                    .map(|t| t.kind(&Interner))
+                {
+                    Some(TyKind::Tuple(_, substs)) => substs
                         .iter(&Interner)
                         .map(|a| a.assert_ty_ref(&Interner).clone())
                         .chain(repeat_with(|| self.table.new_type_var()))
@@ -713,14 +735,16 @@ impl<'a> InferenceContext<'a> {
                 TyKind::Tuple(tys.len(), Substitution::from_iter(&Interner, tys)).intern(&Interner)
             }
             Expr::Array(array) => {
-                let elem_ty = match expected.ty.kind(&Interner) {
-                    TyKind::Array(st, _) | TyKind::Slice(st) => st.clone(),
-                    _ => self.table.new_type_var(),
-                };
+                let elem_ty =
+                    match expected.to_option(&mut self.table).as_ref().map(|t| t.kind(&Interner)) {
+                        Some(TyKind::Array(st, _)) | Some(TyKind::Slice(st)) => st.clone(),
+                        _ => self.table.new_type_var(),
+                    };
 
                 let len = match array {
                     Array::ElementList(items) => {
                         for expr in items.iter() {
+                            // FIXME: use CoerceMany (coerce_merge_branch)
                             self.infer_expr_coerce(*expr, &Expectation::has_type(elem_ty.clone()));
                         }
                         Some(items.len() as u64)
@@ -785,7 +809,6 @@ impl<'a> InferenceContext<'a> {
         };
         // use a new type variable if we got unknown here
         let ty = self.insert_type_vars_shallow(ty);
-        let ty = self.resolve_ty_as_possible(ty);
         self.write_expr_ty(tgt_expr, ty.clone());
         ty
     }
@@ -813,7 +836,6 @@ impl<'a> InferenceContext<'a> {
                         }
                     }
 
-                    let ty = self.resolve_ty_as_possible(ty);
                     self.infer_pat(*pat, &ty, BindingMode::default());
                 }
                 Statement::Expr { expr, .. } => {
@@ -836,7 +858,9 @@ impl<'a> InferenceContext<'a> {
                 // we don't even make an attempt at coercion
                 self.table.new_maybe_never_var()
             } else {
-                self.coerce(&TyBuilder::unit(), &expected.coercion_target());
+                if let Some(t) = expected.only_has_type(&mut self.table) {
+                    self.coerce(&TyBuilder::unit(), &t);
+                }
                 TyBuilder::unit()
             }
         };
@@ -852,7 +876,7 @@ impl<'a> InferenceContext<'a> {
         generic_args: Option<&GenericArgs>,
     ) -> Ty {
         let receiver_ty = self.infer_expr(receiver, &Expectation::none());
-        let canonicalized_receiver = self.canonicalizer().canonicalize_ty(receiver_ty.clone());
+        let canonicalized_receiver = self.canonicalize(receiver_ty.clone());
 
         let traits_in_scope = self.resolver.traits_in_scope(self.db.upcast());
 
@@ -891,7 +915,8 @@ impl<'a> InferenceContext<'a> {
         };
         // Apply autoref so the below unification works correctly
         // FIXME: return correct autorefs from lookup_method
-        let actual_receiver_ty = match expected_receiver_ty.as_reference() {
+        let actual_receiver_ty = match self.resolve_ty_shallow(&expected_receiver_ty).as_reference()
+        {
             Some((_, lifetime, mutability)) => {
                 TyKind::Ref(mutability, lifetime, derefed_receiver_ty).intern(&Interner)
             }
@@ -971,6 +996,7 @@ impl<'a> InferenceContext<'a> {
     }
 
     fn register_obligations_for_call(&mut self, callable_ty: &Ty) {
+        let callable_ty = self.resolve_ty_shallow(&callable_ty);
         if let TyKind::FnDef(fn_def, parameters) = callable_ty.kind(&Interner) {
             let def: CallableDefId = from_chalk(self.db, *fn_def);
             let generic_predicates = self.db.generic_predicates(def.into());
diff --git a/crates/hir_ty/src/infer/pat.rs b/crates/hir_ty/src/infer/pat.rs
index b15f4977d..9c8e3b6ae 100644
--- a/crates/hir_ty/src/infer/pat.rs
+++ b/crates/hir_ty/src/infer/pat.rs
@@ -94,14 +94,15 @@ impl<'a> InferenceContext<'a> {
     pub(super) fn infer_pat(
         &mut self,
         pat: PatId,
-        mut expected: &Ty,
+        expected: &Ty,
         mut default_bm: BindingMode,
     ) -> Ty {
         let body = Arc::clone(&self.body); // avoid borrow checker problem
+        let mut expected = self.resolve_ty_shallow(expected);
 
         if is_non_ref_pat(&body, pat) {
             while let Some((inner, _lifetime, mutability)) = expected.as_reference() {
-                expected = inner;
+                expected = self.resolve_ty_shallow(inner);
                 default_bm = match default_bm {
                     BindingMode::Move => BindingMode::Ref(mutability),
                     BindingMode::Ref(Mutability::Not) => BindingMode::Ref(Mutability::Not),
@@ -147,9 +148,9 @@ impl<'a> InferenceContext<'a> {
             }
             Pat::Or(ref pats) => {
                 if let Some((first_pat, rest)) = pats.split_first() {
-                    let ty = self.infer_pat(*first_pat, expected, default_bm);
+                    let ty = self.infer_pat(*first_pat, &expected, default_bm);
                     for pat in rest {
-                        self.infer_pat(*pat, expected, default_bm);
+                        self.infer_pat(*pat, &expected, default_bm);
                     }
                     ty
                 } else {
@@ -173,13 +174,13 @@ impl<'a> InferenceContext<'a> {
             Pat::TupleStruct { path: p, args: subpats, ellipsis } => self.infer_tuple_struct_pat(
                 p.as_deref(),
                 subpats,
-                expected,
+                &expected,
                 default_bm,
                 pat,
                 *ellipsis,
             ),
             Pat::Record { path: p, args: fields, ellipsis: _ } => {
-                self.infer_record_pat(p.as_deref(), fields, expected, default_bm, pat)
+                self.infer_record_pat(p.as_deref(), fields, &expected, default_bm, pat)
             }
             Pat::Path(path) => {
                 // FIXME use correct resolver for the surrounding expression
@@ -193,7 +194,7 @@ impl<'a> InferenceContext<'a> {
                     BindingMode::convert(*mode)
                 };
                 let inner_ty = if let Some(subpat) = subpat {
-                    self.infer_pat(*subpat, expected, default_bm)
+                    self.infer_pat(*subpat, &expected, default_bm)
                 } else {
                     expected.clone()
                 };
@@ -206,7 +207,6 @@ impl<'a> InferenceContext<'a> {
                     }
                     BindingMode::Move => inner_ty.clone(),
                 };
-                let bound_ty = self.resolve_ty_as_possible(bound_ty);
                 self.write_pat_ty(pat, bound_ty);
                 return inner_ty;
             }
@@ -265,13 +265,12 @@ impl<'a> InferenceContext<'a> {
         };
         // use a new type variable if we got error type here
         let ty = self.insert_type_vars_shallow(ty);
-        if !self.unify(&ty, expected) {
+        if !self.unify(&ty, &expected) {
             self.result.type_mismatches.insert(
                 pat.into(),
                 TypeMismatch { expected: expected.clone(), actual: ty.clone() },
             );
         }
-        let ty = self.resolve_ty_as_possible(ty);
         self.write_pat_ty(pat, ty.clone());
         ty
     }
diff --git a/crates/hir_ty/src/infer/path.rs b/crates/hir_ty/src/infer/path.rs
index 495282eba..14c99eafd 100644
--- a/crates/hir_ty/src/infer/path.rs
+++ b/crates/hir_ty/src/infer/path.rs
@@ -65,7 +65,6 @@ impl<'a> InferenceContext<'a> {
         let typable: ValueTyDefId = match value {
             ValueNs::LocalBinding(pat) => {
                 let ty = self.result.type_of_pat.get(pat)?.clone();
-                let ty = self.resolve_ty_as_possible(ty);
                 return Some(ty);
             }
             ValueNs::FunctionId(it) => it.into(),
@@ -218,14 +217,14 @@ impl<'a> InferenceContext<'a> {
             return Some(result);
         }
 
-        let canonical_ty = self.canonicalizer().canonicalize_ty(ty.clone());
+        let canonical_ty = self.canonicalize(ty.clone());
         let krate = self.resolver.krate()?;
         let traits_in_scope = self.resolver.traits_in_scope(self.db.upcast());
 
         method_resolution::iterate_method_candidates(
             &canonical_ty.value,
             self.db,
-            self.trait_env.clone(),
+            self.table.trait_env.clone(),
             krate,
             &traits_in_scope,
             None,
@@ -275,6 +274,7 @@ impl<'a> InferenceContext<'a> {
         name: &Name,
         id: ExprOrPatId,
     ) -> Option<(ValueNs, Option<Substitution>)> {
+        let ty = self.resolve_ty_shallow(ty);
         let (enum_id, subst) = match ty.as_adt() {
             Some((AdtId::EnumId(e), subst)) => (e, subst),
             _ => return None,
diff --git a/crates/hir_ty/src/infer/unify.rs b/crates/hir_ty/src/infer/unify.rs
index d8e0b4320..21d3fb54e 100644
--- a/crates/hir_ty/src/infer/unify.rs
+++ b/crates/hir_ty/src/infer/unify.rs
@@ -1,177 +1,95 @@
 //! Unification and canonicalization logic.
 
-use std::borrow::Cow;
+use std::{fmt, mem, sync::Arc};
 
 use chalk_ir::{
-    cast::Cast, fold::Fold, interner::HasInterner, FloatTy, IntTy, TyVariableKind, UniverseIndex,
-    VariableKind,
+    cast::Cast, fold::Fold, interner::HasInterner, zip::Zip, FloatTy, IntTy, TyVariableKind,
+    UniverseIndex,
 };
-use ena::unify::{InPlaceUnificationTable, NoError, UnifyKey, UnifyValue};
+use chalk_solve::infer::ParameterEnaVariableExt;
+use ena::unify::UnifyKey;
 
-use super::{DomainGoal, InferenceContext};
+use super::{InferOk, InferResult, InferenceContext, TypeError};
 use crate::{
-    fold_tys, static_lifetime, AliasEq, AliasTy, BoundVar, Canonical, CanonicalVarKinds,
-    DebruijnIndex, FnPointer, FnSubst, InEnvironment, InferenceVar, Interner, Scalar, Substitution,
-    Ty, TyExt, TyKind, WhereClause,
+    db::HirDatabase, fold_tys, static_lifetime, AliasEq, AliasTy, BoundVar, Canonical,
+    DebruijnIndex, GenericArg, Goal, Guidance, InEnvironment, InferenceVar, Interner, ProjectionTy,
+    Scalar, Solution, Substitution, TraitEnvironment, Ty, TyKind, VariableKind,
 };
 
 impl<'a> InferenceContext<'a> {
-    pub(super) fn canonicalizer<'b>(&'b mut self) -> Canonicalizer<'a, 'b>
+    pub(super) fn canonicalize<T: Fold<Interner> + HasInterner<Interner = Interner>>(
+        &mut self,
+        t: T,
+    ) -> Canonicalized<T::Result>
     where
-        'a: 'b,
+        T::Result: HasInterner<Interner = Interner>,
     {
-        Canonicalizer { ctx: self, free_vars: Vec::new(), var_stack: Vec::new() }
+        // try to resolve obligations before canonicalizing, since this might
+        // result in new knowledge about variables
+        self.resolve_obligations_as_possible();
+        self.table.canonicalize(t)
     }
 }
 
-pub(super) struct Canonicalizer<'a, 'b>
-where
-    'a: 'b,
-{
-    ctx: &'b mut InferenceContext<'a>,
-    free_vars: Vec<(InferenceVar, TyVariableKind)>,
-    /// A stack of type variables that is used to detect recursive types (which
-    /// are an error, but we need to protect against them to avoid stack
-    /// overflows).
-    var_stack: Vec<TypeVarId>,
-}
-
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub(super) struct Canonicalized<T>
 where
     T: HasInterner<Interner = Interner>,
 {
     pub(super) value: Canonical<T>,
-    free_vars: Vec<(InferenceVar, TyVariableKind)>,
-}
-
-impl<'a, 'b> Canonicalizer<'a, 'b> {
-    fn add(&mut self, free_var: InferenceVar, kind: TyVariableKind) -> usize {
-        self.free_vars.iter().position(|&(v, _)| v == free_var).unwrap_or_else(|| {
-            let next_index = self.free_vars.len();
-            self.free_vars.push((free_var, kind));
-            next_index
-        })
-    }
-
-    fn do_canonicalize<T: Fold<Interner, Result = T> + HasInterner<Interner = Interner>>(
-        &mut self,
-        t: T,
-        binders: DebruijnIndex,
-    ) -> T {
-        fold_tys(
-            t,
-            |ty, binders| match ty.kind(&Interner) {
-                &TyKind::InferenceVar(var, kind) => {
-                    let inner = from_inference_var(var);
-                    if self.var_stack.contains(&inner) {
-                        // recursive type
-                        return self.ctx.table.type_variable_table.fallback_value(var, kind);
-                    }
-                    if let Some(known_ty) =
-                        self.ctx.table.var_unification_table.inlined_probe_value(inner).known()
-                    {
-                        self.var_stack.push(inner);
-                        let result = self.do_canonicalize(known_ty.clone(), binders);
-                        self.var_stack.pop();
-                        result
-                    } else {
-                        let root = self.ctx.table.var_unification_table.find(inner);
-                        let position = self.add(to_inference_var(root), kind);
-                        TyKind::BoundVar(BoundVar::new(binders, position)).intern(&Interner)
-                    }
-                }
-                _ => ty,
-            },
-            binders,
-        )
-    }
-
-    fn into_canonicalized<T: HasInterner<Interner = Interner>>(
-        self,
-        result: T,
-    ) -> Canonicalized<T> {
-        let kinds = self
-            .free_vars
-            .iter()
-            .map(|&(_, k)| chalk_ir::WithKind::new(VariableKind::Ty(k), UniverseIndex::ROOT));
-        Canonicalized {
-            value: Canonical {
-                value: result,
-                binders: CanonicalVarKinds::from_iter(&Interner, kinds),
-            },
-            free_vars: self.free_vars,
-        }
-    }
-
-    pub(crate) fn canonicalize_ty(mut self, ty: Ty) -> Canonicalized<Ty> {
-        let result = self.do_canonicalize(ty, DebruijnIndex::INNERMOST);
-        self.into_canonicalized(result)
-    }
-
-    pub(crate) fn canonicalize_obligation(
-        mut self,
-        obligation: InEnvironment<DomainGoal>,
-    ) -> Canonicalized<InEnvironment<DomainGoal>> {
-        let result = match obligation.goal {
-            DomainGoal::Holds(wc) => {
-                DomainGoal::Holds(self.do_canonicalize(wc, DebruijnIndex::INNERMOST))
-            }
-            _ => unimplemented!(),
-        };
-        self.into_canonicalized(InEnvironment { goal: result, environment: obligation.environment })
-    }
+    free_vars: Vec<GenericArg>,
 }
 
 impl<T: HasInterner<Interner = Interner>> Canonicalized<T> {
     pub(super) fn decanonicalize_ty(&self, ty: Ty) -> Ty {
-        crate::fold_free_vars(ty, |bound, _binders| {
-            let (v, k) = self.free_vars[bound.index];
-            TyKind::InferenceVar(v, k).intern(&Interner)
-        })
+        chalk_ir::Substitute::apply(&self.free_vars, ty, &Interner)
     }
 
     pub(super) fn apply_solution(
         &self,
-        ctx: &mut InferenceContext<'_>,
+        ctx: &mut InferenceTable,
         solution: Canonical<Substitution>,
     ) {
         // the solution may contain new variables, which we need to convert to new inference vars
         let new_vars = Substitution::from_iter(
             &Interner,
             solution.binders.iter(&Interner).map(|k| match k.kind {
-                VariableKind::Ty(TyVariableKind::General) => {
-                    ctx.table.new_type_var().cast(&Interner)
-                }
-                VariableKind::Ty(TyVariableKind::Integer) => {
-                    ctx.table.new_integer_var().cast(&Interner)
-                }
-                VariableKind::Ty(TyVariableKind::Float) => {
-                    ctx.table.new_float_var().cast(&Interner)
-                }
+                VariableKind::Ty(TyVariableKind::General) => ctx.new_type_var().cast(&Interner),
+                VariableKind::Ty(TyVariableKind::Integer) => ctx.new_integer_var().cast(&Interner),
+                VariableKind::Ty(TyVariableKind::Float) => ctx.new_float_var().cast(&Interner),
                 // Chalk can sometimes return new lifetime variables. We just use the static lifetime everywhere
                 VariableKind::Lifetime => static_lifetime().cast(&Interner),
                 _ => panic!("const variable in solution"),
             }),
         );
-        for (i, ty) in solution.value.iter(&Interner).enumerate() {
-            let (v, k) = self.free_vars[i];
-            // eagerly replace projections in the type; we may be getting types
-            // e.g. from where clauses where this hasn't happened yet
-            let ty = ctx.normalize_associated_types_in(
-                new_vars.apply(ty.assert_ty_ref(&Interner).clone(), &Interner),
-            );
-            ctx.table.unify(&TyKind::InferenceVar(v, k).intern(&Interner), &ty);
+        for (i, v) in solution.value.iter(&Interner).enumerate() {
+            let var = self.free_vars[i].clone();
+            if let Some(ty) = v.ty(&Interner) {
+                // eagerly replace projections in the type; we may be getting types
+                // e.g. from where clauses where this hasn't happened yet
+                let ty = ctx.normalize_associated_types_in(new_vars.apply(ty.clone(), &Interner));
+                ctx.unify(var.assert_ty_ref(&Interner), &ty);
+            } else {
+                let _ = ctx.try_unify(&var, &new_vars.apply(v.clone(), &Interner));
+            }
         }
     }
 }
 
-pub fn could_unify(t1: &Ty, t2: &Ty) -> bool {
-    InferenceTable::new().unify(t1, t2)
+pub fn could_unify(
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    tys: &Canonical<(Ty, Ty)>,
+) -> bool {
+    unify(db, env, tys).is_some()
 }
 
-pub(crate) fn unify(tys: &Canonical<(Ty, Ty)>) -> Option<Substitution> {
-    let mut table = InferenceTable::new();
+pub(crate) fn unify(
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    tys: &Canonical<(Ty, Ty)>,
+) -> Option<Substitution> {
+    let mut table = InferenceTable::new(db, env);
     let vars = Substitution::from_iter(
         &Interner,
         tys.binders
@@ -187,77 +105,151 @@ pub(crate) fn unify(tys: &Canonical<(Ty, Ty)>) -> Option<Substitution> {
     }
     // default any type vars that weren't unified back to their original bound vars
     // (kind of hacky)
-    for (i, var) in vars.iter(&Interner).enumerate() {
-        let var = var.assert_ty_ref(&Interner);
-        if &*table.resolve_ty_shallow(var) == var {
-            table.unify(
-                var,
-                &TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, i)).intern(&Interner),
-            );
-        }
-    }
+    let find_var = |iv| {
+        vars.iter(&Interner).position(|v| match v.interned() {
+            chalk_ir::GenericArgData::Ty(ty) => ty.inference_var(&Interner),
+            chalk_ir::GenericArgData::Lifetime(lt) => lt.inference_var(&Interner),
+            chalk_ir::GenericArgData::Const(c) => c.inference_var(&Interner),
+        } == Some(iv))
+    };
+    let fallback = |iv, kind, default, binder| match kind {
+        chalk_ir::VariableKind::Ty(_ty_kind) => find_var(iv)
+            .map_or(default, |i| BoundVar::new(binder, i).to_ty(&Interner).cast(&Interner)),
+        chalk_ir::VariableKind::Lifetime => find_var(iv)
+            .map_or(default, |i| BoundVar::new(binder, i).to_lifetime(&Interner).cast(&Interner)),
+        chalk_ir::VariableKind::Const(ty) => find_var(iv)
+            .map_or(default, |i| BoundVar::new(binder, i).to_const(&Interner, ty).cast(&Interner)),
+    };
     Some(Substitution::from_iter(
         &Interner,
         vars.iter(&Interner)
-            .map(|v| table.resolve_ty_completely(v.assert_ty_ref(&Interner).clone())),
+            .map(|v| table.resolve_with_fallback(v.assert_ty_ref(&Interner).clone(), fallback)),
     ))
 }
 
-#[derive(Clone, Debug)]
-pub(super) struct TypeVariableTable {
-    inner: Vec<TypeVariableData>,
+#[derive(Copy, Clone, Debug)]
+pub(crate) struct TypeVariableData {
+    diverging: bool,
 }
 
-impl TypeVariableTable {
-    fn push(&mut self, data: TypeVariableData) {
-        self.inner.push(data);
+type ChalkInferenceTable = chalk_solve::infer::InferenceTable<Interner>;
+
+#[derive(Clone)]
+pub(crate) struct InferenceTable<'a> {
+    pub(crate) db: &'a dyn HirDatabase,
+    pub(crate) trait_env: Arc<TraitEnvironment>,
+    var_unification_table: ChalkInferenceTable,
+    type_variable_table: Vec<TypeVariableData>,
+    pending_obligations: Vec<Canonicalized<InEnvironment<Goal>>>,
+}
+
+impl<'a> InferenceTable<'a> {
+    pub(crate) fn new(db: &'a dyn HirDatabase, trait_env: Arc<TraitEnvironment>) -> Self {
+        InferenceTable {
+            db,
+            trait_env,
+            var_unification_table: ChalkInferenceTable::new(),
+            type_variable_table: Vec::new(),
+            pending_obligations: Vec::new(),
+        }
     }
 
-    pub(super) fn set_diverging(&mut self, iv: InferenceVar, diverging: bool) {
-        self.inner[from_inference_var(iv).0 as usize].diverging = diverging;
+    /// Chalk doesn't know about the `diverging` flag, so when it unifies two
+    /// type variables of which one is diverging, the chosen root might not be
+    /// diverging and we have no way of marking it as such at that time. This
+    /// function goes through all type variables and make sure their root is
+    /// marked as diverging if necessary, so that resolving them gives the right
+    /// result.
+    pub(super) fn propagate_diverging_flag(&mut self) {
+        for i in 0..self.type_variable_table.len() {
+            if !self.type_variable_table[i].diverging {
+                continue;
+            }
+            let v = InferenceVar::from(i as u32);
+            let root = self.var_unification_table.inference_var_root(v);
+            if let Some(data) = self.type_variable_table.get_mut(root.index() as usize) {
+                data.diverging = true;
+            }
+        }
     }
 
-    fn is_diverging(&mut self, iv: InferenceVar) -> bool {
-        self.inner[from_inference_var(iv).0 as usize].diverging
+    pub(super) fn set_diverging(&mut self, iv: InferenceVar, diverging: bool) {
+        self.type_variable_table[iv.index() as usize].diverging = diverging;
     }
 
     fn fallback_value(&self, iv: InferenceVar, kind: TyVariableKind) -> Ty {
         match kind {
-            _ if self.inner[from_inference_var(iv).0 as usize].diverging => TyKind::Never,
+            _ if self
+                .type_variable_table
+                .get(iv.index() as usize)
+                .map_or(false, |data| data.diverging) =>
+            {
+                TyKind::Never
+            }
             TyVariableKind::General => TyKind::Error,
             TyVariableKind::Integer => TyKind::Scalar(Scalar::Int(IntTy::I32)),
             TyVariableKind::Float => TyKind::Scalar(Scalar::Float(FloatTy::F64)),
         }
         .intern(&Interner)
     }
-}
 
-#[derive(Copy, Clone, Debug)]
-pub(crate) struct TypeVariableData {
-    diverging: bool,
-}
+    pub(super) fn canonicalize<T: Fold<Interner> + HasInterner<Interner = Interner>>(
+        &mut self,
+        t: T,
+    ) -> Canonicalized<T::Result>
+    where
+        T::Result: HasInterner<Interner = Interner>,
+    {
+        let result = self.var_unification_table.canonicalize(&Interner, t);
+        let free_vars = result
+            .free_vars
+            .into_iter()
+            .map(|free_var| free_var.to_generic_arg(&Interner))
+            .collect();
+        Canonicalized { value: result.quantified, free_vars }
+    }
+
+    /// Recurses through the given type, normalizing associated types mentioned
+    /// in it by replacing them by type variables and registering obligations to
+    /// resolve later. This should be done once for every type we get from some
+    /// type annotation (e.g. from a let type annotation, field type or function
+    /// call). `make_ty` handles this already, but e.g. for field types we need
+    /// to do it as well.
+    pub(super) fn normalize_associated_types_in(&mut self, ty: Ty) -> Ty {
+        fold_tys(
+            ty,
+            |ty, _| match ty.kind(&Interner) {
+                TyKind::Alias(AliasTy::Projection(proj_ty)) => {
+                    self.normalize_projection_ty(proj_ty.clone())
+                }
+                _ => ty,
+            },
+            DebruijnIndex::INNERMOST,
+        )
+    }
 
-#[derive(Clone, Debug)]
-pub(crate) struct InferenceTable {
-    pub(super) var_unification_table: InPlaceUnificationTable<TypeVarId>,
-    pub(super) type_variable_table: TypeVariableTable,
-    pub(super) revision: u32,
-}
+    pub(super) fn normalize_projection_ty(&mut self, proj_ty: ProjectionTy) -> Ty {
+        let var = self.new_type_var();
+        let alias_eq = AliasEq { alias: AliasTy::Projection(proj_ty), ty: var.clone() };
+        let obligation = alias_eq.cast(&Interner);
+        self.register_obligation(obligation);
+        var
+    }
 
-impl InferenceTable {
-    pub(crate) fn new() -> Self {
-        InferenceTable {
-            var_unification_table: InPlaceUnificationTable::new(),
-            type_variable_table: TypeVariableTable { inner: Vec::new() },
-            revision: 0,
-        }
+    fn extend_type_variable_table(&mut self, to_index: usize) {
+        self.type_variable_table.extend(
+            (0..1 + to_index - self.type_variable_table.len())
+                .map(|_| TypeVariableData { diverging: false }),
+        );
     }
 
     fn new_var(&mut self, kind: TyVariableKind, diverging: bool) -> Ty {
-        self.type_variable_table.push(TypeVariableData { diverging });
-        let key = self.var_unification_table.new_key(TypeVarValue::Unknown);
-        assert_eq!(key.0 as usize, self.type_variable_table.inner.len() - 1);
-        TyKind::InferenceVar(to_inference_var(key), kind).intern(&Interner)
+        let var = self.var_unification_table.new_variable(UniverseIndex::ROOT);
+        // Chalk might have created some type variables for its own purposes that we don't know about...
+        self.extend_type_variable_table(var.index() as usize);
+        assert_eq!(var.index() as usize, self.type_variable_table.len() - 1);
+        self.type_variable_table[var.index() as usize].diverging = diverging;
+        var.to_ty_with_kind(&Interner, kind)
     }
 
     pub(crate) fn new_type_var(&mut self) -> Ty {
@@ -276,350 +268,261 @@ impl InferenceTable {
         self.new_var(TyVariableKind::General, true)
     }
 
-    pub(crate) fn resolve_ty_completely(&mut self, ty: Ty) -> Ty {
-        self.resolve_ty_completely_inner(&mut Vec::new(), ty)
+    pub(crate) fn resolve_with_fallback<T>(
+        &mut self,
+        t: T,
+        fallback: impl Fn(InferenceVar, VariableKind, GenericArg, DebruijnIndex) -> GenericArg,
+    ) -> T::Result
+    where
+        T: HasInterner<Interner = Interner> + Fold<Interner>,
+    {
+        self.resolve_with_fallback_inner(&mut Vec::new(), t, &fallback)
     }
 
-    pub(crate) fn resolve_ty_as_possible(&mut self, ty: Ty) -> Ty {
-        self.resolve_ty_as_possible_inner(&mut Vec::new(), ty)
+    fn resolve_with_fallback_inner<T>(
+        &mut self,
+        var_stack: &mut Vec<InferenceVar>,
+        t: T,
+        fallback: &impl Fn(InferenceVar, VariableKind, GenericArg, DebruijnIndex) -> GenericArg,
+    ) -> T::Result
+    where
+        T: HasInterner<Interner = Interner> + Fold<Interner>,
+    {
+        t.fold_with(
+            &mut resolve::Resolver { table: self, var_stack, fallback },
+            DebruijnIndex::INNERMOST,
+        )
+        .expect("fold failed unexpectedly")
     }
 
-    pub(crate) fn unify(&mut self, ty1: &Ty, ty2: &Ty) -> bool {
-        self.unify_inner(ty1, ty2, 0)
+    pub(crate) fn resolve_ty_completely(&mut self, ty: Ty) -> Ty {
+        self.resolve_with_fallback(ty, |_, _, d, _| d)
     }
 
-    pub(crate) fn unify_substs(
-        &mut self,
-        substs1: &Substitution,
-        substs2: &Substitution,
-        depth: usize,
-    ) -> bool {
-        substs1.iter(&Interner).zip(substs2.iter(&Interner)).all(|(t1, t2)| {
-            self.unify_inner(t1.assert_ty_ref(&Interner), t2.assert_ty_ref(&Interner), depth)
-        })
+    /// Unify two types and register new trait goals that arise from that.
+    pub(crate) fn unify(&mut self, ty1: &Ty, ty2: &Ty) -> bool {
+        let result = if let Ok(r) = self.try_unify(ty1, ty2) {
+            r
+        } else {
+            return false;
+        };
+        self.register_infer_ok(result);
+        true
     }
 
-    fn unify_inner(&mut self, ty1: &Ty, ty2: &Ty, depth: usize) -> bool {
-        if depth > 1000 {
-            // prevent stackoverflows
-            panic!("infinite recursion in unification");
-        }
-        if ty1 == ty2 {
-            return true;
-        }
-        // try to resolve type vars first
-        let ty1 = self.resolve_ty_shallow(ty1);
-        let ty2 = self.resolve_ty_shallow(ty2);
-        if ty1.equals_ctor(&ty2) {
-            match (ty1.kind(&Interner), ty2.kind(&Interner)) {
-                (TyKind::Adt(_, substs1), TyKind::Adt(_, substs2))
-                | (TyKind::FnDef(_, substs1), TyKind::FnDef(_, substs2))
-                | (
-                    TyKind::Function(FnPointer { substitution: FnSubst(substs1), .. }),
-                    TyKind::Function(FnPointer { substitution: FnSubst(substs2), .. }),
-                )
-                | (TyKind::Tuple(_, substs1), TyKind::Tuple(_, substs2))
-                | (TyKind::OpaqueType(_, substs1), TyKind::OpaqueType(_, substs2))
-                | (TyKind::AssociatedType(_, substs1), TyKind::AssociatedType(_, substs2))
-                | (TyKind::Closure(.., substs1), TyKind::Closure(.., substs2)) => {
-                    self.unify_substs(substs1, substs2, depth + 1)
-                }
-                (TyKind::Array(ty1, c1), TyKind::Array(ty2, c2)) if c1 == c2 => {
-                    self.unify_inner(ty1, ty2, depth + 1)
-                }
-                (TyKind::Ref(_, _, ty1), TyKind::Ref(_, _, ty2))
-                | (TyKind::Raw(_, ty1), TyKind::Raw(_, ty2))
-                | (TyKind::Slice(ty1), TyKind::Slice(ty2)) => self.unify_inner(ty1, ty2, depth + 1),
-                _ => true, /* we checked equals_ctor already */
-            }
-        } else if let (TyKind::Closure(.., substs1), TyKind::Closure(.., substs2)) =
-            (ty1.kind(&Interner), ty2.kind(&Interner))
-        {
-            self.unify_substs(substs1, substs2, depth + 1)
-        } else {
-            self.unify_inner_trivial(&ty1, &ty2, depth)
+    /// Unify two types and return new trait goals arising from it, so the
+    /// caller needs to deal with them.
+    pub(crate) fn try_unify<T: Zip<Interner>>(&mut self, t1: &T, t2: &T) -> InferResult {
+        match self.var_unification_table.relate(
+            &Interner,
+            &self.db,
+            &self.trait_env.env,
+            chalk_ir::Variance::Invariant,
+            t1,
+            t2,
+        ) {
+            Ok(result) => Ok(InferOk { goals: result.goals }),
+            Err(chalk_ir::NoSolution) => Err(TypeError),
         }
     }
 
-    pub(super) fn unify_inner_trivial(&mut self, ty1: &Ty, ty2: &Ty, depth: usize) -> bool {
-        match (ty1.kind(&Interner), ty2.kind(&Interner)) {
-            (TyKind::Error, _) | (_, TyKind::Error) => true,
+    /// If `ty` is a type variable with known type, returns that type;
+    /// otherwise, return ty.
+    pub(crate) fn resolve_ty_shallow(&mut self, ty: &Ty) -> Ty {
+        self.var_unification_table.normalize_ty_shallow(&Interner, ty).unwrap_or_else(|| ty.clone())
+    }
 
-            (TyKind::Placeholder(p1), TyKind::Placeholder(p2)) if *p1 == *p2 => true,
+    pub(crate) fn register_obligation(&mut self, goal: Goal) {
+        let in_env = InEnvironment::new(&self.trait_env.env, goal);
+        self.register_obligation_in_env(in_env)
+    }
 
-            (TyKind::Dyn(dyn1), TyKind::Dyn(dyn2))
-                if dyn1.bounds.skip_binders().interned().len()
-                    == dyn2.bounds.skip_binders().interned().len() =>
-            {
-                for (pred1, pred2) in dyn1
-                    .bounds
-                    .skip_binders()
-                    .interned()
-                    .iter()
-                    .zip(dyn2.bounds.skip_binders().interned().iter())
-                {
-                    if !self.unify_preds(pred1.skip_binders(), pred2.skip_binders(), depth + 1) {
-                        return false;
-                    }
-                }
-                true
-            }
+    fn register_obligation_in_env(&mut self, goal: InEnvironment<Goal>) {
+        let canonicalized = self.canonicalize(goal);
+        if !self.try_resolve_obligation(&canonicalized) {
+            self.pending_obligations.push(canonicalized);
+        }
+    }
 
-            (
-                TyKind::InferenceVar(tv1, TyVariableKind::General),
-                TyKind::InferenceVar(tv2, TyVariableKind::General),
-            )
-            | (
-                TyKind::InferenceVar(tv1, TyVariableKind::Integer),
-                TyKind::InferenceVar(tv2, TyVariableKind::Integer),
-            )
-            | (
-                TyKind::InferenceVar(tv1, TyVariableKind::Float),
-                TyKind::InferenceVar(tv2, TyVariableKind::Float),
-            ) if self.type_variable_table.is_diverging(*tv1)
-                == self.type_variable_table.is_diverging(*tv2) =>
-            {
-                // both type vars are unknown since we tried to resolve them
-                if !self
-                    .var_unification_table
-                    .unioned(from_inference_var(*tv1), from_inference_var(*tv2))
-                {
-                    self.var_unification_table
-                        .union(from_inference_var(*tv1), from_inference_var(*tv2));
-                    self.revision += 1;
-                }
-                true
-            }
+    pub(crate) fn register_infer_ok(&mut self, infer_ok: InferOk) {
+        infer_ok.goals.into_iter().for_each(|goal| self.register_obligation_in_env(goal));
+    }
 
-            // The order of MaybeNeverTypeVar matters here.
-            // Unifying MaybeNeverTypeVar and TypeVar will let the latter become MaybeNeverTypeVar.
-            // Unifying MaybeNeverTypeVar and other concrete type will let the former become it.
-            (TyKind::InferenceVar(tv, TyVariableKind::General), other)
-            | (other, TyKind::InferenceVar(tv, TyVariableKind::General))
-            | (
-                TyKind::InferenceVar(tv, TyVariableKind::Integer),
-                other @ TyKind::Scalar(Scalar::Int(_)),
-            )
-            | (
-                other @ TyKind::Scalar(Scalar::Int(_)),
-                TyKind::InferenceVar(tv, TyVariableKind::Integer),
-            )
-            | (
-                TyKind::InferenceVar(tv, TyVariableKind::Integer),
-                other @ TyKind::Scalar(Scalar::Uint(_)),
-            )
-            | (
-                other @ TyKind::Scalar(Scalar::Uint(_)),
-                TyKind::InferenceVar(tv, TyVariableKind::Integer),
-            )
-            | (
-                TyKind::InferenceVar(tv, TyVariableKind::Float),
-                other @ TyKind::Scalar(Scalar::Float(_)),
-            )
-            | (
-                other @ TyKind::Scalar(Scalar::Float(_)),
-                TyKind::InferenceVar(tv, TyVariableKind::Float),
-            ) => {
-                // the type var is unknown since we tried to resolve it
-                self.var_unification_table.union_value(
-                    from_inference_var(*tv),
-                    TypeVarValue::Known(other.clone().intern(&Interner)),
+    pub(crate) fn resolve_obligations_as_possible(&mut self) {
+        let _span = profile::span("resolve_obligations_as_possible");
+        let mut changed = true;
+        let mut obligations = Vec::new();
+        while changed {
+            changed = false;
+            mem::swap(&mut self.pending_obligations, &mut obligations);
+            for canonicalized in obligations.drain(..) {
+                if !self.check_changed(&canonicalized) {
+                    self.pending_obligations.push(canonicalized);
+                    continue;
+                }
+                changed = true;
+                let uncanonical = chalk_ir::Substitute::apply(
+                    &canonicalized.free_vars,
+                    canonicalized.value.value,
+                    &Interner,
                 );
-                self.revision += 1;
-                true
+                self.register_obligation_in_env(uncanonical);
             }
-
-            _ => false,
         }
     }
 
-    fn unify_preds(&mut self, pred1: &WhereClause, pred2: &WhereClause, depth: usize) -> bool {
-        match (pred1, pred2) {
-            (WhereClause::Implemented(tr1), WhereClause::Implemented(tr2))
-                if tr1.trait_id == tr2.trait_id =>
-            {
-                self.unify_substs(&tr1.substitution, &tr2.substitution, depth + 1)
+    /// This checks whether any of the free variables in the `canonicalized`
+    /// have changed (either been unified with another variable, or with a
+    /// value). If this is not the case, we don't need to try to solve the goal
+    /// again -- it'll give the same result as last time.
+    fn check_changed(&mut self, canonicalized: &Canonicalized<InEnvironment<Goal>>) -> bool {
+        canonicalized.free_vars.iter().any(|var| {
+            let iv = match var.data(&Interner) {
+                chalk_ir::GenericArgData::Ty(ty) => ty.inference_var(&Interner),
+                chalk_ir::GenericArgData::Lifetime(lt) => lt.inference_var(&Interner),
+                chalk_ir::GenericArgData::Const(c) => c.inference_var(&Interner),
             }
-            (
-                WhereClause::AliasEq(AliasEq { alias: alias1, ty: ty1 }),
-                WhereClause::AliasEq(AliasEq { alias: alias2, ty: ty2 }),
-            ) => {
-                let (substitution1, substitution2) = match (alias1, alias2) {
-                    (AliasTy::Projection(projection_ty1), AliasTy::Projection(projection_ty2))
-                        if projection_ty1.associated_ty_id == projection_ty2.associated_ty_id =>
-                    {
-                        (&projection_ty1.substitution, &projection_ty2.substitution)
-                    }
-                    (AliasTy::Opaque(opaque1), AliasTy::Opaque(opaque2))
-                        if opaque1.opaque_ty_id == opaque2.opaque_ty_id =>
-                    {
-                        (&opaque1.substitution, &opaque2.substitution)
-                    }
-                    _ => return false,
-                };
-                self.unify_substs(&substitution1, &substitution2, depth + 1)
-                    && self.unify_inner(&ty1, &ty2, depth + 1)
+            .expect("free var is not inference var");
+            if self.var_unification_table.probe_var(iv).is_some() {
+                return true;
             }
-            _ => false,
-        }
+            let root = self.var_unification_table.inference_var_root(iv);
+            iv != root
+        })
     }
 
-    /// If `ty` is a type variable with known type, returns that type;
-    /// otherwise, return ty.
-    pub(crate) fn resolve_ty_shallow<'b>(&mut self, ty: &'b Ty) -> Cow<'b, Ty> {
-        let mut ty = Cow::Borrowed(ty);
-        // The type variable could resolve to a int/float variable. Hence try
-        // resolving up to three times; each type of variable shouldn't occur
-        // more than once
-        for i in 0..3 {
-            if i > 0 {
-                cov_mark::hit!(type_var_resolves_to_int_var);
+    fn try_resolve_obligation(
+        &mut self,
+        canonicalized: &Canonicalized<InEnvironment<Goal>>,
+    ) -> bool {
+        let solution = self.db.trait_solve(self.trait_env.krate, canonicalized.value.clone());
+
+        match solution {
+            Some(Solution::Unique(canonical_subst)) => {
+                canonicalized.apply_solution(
+                    self,
+                    Canonical {
+                        binders: canonical_subst.binders,
+                        // FIXME: handle constraints
+                        value: canonical_subst.value.subst,
+                    },