9 files changed, 1140 insertions, 145 deletions

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);