373 files changed, 7161 insertions, 758 deletions

diff --git a/crates/ra_assists/src/doc_tests/generated.rs b/crates/ra_assists/src/doc_tests/generated.rs
index 0848ab6bc..64444ee3a 100644
--- a/crates/ra_assists/src/doc_tests/generated.rs
+++ b/crates/ra_assists/src/doc_tests/generated.rs
@@ -59,6 +59,33 @@ fn main() {
 }
 
 #[test]
+fn doctest_add_function() {
+    check(
+        "add_function",
+        r#####"
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+     bar<|>("", baz());
+}
+
+"#####,
+        r#####"
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+     bar("", baz());
+}
+
+fn bar(arg: &str, baz: Baz) {
+    unimplemented!()
+}
+
+"#####,
+    )
+}
+
+#[test]
 fn doctest_add_hash() {
     check(
         "add_hash",
diff --git a/crates/ra_assists/src/handlers/add_from_impl_for_enum.rs b/crates/ra_assists/src/handlers/add_from_impl_for_enum.rs
new file mode 100644
index 000000000..864373aa5
--- /dev/null
+++ b/crates/ra_assists/src/handlers/add_from_impl_for_enum.rs
@@ -0,0 +1,206 @@
+use ra_syntax::{
+    ast::{self, AstNode, NameOwner},
+    TextUnit,
+};
+use stdx::format_to;
+
+use crate::{Assist, AssistCtx, AssistId};
+use ra_ide_db::RootDatabase;
+
+// Assist add_from_impl_for_enum
+//
+// Adds a From impl for an enum variant with one tuple field
+//
+// ```
+// enum A { <|>One(u32) }
+// ```
+// ->
+// ```
+// enum A { One(u32) }
+//
+// impl From<u32> for A {
+//     fn from(v: u32) -> Self {
+//         A::One(v)
+//     }
+// }
+// ```
+pub(crate) fn add_from_impl_for_enum(ctx: AssistCtx) -> Option<Assist> {
+    let variant = ctx.find_node_at_offset::<ast::EnumVariant>()?;
+    let variant_name = variant.name()?;
+    let enum_name = variant.parent_enum().name()?;
+    let field_list = match variant.kind() {
+        ast::StructKind::Tuple(field_list) => field_list,
+        _ => return None,
+    };
+    if field_list.fields().count() != 1 {
+        return None;
+    }
+    let field_type = field_list.fields().next()?.type_ref()?;
+    let path = match field_type {
+        ast::TypeRef::PathType(p) => p,
+        _ => return None,
+    };
+
+    if already_has_from_impl(ctx.sema, &variant) {
+        return None;
+    }
+
+    ctx.add_assist(
+        AssistId("add_from_impl_for_enum"),
+        "Add From impl for this enum variant",
+        |edit| {
+            let start_offset = variant.parent_enum().syntax().text_range().end();
+            let mut buf = String::new();
+            format_to!(
+                buf,
+                r#"
+
+impl From<{0}> for {1} {{
+    fn from(v: {0}) -> Self {{
+        {1}::{2}(v)
+    }}
+}}"#,
+                path.syntax(),
+                enum_name,
+                variant_name
+            );
+            edit.insert(start_offset, buf);
+            edit.set_cursor(start_offset + TextUnit::of_str("\n\n"));
+        },
+    )
+}
+
+fn already_has_from_impl(
+    sema: &'_ hir::Semantics<'_, RootDatabase>,
+    variant: &ast::EnumVariant,
+) -> bool {
+    let scope = sema.scope(&variant.syntax());
+
+    let from_path = ast::make::path_from_text("From");
+    let from_hir_path = match hir::Path::from_ast(from_path) {
+        Some(p) => p,
+        None => return false,
+    };
+    let from_trait = match scope.resolve_hir_path(&from_hir_path) {
+        Some(hir::PathResolution::Def(hir::ModuleDef::Trait(t))) => t,
+        _ => return false,
+    };
+
+    let e: hir::Enum = match sema.to_def(&variant.parent_enum()) {
+        Some(e) => e,
+        None => return false,
+    };
+    let e_ty = e.ty(sema.db);
+
+    let hir_enum_var: hir::EnumVariant = match sema.to_def(variant) {
+        Some(ev) => ev,
+        None => return false,
+    };
+    let var_ty = hir_enum_var.fields(sema.db)[0].signature_ty(sema.db);
+
+    e_ty.impls_trait(sema.db, from_trait, &[var_ty.clone()])
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::helpers::{check_assist, check_assist_not_applicable};
+
+    #[test]
+    fn test_add_from_impl_for_enum() {
+        check_assist(
+            add_from_impl_for_enum,
+            "enum A { <|>One(u32) }",
+            r#"enum A { One(u32) }
+
+<|>impl From<u32> for A {
+    fn from(v: u32) -> Self {
+        A::One(v)
+    }
+}"#,
+        );
+    }
+
+    #[test]
+    fn test_add_from_impl_for_enum_complicated_path() {
+        check_assist(
+            add_from_impl_for_enum,
+            "enum A { <|>One(foo::bar::baz::Boo) }",
+            r#"enum A { One(foo::bar::baz::Boo) }
+
+<|>impl From<foo::bar::baz::Boo> for A {
+    fn from(v: foo::bar::baz::Boo) -> Self {
+        A::One(v)
+    }
+}"#,
+        );
+    }
+
+    #[test]
+    fn test_add_from_impl_no_element() {
+        check_assist_not_applicable(add_from_impl_for_enum, "enum A { <|>One }");
+    }
+
+    #[test]
+    fn test_add_from_impl_more_than_one_element_in_tuple() {
+        check_assist_not_applicable(add_from_impl_for_enum, "enum A { <|>One(u32, String) }");
+    }
+
+    #[test]
+    fn test_add_from_impl_struct_variant() {
+        check_assist_not_applicable(add_from_impl_for_enum, "enum A { <|>One { x: u32 } }");
+    }
+
+    #[test]
+    fn test_add_from_impl_already_exists() {
+        check_assist_not_applicable(
+            add_from_impl_for_enum,
+            r#"enum A { <|>One(u32), }
+
+impl From<u32> for A {
+    fn from(v: u32) -> Self {
+        A::One(v)
+    }
+}
+
+pub trait From<T> {
+    fn from(T) -> Self;
+}"#,
+        );
+    }
+
+    #[test]
+    fn test_add_from_impl_different_variant_impl_exists() {
+        check_assist(
+            add_from_impl_for_enum,
+            r#"enum A { <|>One(u32), Two(String), }
+
+impl From<String> for A {
+    fn from(v: String) -> Self {
+        A::Two(v)
+    }
+}
+
+pub trait From<T> {
+    fn from(T) -> Self;
+}"#,
+            r#"enum A { One(u32), Two(String), }
+
+<|>impl From<u32> for A {
+    fn from(v: u32) -> Self {
+        A::One(v)
+    }
+}
+
+impl From<String> for A {
+    fn from(v: String) -> Self {
+        A::Two(v)
+    }
+}
+
+pub trait From<T> {
+    fn from(T) -> Self;
+}"#,
+        );
+    }
+}
diff --git a/crates/ra_assists/src/handlers/add_function.rs b/crates/ra_assists/src/handlers/add_function.rs
new file mode 100644
index 000000000..488bae08f
--- /dev/null
+++ b/crates/ra_assists/src/handlers/add_function.rs
@@ -0,0 +1,788 @@
+use ra_syntax::{
+    ast::{self, AstNode},
+    SyntaxKind, SyntaxNode, TextUnit,
+};
+
+use crate::{Assist, AssistCtx, AssistId};
+use ast::{edit::IndentLevel, ArgListOwner, CallExpr, Expr};
+use hir::HirDisplay;
+use rustc_hash::{FxHashMap, FxHashSet};
+
+// Assist: add_function
+//
+// Adds a stub function with a signature matching the function under the cursor.
+//
+// ```
+// struct Baz;
+// fn baz() -> Baz { Baz }
+// fn foo() {
+//      bar<|>("", baz());
+// }
+//
+// ```
+// ->
+// ```
+// struct Baz;
+// fn baz() -> Baz { Baz }
+// fn foo() {
+//      bar("", baz());
+// }
+//
+// fn bar(arg: &str, baz: Baz) {
+//     unimplemented!()
+// }
+//
+// ```
+pub(crate) fn add_function(ctx: AssistCtx) -> Option<Assist> {
+    let path_expr: ast::PathExpr = ctx.find_node_at_offset()?;
+    let call = path_expr.syntax().parent().and_then(ast::CallExpr::cast)?;
+    let path = path_expr.path()?;
+
+    if path.qualifier().is_some() {
+        return None;
+    }
+
+    if ctx.sema.resolve_path(&path).is_some() {
+        // The function call already resolves, no need to add a function
+        return None;
+    }
+
+    let function_builder = FunctionBuilder::from_call(&ctx, &call)?;
+
+    ctx.add_assist(AssistId("add_function"), "Add function", |edit| {
+        edit.target(call.syntax().text_range());
+
+        if let Some(function_template) = function_builder.render() {
+            edit.set_cursor(function_template.cursor_offset);
+            edit.insert(function_template.insert_offset, function_template.fn_def.to_string());
+        }
+    })
+}
+
+struct FunctionTemplate {
+    insert_offset: TextUnit,
+    cursor_offset: TextUnit,
+    fn_def: ast::SourceFile,
+}
+
+struct FunctionBuilder {
+    append_fn_at: SyntaxNode,
+    fn_name: ast::Name,
+    type_params: Option<ast::TypeParamList>,
+    params: ast::ParamList,
+}
+
+impl FunctionBuilder {
+    fn from_call(ctx: &AssistCtx, call: &ast::CallExpr) -> Option<Self> {
+        let append_fn_at = next_space_for_fn(&call)?;
+        let fn_name = fn_name(&call)?;
+        let (type_params, params) = fn_args(ctx, &call)?;
+        Some(Self { append_fn_at, fn_name, type_params, params })
+    }
+    fn render(self) -> Option<FunctionTemplate> {
+        let placeholder_expr = ast::make::expr_unimplemented();
+        let fn_body = ast::make::block_expr(vec![], Some(placeholder_expr));
+        let fn_def = ast::make::fn_def(self.fn_name, self.type_params, self.params, fn_body);
+        let fn_def = ast::make::add_newlines(2, fn_def);
+        let fn_def = IndentLevel::from_node(&self.append_fn_at).increase_indent(fn_def);
+        let insert_offset = self.append_fn_at.text_range().end();
+        let cursor_offset_from_fn_start = fn_def
+            .syntax()
+            .descendants()
+            .find_map(ast::MacroCall::cast)?
+            .syntax()
+            .text_range()
+            .start();
+        let cursor_offset = insert_offset + cursor_offset_from_fn_start;
+        Some(FunctionTemplate { insert_offset, cursor_offset, fn_def })
+    }
+}
+
+fn fn_name(call: &CallExpr) -> Option<ast::Name> {
+    let name = call.expr()?.syntax().to_string();
+    Some(ast::make::name(&name))
+}
+
+/// Computes the type variables and arguments required for the generated function
+fn fn_args(
+    ctx: &AssistCtx,
+    call: &CallExpr,
+) -> Option<(Option<ast::TypeParamList>, ast::ParamList)> {
+    let mut arg_names = Vec::new();
+    let mut arg_types = Vec::new();
+    for arg in call.arg_list()?.args() {
+        let arg_name = match fn_arg_name(&arg) {
+            Some(name) => name,
+            None => String::from("arg"),
+        };
+        arg_names.push(arg_name);
+        arg_types.push(match fn_arg_type(ctx, &arg) {
+            Some(ty) => ty,
+            None => String::from("()"),
+        });
+    }
+    deduplicate_arg_names(&mut arg_names);
+    let params = arg_names.into_iter().zip(arg_types).map(|(name, ty)| ast::make::param(name, ty));
+    Some((None, ast::make::param_list(params)))
+}
+
+/// Makes duplicate argument names unique by appending incrementing numbers.
+///
+/// ```
+/// let mut names: Vec<String> =
+///     vec!["foo".into(), "foo".into(), "bar".into(), "baz".into(), "bar".into()];
+/// deduplicate_arg_names(&mut names);
+/// let expected: Vec<String> =
+///     vec!["foo_1".into(), "foo_2".into(), "bar_1".into(), "baz".into(), "bar_2".into()];
+/// assert_eq!(names, expected);
+/// ```
+fn deduplicate_arg_names(arg_names: &mut Vec<String>) {
+    let arg_name_counts = arg_names.iter().fold(FxHashMap::default(), |mut m, name| {
+        *m.entry(name).or_insert(0) += 1;
+        m
+    });
+    let duplicate_arg_names: FxHashSet<String> = arg_name_counts
+        .into_iter()
+        .filter(|(_, count)| *count >= 2)
+        .map(|(name, _)| name.clone())
+        .collect();
+
+    let mut counter_per_name = FxHashMap::default();
+    for arg_name in arg_names.iter_mut() {
+        if duplicate_arg_names.contains(arg_name) {
+            let counter = counter_per_name.entry(arg_name.clone()).or_insert(1);
+            arg_name.push('_');
+            arg_name.push_str(&counter.to_string());
+            *counter += 1;
+        }
+    }
+}
+
+fn fn_arg_name(fn_arg: &Expr) -> Option<String> {
+    match fn_arg {
+        Expr::CastExpr(cast_expr) => fn_arg_name(&cast_expr.expr()?),
+        _ => Some(
+            fn_arg
+                .syntax()
+                .descendants()
+                .filter(|d| ast::NameRef::can_cast(d.kind()))
+                .last()?
+                .to_string(),
+        ),
+    }
+}
+
+fn fn_arg_type(ctx: &AssistCtx, fn_arg: &Expr) -> Option<String> {
+    let ty = ctx.sema.type_of_expr(fn_arg)?;
+    if ty.is_unknown() {
+        return None;
+    }
+    Some(ty.display(ctx.sema.db).to_string())
+}
+
+/// Returns the position inside the current mod or file
+/// directly after the current block
+/// We want to write the generated function directly after
+/// fns, impls or macro calls, but inside mods
+fn next_space_for_fn(expr: &CallExpr) -> Option<SyntaxNode> {
+    let mut ancestors = expr.syntax().ancestors().peekable();
+    let mut last_ancestor: Option<SyntaxNode> = None;
+    while let Some(next_ancestor) = ancestors.next() {
+        match next_ancestor.kind() {
+            SyntaxKind::SOURCE_FILE => {
+                break;
+            }
+            SyntaxKind::ITEM_LIST => {
+                if ancestors.peek().map(|a| a.kind()) == Some(SyntaxKind::MODULE) {
+                    break;
+                }
+            }
+            _ => {}
+        }
+        last_ancestor = Some(next_ancestor);
+    }
+    last_ancestor
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::helpers::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn add_function_with_no_args() {
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    bar<|>();
+}
+",
+            r"
+fn foo() {
+    bar();
+}
+
+fn bar() {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_from_method() {
+        // This ensures that the function is correctly generated
+        // in the next outer mod or file
+        check_assist(
+            add_function,
+            r"
+impl Foo {
+    fn foo() {
+        bar<|>();
+    }
+}
+",
+            r"
+impl Foo {
+    fn foo() {
+        bar();
+    }
+}
+
+fn bar() {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_directly_after_current_block() {
+        // The new fn should not be created at the end of the file or module
+        check_assist(
+            add_function,
+            r"
+fn foo1() {
+    bar<|>();
+}
+
+fn foo2() {}
+",
+            r"
+fn foo1() {
+    bar();
+}
+
+fn bar() {
+    <|>unimplemented!()
+}
+
+fn foo2() {}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_with_no_args_in_same_module() {
+        check_assist(
+            add_function,
+            r"
+mod baz {
+    fn foo() {
+        bar<|>();
+    }
+}
+",
+            r"
+mod baz {
+    fn foo() {
+        bar();
+    }
+
+    fn bar() {
+        <|>unimplemented!()
+    }
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_with_function_call_arg() {
+        check_assist(
+            add_function,
+            r"
+struct Baz;
+fn baz() -> Baz { unimplemented!() }
+fn foo() {
+    bar<|>(baz());
+}
+",
+            r"
+struct Baz;
+fn baz() -> Baz { unimplemented!() }
+fn foo() {
+    bar(baz());
+}
+
+fn bar(baz: Baz) {
+    <|>unimplemented!()
+}
+",
+        );
+    }
+
+    #[test]
+    fn add_function_with_method_call_arg() {
+        check_assist(
+            add_function,
+            r"
+struct Baz;
+impl Baz {
+    fn foo(&self) -> Baz {
+        ba<|>r(self.baz())
+    }
+    fn baz(&self) -> Baz {
+        Baz
+    }
+}
+",
+            r"
+struct Baz;
+impl Baz {
+    fn foo(&self) -> Baz {
+        bar(self.baz())
+    }
+    fn baz(&self) -> Baz {
+        Baz
+    }
+}
+
+fn bar(baz: Baz) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_with_string_literal_arg() {
+        check_assist(
+            add_function,
+            r#"
+fn foo() {
+    <|>bar("bar")
+}
+"#,
+            r#"
+fn foo() {
+    bar("bar")
+}
+
+fn bar(arg: &str) {
+    <|>unimplemented!()
+}
+"#,
+        )
+    }
+
+    #[test]
+    fn add_function_with_char_literal_arg() {
+        check_assist(
+            add_function,
+            r#"
+fn foo() {
+    <|>bar('x')
+}
+"#,
+            r#"
+fn foo() {
+    bar('x')
+}
+
+fn bar(arg: char) {
+    <|>unimplemented!()
+}
+"#,
+        )
+    }
+
+    #[test]
+    fn add_function_with_int_literal_arg() {
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    <|>bar(42)
+}
+",
+            r"
+fn foo() {
+    bar(42)
+}
+
+fn bar(arg: i32) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_with_cast_int_literal_arg() {
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    <|>bar(42 as u8)
+}
+",
+            r"
+fn foo() {
+    bar(42 as u8)
+}
+
+fn bar(arg: u8) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn name_of_cast_variable_is_used() {
+        // Ensures that the name of the cast type isn't used
+        // in the generated function signature.
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    let x = 42;
+    bar<|>(x as u8)
+}
+",
+            r"
+fn foo() {
+    let x = 42;
+    bar(x as u8)
+}
+
+fn bar(x: u8) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_with_variable_arg() {
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    let worble = ();
+    <|>bar(worble)
+}
+",
+            r"
+fn foo() {
+    let worble = ();
+    bar(worble)
+}
+
+fn bar(worble: ()) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_with_impl_trait_arg() {
+        check_assist(
+            add_function,
+            r"
+trait Foo {}
+fn foo() -> impl Foo {
+    unimplemented!()
+}
+fn baz() {
+    <|>bar(foo())
+}
+",
+            r"
+trait Foo {}
+fn foo() -> impl Foo {
+    unimplemented!()
+}
+fn baz() {
+    bar(foo())
+}
+
+fn bar(foo: impl Foo) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    #[ignore]
+    // FIXME print paths properly to make this test pass
+    fn add_function_with_qualified_path_arg() {
+        check_assist(
+            add_function,
+            r"
+mod Baz {
+    pub struct Bof;
+    pub fn baz() -> Bof { Bof }
+}
+mod Foo {
+    fn foo() {
+        <|>bar(super::Baz::baz())
+    }
+}
+",
+            r"
+mod Baz {
+    pub struct Bof;
+    pub fn baz() -> Bof { Bof }
+}
+mod Foo {
+    fn foo() {
+        bar(super::Baz::baz())
+    }
+
+    fn bar(baz: super::Baz::Bof) {
+        <|>unimplemented!()
+    }
+}
+",
+        )
+    }
+
+    #[test]
+    #[ignore]
+    // FIXME fix printing the generics of a `Ty` to make this test pass
+    fn add_function_with_generic_arg() {
+        check_assist(
+            add_function,
+            r"
+fn foo<T>(t: T) {
+    <|>bar(t)
+}
+",
+            r"
+fn foo<T>(t: T) {
+    bar(t)
+}
+
+fn bar<T>(t: T) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    #[ignore]
+    // FIXME Fix function type printing to make this test pass
+    fn add_function_with_fn_arg() {
+        check_assist(
+            add_function,
+            r"
+struct Baz;
+impl Baz {
+    fn new() -> Self { Baz }
+}
+fn foo() {
+    <|>bar(Baz::new);
+}
+",
+            r"
+struct Baz;
+impl Baz {
+    fn new() -> Self { Baz }
+}
+fn foo() {
+    bar(Baz::new);
+}
+
+fn bar(arg: fn() -> Baz) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    #[ignore]
+    // FIXME Fix closure type printing to make this test pass
+    fn add_function_with_closure_arg() {
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    let closure = |x: i64| x - 1;
+    <|>bar(closure)
+}
+",
+            r"
+fn foo() {
+    let closure = |x: i64| x - 1;
+    bar(closure)
+}
+
+fn bar(closure: impl Fn(i64) -> i64) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn unresolveable_types_default_to_unit() {
+        check_assist(
+            add_function,
+            r"
+fn foo() {
+    <|>bar(baz)
+}
+",
+            r"
+fn foo() {
+    bar(baz)
+}
+
+fn bar(baz: ()) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn arg_names_dont_overlap() {
+        check_assist(
+            add_function,
+            r"
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+    <|>bar(baz(), baz())
+}
+",
+            r"
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+    bar(baz(), baz())
+}
+
+fn bar(baz_1: Baz, baz_2: Baz) {
+    <|>unimplemented!()
+}
+",
+        )
+    }
+
+    #[test]
+    fn arg_name_counters_start_at_1_per_name() {
+        check_assist(
+            add_function,
+            r#"
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+    <|>bar(baz(), baz(), "foo", "bar")
+}
+"#,
+            r#"
+struct Baz;
+fn baz() -> Baz { Baz }
+fn foo() {
+    bar(baz(), baz(), "foo", "bar")
+}
+
+fn bar(baz_1: Baz, baz_2: Baz, arg_1: &str, arg_2: &str) {
+    <|>unimplemented!()
+}
+"#,
+        )
+    }
+
+    #[test]
+    fn add_function_not_applicable_if_function_already_exists() {
+        check_assist_not_applicable(
+            add_function,
+            r"
+fn foo() {
+    bar<|>();
+}
+
+fn bar() {}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_not_applicable_if_unresolved_variable_in_call_is_selected() {
+        check_assist_not_applicable(
+            // bar is resolved, but baz isn't.
+            // The assist is only active if the cursor is on an unresolved path,
+            // but the assist should only be offered if the path is a function call.
+            add_function,
+            r"
+fn foo() {
+    bar(b<|>az);
+}
+
+fn bar(baz: ()) {}
+",
+        )
+    }
+
+    #[test]
+    fn add_function_not_applicable_if_function_path_not_singleton() {
+        // In the future this assist could be extended to generate functions
+        // if the path is in the same crate (or even the same workspace).
+        // For the beginning, I think this is fine.
+        check_assist_not_applicable(
+            add_function,
+            r"
+fn foo() {
+    other_crate::bar<|>();
+}
+        ",
+        )
+    }
+
+    #[test]
+    #[ignore]
+    fn create_method_with_no_args() {
+        check_assist(
+            add_function,
+            r"
+struct Foo;
+impl Foo {
+    fn foo(&self) {
+        self.bar()<|>;
+    }
+}
+        ",
+            r"
+struct Foo;
+impl Foo {
+    fn foo(&self) {
+        self.bar();
+    }
+    fn bar(&self) {
+        unimplemented!();
+    }
+}
+        ",
+        )
+    }
+}
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index fa1f3dd26..c698d6e8c 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -5,6 +5,11 @@
 //! certain context. For example, if the cursor is over `,`, a "swap `,`" assist
 //! becomes available.
 
+#[allow(unused)]
+macro_rules! eprintln {
+    ($($tt:tt)*) => { stdx::eprintln!($($tt)*) };
+}
+
 mod assist_ctx;
 mod marks;
 #[cfg(test)]
@@ -96,6 +101,7 @@ mod handlers {
     mod add_custom_impl;
     mod add_derive;
     mod add_explicit_type;
+    mod add_function;
     mod add_impl;
     mod add_missing_impl_members;
     mod add_new;
@@ -122,12 +128,14 @@ mod handlers {
     mod replace_qualified_name_with_use;
     mod replace_unwrap_with_match;
     mod split_import;
+    mod add_from_impl_for_enum;
 
     pub(crate) fn all() -> &'static [AssistHandler] {
         &[
             add_custom_impl::add_custom_impl,
             add_derive::add_derive,
             add_explicit_type::add_explicit_type,
+            add_function::add_function,
             add_impl::add_impl,
             add_missing_impl_members::add_missing_default_members,
             add_missing_impl_members::add_missing_impl_members,
@@ -159,6 +167,7 @@ mod handlers {
             replace_qualified_name_with_use::replace_qualified_name_with_use,
             replace_unwrap_with_match::replace_unwrap_with_match,
             split_import::split_import,
+            add_from_impl_for_enum::add_from_impl_for_enum,
         ]
     }
 }
diff --git a/crates/ra_hir/src/code_model.rs b/crates/ra_hir/src/code_model.rs
index cd2a8fc62..9baebf643 100644
--- a/crates/ra_hir/src/code_model.rs
+++ b/crates/ra_hir/src/code_model.rs
@@ -1027,8 +1027,16 @@ impl Type {
         ty: Ty,
     ) -> Option<Type> {
         let krate = resolver.krate()?;
+        Some(Type::new_with_resolver_inner(db, krate, resolver, ty))
+    }
+    pub(crate) fn new_with_resolver_inner(
+        db: &dyn HirDatabase,
+        krate: CrateId,
+        resolver: &Resolver,
+        ty: Ty,
+    ) -> Type {
         let environment = TraitEnvironment::lower(db, &resolver);
-        Some(Type { krate, ty: InEnvironment { value: ty, environment } })
+        Type { krate, ty: InEnvironment { value: ty, environment } }
     }
 
     fn new(db: &dyn HirDatabase, krate: CrateId, lexical_env: impl HasResolver, ty: Ty) -> Type {
@@ -1084,6 +1092,26 @@ impl Type {
         )
     }
 
+    pub fn impls_trait(&self, db: &dyn HirDatabase, trait_: Trait, args: &[Type]) -> bool {
+        let trait_ref = hir_ty::TraitRef {
+            trait_: trait_.id,
+            substs: Substs::build_for_def(db, trait_.id)
+                .push(self.ty.value.clone())
+                .fill(args.iter().map(|t| t.ty.value.clone()))
+                .build(),
+        };
+
+        let goal = Canonical {
+            value: hir_ty::InEnvironment::new(
+                self.ty.environment.clone(),
+                hir_ty::Obligation::Trait(trait_ref),
+            ),
+            num_vars: 0,
+        };
+
+        db.trait_solve(self.krate, goal).is_some()
+    }
+
     // FIXME: this method is broken, as it doesn't take closures into account.
     pub fn as_callable(&self) -> Option<CallableDef> {
         Some(self.ty.value.as_callable()?.0)
@@ -1132,27 +1160,6 @@ impl Type {
         res
     }
 
-    pub fn variant_fields(
-        &self,
-        db: &dyn HirDatabase,
-        def: VariantDef,
-    ) -> Vec<(StructField, Type)> {
-        // FIXME: check that ty and def match
-        match &self.ty.value {
-            Ty::Apply(a_ty) => {
-                let field_types = db.field_types(def.into());
-                def.fields(db)
-                    .into_iter()
-                    .map(|it| {
-                        let ty = field_types[it.id].clone().subst(&a_ty.parameters);
-                        (it, self.derived(ty))
-                    })
-                    .collect()
-            }
-            _ => Vec::new(),
-        }
-    }
-
     pub fn autoderef<'a>(&'a self, db: &'a dyn HirDatabase) -> impl Iterator<Item = Type> + 'a {
         // There should be no inference vars in types passed here
         // FIXME check that?
diff --git a/crates/ra_hir/src/diagnostics.rs b/crates/ra_hir/src/diagnostics.rs
index a9040ea3d..c82883d0c 100644
--- a/crates/ra_hir/src/diagnostics.rs
+++ b/crates/ra_hir/src/diagnostics.rs
@@ -1,4 +1,4 @@
 //! FIXME: write short doc here
 pub use hir_def::diagnostics::UnresolvedModule;
 pub use hir_expand::diagnostics::{AstDiagnostic, Diagnostic, DiagnosticSink};
-pub use hir_ty::diagnostics::{MissingFields, MissingOkInTailExpr, NoSuchField};
+pub use hir_ty::diagnostics::{MissingFields, MissingMatchArms, MissingOkInTailExpr, NoSuchField};
diff --git a/crates/ra_hir/src/semantics.rs b/crates/ra_hir/src/semantics.rs
index 16a5fe968..2707e422d 100644
--- a/crates/ra_hir/src/semantics.rs
+++ b/crates/ra_hir/src/semantics.rs
@@ -9,6 +9,7 @@ use hir_def::{
     AsMacroCall, TraitId,
 };
 use hir_expand::ExpansionInfo;
+use itertools::Itertools;
 use ra_db::{FileId, FileRange};
 use ra_prof::profile;
 use ra_syntax::{
@@ -22,7 +23,7 @@ use crate::{
     semantics::source_to_def::{ChildContainer, SourceToDefCache, SourceToDefCtx},
     source_analyzer::{resolve_hir_path, SourceAnalyzer},
     AssocItem, Function, HirFileId, ImplDef, InFile, Local, MacroDef, Module, ModuleDef, Name,
-    Origin, Path, ScopeDef, StructField, Trait, Type, TypeParam, VariantDef,
+    Origin, Path, ScopeDef, StructField, Trait, Type, TypeParam,
 };
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -135,7 +136,6 @@ impl<'db, DB: HirDatabase> Semantics<'db, DB> {
         node: &SyntaxNode,
         offset: TextUnit,
     ) -> impl Iterator<Item = SyntaxNode> + '_ {
-        use itertools::Itertools;
         node.token_at_offset(offset)
             .map(|token| self.ancestors_with_macros(token.parent()))
             .kmerge_by(|node1, node2| node1.text_range().len() < node2.text_range().len())
@@ -187,14 +187,6 @@ impl<'db, DB: HirDatabase> Semantics<'db, DB> {
         self.analyze(field.syntax()).resolve_record_field(self.db, field)
     }
 
-    pub fn resolve_record_literal(&self, record_lit: &ast::RecordLit) -> Option<VariantDef> {
-        self.analyze(record_lit.syntax()).resolve_record_literal(self.db, record_lit)
-    }
-
-    pub fn resolve_record_pattern(&self, record_pat: &ast::RecordPat) -> Option<VariantDef> {
-        self.analyze(record_pat.syntax()).resolve_record_pattern(record_pat)
-    }
-
     pub fn resolve_macro_call(&self, macro_call: &ast::MacroCall) -> Option<MacroDef> {
         let sa = self.analyze(macro_call.syntax());
         let macro_call = self.find_file(macro_call.syntax().clone()).with_value(macro_call);
@@ -212,6 +204,24 @@ impl<'db, DB: HirDatabase> Semantics<'db, DB> {
     // FIXME: use this instead?
     // pub fn resolve_name_ref(&self, name_ref: &ast::NameRef) -> Option<???>;
 
+    pub fn record_literal_missing_fields(
+        &self,
+        literal: &ast::RecordLit,
+    ) -> Vec<(StructField, Type)> {
+        self.analyze(literal.syntax())
+            .record_literal_missing_fields(self.db, literal)
+            .unwrap_or_default()
+    }
+
+    pub fn record_pattern_missing_fields(
+        &self,
+        pattern: &ast::RecordPat,
+    ) -> Vec<(StructField, Type)> {
+        self.analyze(pattern.syntax())
+            .record_pattern_missing_fields(self.db, pattern)
+            .unwrap_or_default()
+    }
+
     pub fn to_def<T: ToDef>(&self, src: &T) -> Option<T::Def> {
         let src = self.find_file(src.syntax().clone()).with_value(src).cloned();
         T::to_def(self, src)
diff --git a/crates/ra_hir/src/semantics/source_to_def.rs b/crates/ra_hir/src/semantics/source_to_def.rs
index 8843f2835..66724919b 100644
--- a/crates/ra_hir/src/semantics/source_to_def.rs
+++ b/crates/ra_hir/src/semantics/source_to_def.rs
@@ -208,12 +208,12 @@ impl SourceToDefCtx<'_, '_> {
         for container in src.cloned().ancestors_with_macros(self.db.upcast()).skip(1) {
             let res: GenericDefId = match_ast! {
                 match (container.value) {
-                    ast::FnDef(it) => { self.fn_to_def(container.with_value(it))?.into() },
-                    ast::StructDef(it) => { self.struct_to_def(container.with_value(it))?.into() },
-                    ast::EnumDef(it) => { self.enum_to_def(container.with_value(it))?.into() },
-                    ast::TraitDef(it) => { self.trait_to_def(container.with_value(it))?.into() },
-                    ast::TypeAliasDef(it) => { self.type_alias_to_def(container.with_value(it))?.into() },
-                    ast::ImplDef(it) => { self.impl_to_def(container.with_value(it))?.into() },
+                    ast::FnDef(it) => self.fn_to_def(container.with_value(it))?.into(),
+                    ast::StructDef(it) => self.struct_to_def(container.with_value(it))?.into(),
+                    ast::EnumDef(it) => self.enum_to_def(container.with_value(it))?.into(),
+                    ast::TraitDef(it) => self.trait_to_def(container.with_value(it))?.into(),
+                    ast::TypeAliasDef(it) => self.type_alias_to_def(container.with_value(it))?.into(),
+                    ast::ImplDef(it) => self.impl_to_def(container.with_value(it))?.into(),
                     _ => continue,
                 }
             };
@@ -226,9 +226,9 @@ impl SourceToDefCtx<'_, '_> {
         for container in src.cloned().ancestors_with_macros(self.db.upcast()).skip(1) {
             let res: DefWithBodyId = match_ast! {
                 match (container.value) {
-                    ast::ConstDef(it) => { self.const_to_def(container.with_value(it))?.into() },
-                    ast::StaticDef(it) => { self.static_to_def(container.with_value(it))?.into() },
-                    ast::FnDef(it) => { self.fn_to_def(container.with_value(it))?.into() },
+                    ast::ConstDef(it) => self.const_to_def(container.with_value(it))?.into(),
+                    ast::StaticDef(it) => self.static_to_def(container.with_value(it))?.into(),
+                    ast::FnDef(it) => self.fn_to_def(container.with_value(it))?.into(),
                     _ => continue,
                 }
             };
diff --git a/crates/ra_hir/src/source_analyzer.rs b/crates/ra_hir/src/source_analyzer.rs
index 815ca158c..45631f8fd 100644
--- a/crates/ra_hir/src/source_analyzer.rs
+++ b/crates/ra_hir/src/source_analyzer.rs
@@ -14,10 +14,13 @@ use hir_def::{
     },
     expr::{ExprId, Pat, PatId},
     resolver::{resolver_for_scope, Resolver, TypeNs, ValueNs},
-    AsMacroCall, DefWithBodyId,
+    AsMacroCall, DefWithBodyId, LocalStructFieldId, StructFieldId, VariantId,
 };
 use hir_expand::{hygiene::Hygiene, name::AsName, HirFileId, InFile};
-use hir_ty::InferenceResult;
+use hir_ty::{
+    expr::{record_literal_missing_fields, record_pattern_missing_fields},
+    InferenceResult, Substs, Ty,
+};
 use ra_syntax::{
     ast::{self, AstNode},
     SyntaxNode, SyntaxNodePtr, TextUnit,
@@ -25,8 +28,10 @@ use ra_syntax::{
 
 use crate::{
     db::HirDatabase, semantics::PathResolution, Adt, Const, EnumVariant, Function, Local, MacroDef,
-    ModPath, ModuleDef, Path, PathKind, Static, Struct, Trait, Type, TypeAlias, TypeParam,
+    ModPath, ModuleDef, Path, PathKind, Static, Struct, StructField, Trait, Type, TypeAlias,
+    TypeParam,
 };
+use ra_db::CrateId;
 
 /// `SourceAnalyzer` is a convenience wrapper which exposes HIR API in terms of
 /// original source files. It should not be used inside the HIR itself.
@@ -164,23 +169,6 @@ impl SourceAnalyzer {
         Some((struct_field.into(), local))
     }
 
-    pub(crate) fn resolve_record_literal(
-        &self,
-        db: &dyn HirDatabase,
-        record_lit: &ast::RecordLit,
-    ) -> Option<crate::VariantDef> {
-        let expr_id = self.expr_id(db, &record_lit.clone().into())?;
-        self.infer.as_ref()?.variant_resolution_for_expr(expr_id).map(|it| it.into())
-    }
-
-    pub(crate) fn resolve_record_pattern(
-        &self,
-        record_pat: &ast::RecordPat,
-    ) -> Option<crate::VariantDef> {
-        let pat_id = self.pat_id(&record_pat.clone().into())?;
-        self.infer.as_ref()?.variant_resolution_for_pat(pat_id).map(|it| it.into())
-    }
-
     pub(crate) fn resolve_macro_call(
         &self,
         db: &dyn HirDatabase,
@@ -231,6 +219,68 @@ impl SourceAnalyzer {
         resolve_hir_path(db, &self.resolver, &hir_path)
     }
 
+    pub(crate) fn record_literal_missing_fields(
+        &self,
+        db: &dyn HirDatabase,
+        literal: &ast::RecordLit,
+    ) -> Option<Vec<(StructField, Type)>> {
+        let krate = self.resolver.krate()?;
+        let body = self.body.as_ref()?;
+        let infer = self.infer.as_ref()?;
+
+        let expr_id = self.expr_id(db, &literal.clone().into())?;
+        let substs = match &infer.type_of_expr[expr_id] {
+            Ty::Apply(a_ty) => &a_ty.parameters,
+            _ => return None,
+        };
+
+        let (variant, missing_fields, _exhaustive) =
+            record_literal_missing_fields(db, infer, expr_id, &body[expr_id])?;
+        let res = self.missing_fields(db, krate, substs, variant, missing_fields);
+        Some(res)
+    }
+
+    pub(crate) fn record_pattern_missing_fields(
+        &self,
+        db: &dyn HirDatabase,
+        pattern: &ast::RecordPat,
+    ) -> Option<Vec<(StructField, Type)>> {
+        let krate = self.resolver.krate()?;
+        let body = self.body.as_ref()?;
+        let infer = self.infer.as_ref()?;
+
+        let pat_id = self.pat_id(&pattern.clone().into())?;
+        let substs = match &infer.type_of_pat[pat_id] {
+            Ty::Apply(a_ty) => &a_ty.parameters,
+            _ => return None,
+        };
+
+        let (variant, missing_fields) =
+            record_pattern_missing_fields(db, infer, pat_id, &body[pat_id])?;
+        let res = self.missing_fields(db, krate, substs, variant, missing_fields);
+        Some(res)
+    }
+
+    fn missing_fields(
+        &self,
+        db: &dyn HirDatabase,
+        krate: CrateId,
+        substs: &Substs,
+        variant: VariantId,
+        missing_fields: Vec<LocalStructFieldId>,
+    ) -> Vec<(StructField, Type)> {
+        let field_types = db.field_types(variant);
+
+        missing_fields
+            .into_iter()
+            .map(|local_id| {
+                let field = StructFieldId { parent: variant, local_id };
+                let ty = field_types[local_id].clone().subst(substs);
+                (field.into(), Type::new_with_resolver_inner(db, krate, &self.resolver, ty))
+            })
+            .collect()
+    }
+
     pub(crate) fn expand(
         &self,
         db: &dyn HirDatabase,
diff --git a/crates/ra_hir_def/src/body/lower.rs b/crates/ra_hir_def/src/body/lower.rs
index 28c570c76..8d4b8b0f0 100644
--- a/crates/ra_hir_def/src/body/lower.rs
+++ b/crates/ra_hir_def/src/body/lower.rs
@@ -672,8 +672,7 @@ impl ExprCollector<'_> {
             }
 
             // FIXME: implement
-            ast::Pat::BoxPat(_) => Pat::Missing,
-            ast::Pat::RangePat(_) => Pat::Missing,
+            ast::Pat::BoxPat(_) | ast::Pat::RangePat(_) | ast::Pat::MacroPat(_) => Pat::Missing,
         };
         let ptr = AstPtr::new(&pat);
         self.alloc_pat(pattern, Either::Left(ptr))
diff --git a/crates/ra_hir_def/src/lib.rs b/crates/ra_hir_def/src/lib.rs
index bd32ac20a..2d27bbdf8 100644
--- a/crates/ra_hir_def/src/lib.rs
+++ b/crates/ra_hir_def/src/lib.rs
@@ -7,6 +7,11 @@
 //! Note that `hir_def` is a work in progress, so not all of the above is
 //! actually true.
 
+#[allow(unused)]
+macro_rules! eprintln {
+    ($($tt:tt)*) => { stdx::eprintln!($($tt)*) };
+}
+
 pub mod db;
 
 pub mod attr;
diff --git a/crates/ra_hir_def/src/type_ref.rs b/crates/ra_hir_def/src/type_ref.rs
index 102fdb13d..01cc392db 100644
--- a/crates/ra_hir_def/src/type_ref.rs
+++ b/crates/ra_hir_def/src/type_ref.rs
@@ -93,7 +93,11 @@ impl TypeRef {
             }
             ast::TypeRef::PlaceholderType(_inner) => TypeRef::Placeholder,
             ast::TypeRef::FnPointerType(inner) => {
-                let ret_ty = TypeRef::from_ast_opt(inner.ret_type().and_then(|rt| rt.type_ref()));
+                let ret_ty = inner
+                    .ret_type()
+                    .and_then(|rt| rt.type_ref())
+                    .map(TypeRef::from_ast)
+                    .unwrap_or_else(|| TypeRef::Tuple(Vec::new()));
                 let mut params = if let Some(pl) = inner.param_list() {
                     pl.params().map(|p| p.ascribed_type()).map(TypeRef::from_ast_opt).collect()
                 } else {
diff --git a/crates/ra_hir_expand/src/builtin_derive.rs b/crates/ra_hir_expand/src/builtin_derive.rs
index 79aea5806..bb45b0f1d 100644
--- a/crates/ra_hir_expand/src/builtin_derive.rs
+++ b/crates/ra_hir_expand/src/builtin_derive.rs
@@ -73,9 +73,9 @@ fn parse_adt(tt: &tt::Subtree) -> Result<BasicAdtInfo, mbe::ExpandError> {
     let node = item.syntax();
     let (name, params) = match_ast! {
         match node {
-            ast::StructDef(it) => { (it.name(), it.type_param_list()) },
-            ast::EnumDef(it) => { (it.name(), it.type_param_list()) },
-            ast::UnionDef(it) => { (it.name(), it.type_param_list()) },
+            ast::StructDef(it) => (it.name(), it.type_param_list()),
+            ast::EnumDef(it) => (it.name(), it.type_param_list()),
+            ast::UnionDef(it) => (it.name(), it.type_param_list()),
             _ => {
                 debug!("unexpected node is {:?}", node);
                 return Err(mbe::ExpandError::ConversionError)
diff --git a/crates/ra_hir_ty/Cargo.toml b/crates/ra_hir_ty/Cargo.toml
index 5a58d70cf..9a4a7aa6f 100644
--- a/crates/ra_hir_ty/Cargo.toml
+++ b/crates/ra_hir_ty/Cargo.toml
@@ -9,6 +9,7 @@ doctest = false
 
 [dependencies]
 arrayvec = "0.5.1"
+smallvec = "1.2.0"
 ena = "0.13.1"
 log = "0.4.8"
 rustc-hash = "1.1.0"
@@ -23,9 +24,9 @@ ra_prof = { path = "../ra_prof" }
 ra_syntax = { path = "../ra_syntax" }
 test_utils = { path = "../test_utils" }
 
-chalk-solve =   { git = "https://github.com/rust-lang/chalk.git", rev = "177d71340acc7a7204a33115fc63075d86452179" }
-chalk-rust-ir = { git = "https://github.com/rust-lang/chalk.git", rev = "177d71340acc7a7204a33115fc63075d86452179" }
-chalk-ir =      { git = "https://github.com/rust-lang/chalk.git", rev = "177d71340acc7a7204a33115fc63075d86452179" }
+chalk-solve =   { git = "https://github.com/rust-lang/chalk.git", rev = "039fc904a05f8cb3d0c682c9a57a63dda7a35356" }
+chalk-rust-ir = { git = "https://github.com/rust-lang/chalk.git", rev = "039fc904a05f8cb3d0c682c9a57a63dda7a35356" }
+chalk-ir =      { git = "https://github.com/rust-lang/chalk.git", rev = "039fc904a05f8cb3d0c682c9a57a63dda7a35356" }
 
 [dev-dependencies]
 insta = "0.15.0"
diff --git a/crates/ra_hir_ty/src/_match.rs b/crates/ra_hir_ty/src/_match.rs
new file mode 100644
index 000000000..f29a25505
--- /dev/null
+++ b/crates/ra_hir_ty/src/_match.rs
@@ -0,0 +1,1411 @@
+//! This module implements match statement exhaustiveness checking and usefulness checking
+//! for match arms.
+//!
+//! It is modeled on the rustc module `librustc_mir_build::hair::pattern::_match`, which
+//! contains very detailed documentation about the algorithms used here. I've duplicated
+//! most of that documentation below.
+//!
+//! This file includes the logic for exhaustiveness and usefulness checking for
+//! pattern-matching. Specifically, given a list of patterns for a type, we can
+//! tell whether:
+//! (a) the patterns cover every possible constructor for the type [exhaustiveness]
+//! (b) each pattern is necessary [usefulness]
+//!
+//! The algorithm implemented here is a modified version of the one described in:
+//! http://moscova.inria.fr/~maranget/papers/warn/index.html
+//! However, to save future implementors from reading the original paper, we
+//! summarise the algorithm here to hopefully save time and be a little clearer
+//! (without being so rigorous).
+//!
+//! The core of the algorithm revolves about a "usefulness" check. In particular, we
+//! are trying to compute a predicate `U(P, p)` where `P` is a list of patterns (we refer to this as
+//! a matrix). `U(P, p)` represents whether, given an existing list of patterns
+//! `P_1 ..= P_m`, adding a new pattern `p` will be "useful" (that is, cover previously-
+//! uncovered values of the type).
+//!
+//! If we have this predicate, then we can easily compute both exhaustiveness of an
+//! entire set of patterns and the individual usefulness of each one.
+//! (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e., adding a wildcard
+//! match doesn't increase the number of values we're matching)
+//! (b) a pattern `P_i` is not useful if `U(P[0..=(i-1), P_i)` is false (i.e., adding a
+//! pattern to those that have come before it doesn't increase the number of values
+//! we're matching).
+//!
+//! During the course of the algorithm, the rows of the matrix won't just be individual patterns,
+//! but rather partially-deconstructed patterns in the form of a list of patterns. The paper
+//! calls those pattern-vectors, and we will call them pattern-stacks. The same holds for the
+//! new pattern `p`.
+//!
+//! For example, say we have the following:
+//! ```
+//!     // x: (Option<bool>, Result<()>)
+//!     match x {
+//!         (Some(true), _) => {}
+//!         (None, Err(())) => {}
+//!         (None, Err(_)) => {}
+//!     }
+//! ```
+//! Here, the matrix `P` starts as:
+//! [
+//!     [(Some(true), _)],
+//!     [(None, Err(()))],
+//!     [(None, Err(_))],
+//! ]
+//! We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering
+//! `[(Some(false), _)]`, for instance). In addition, row 3 is not useful, because
+//! all the values it covers are already covered by row 2.
+//!
+//! A list of patterns can be thought of as a stack, because we are mainly interested in the top of
+//! the stack at any given point, and we can pop or apply constructors to get new pattern-stacks.
+//! To match the paper, the top of the stack is at the beginning / on the left.
+//!
+//! There are two important operations on pattern-stacks necessary to understand the algorithm:
+//!     1. We can pop a given constructor off the top of a stack. This operation is called
+//!        `specialize`, and is denoted `S(c, p)` where `c` is a constructor (like `Some` or
+//!        `None`) and `p` a pattern-stack.
+//!        If the pattern on top of the stack can cover `c`, this removes the constructor and
+//!        pushes its arguments onto the stack. It also expands OR-patterns into distinct patterns.
+//!        Otherwise the pattern-stack is discarded.
+//!        This essentially filters those pattern-stacks whose top covers the constructor `c` and
+//!        discards the others.
+//!
+//!        For example, the first pattern above initially gives a stack `[(Some(true), _)]`. If we
+//!        pop the tuple constructor, we are left with `[Some(true), _]`, and if we then pop the
+//!        `Some` constructor we get `[true, _]`. If we had popped `None` instead, we would get
+//!        nothing back.
+//!
+//!        This returns zero or more new pattern-stacks, as follows. We look at the pattern `p_1`
+//!        on top of the stack, and we have four cases:
+//!             1.1. `p_1 = c(r_1, .., r_a)`, i.e. the top of the stack has constructor `c`. We
+//!                  push onto the stack the arguments of this constructor, and return the result:
+//!                     r_1, .., r_a, p_2, .., p_n
+//!             1.2. `p_1 = c'(r_1, .., r_a')` where `c ≠ c'`. We discard the current stack and
+//!                  return nothing.
+//!             1.3. `p_1 = _`. We push onto the stack as many wildcards as the constructor `c` has
+//!                  arguments (its arity), and return the resulting stack:
+//!                     _, .., _, p_2, .., p_n
+//!             1.4. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting
+//!                  stack:
+//!                     S(c, (r_1, p_2, .., p_n))
+//!                     S(c, (r_2, p_2, .., p_n))
+//!
+//!     2. We can pop a wildcard off the top of the stack. This is called `D(p)`, where `p` is
+//!        a pattern-stack.
+//!        This is used when we know there are missing constructor cases, but there might be
+//!        existing wildcard patterns, so to check the usefulness of the matrix, we have to check
+//!        all its *other* components.
+//!
+//!        It is computed as follows. We look at the pattern `p_1` on top of the stack,
+//!        and we have three cases:
+//!             1.1. `p_1 = c(r_1, .., r_a)`. We discard the current stack and return nothing.
+//!             1.2. `p_1 = _`. We return the rest of the stack:
+//!                     p_2, .., p_n
+//!             1.3. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting
+//!               stack.
+//!                     D((r_1, p_2, .., p_n))
+//!                     D((r_2, p_2, .., p_n))
+//!
+//!     Note that the OR-patterns are not always used directly in Rust, but are used to derive the
+//!     exhaustive integer matching rules, so they're written here for posterity.
+//!
+//! Both those operations extend straightforwardly to a list or pattern-stacks, i.e. a matrix, by
+//! working row-by-row. Popping a constructor ends up keeping only the matrix rows that start with
+//! the given constructor, and popping a wildcard keeps those rows that start with a wildcard.
+//!
+//!
+//! The algorithm for computing `U`
+//! -------------------------------
+//! The algorithm is inductive (on the number of columns: i.e., components of tuple patterns).
+//! That means we're going to check the components from left-to-right, so the algorithm
+//! operates principally on the first component of the matrix and new pattern-stack `p`.
+//! This algorithm is realised in the `is_useful` function.
+//!
+//! Base case. (`n = 0`, i.e., an empty tuple pattern)
+//!     - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`),
+//!       then `U(P, p)` is false.
+//!     - Otherwise, `P` must be empty, so `U(P, p)` is true.
+//!
+//! Inductive step. (`n > 0`, i.e., whether there's at least one column
+//!                  [which may then be expanded into further columns later])
+//!     We're going to match on the top of the new pattern-stack, `p_1`.
+//!         - If `p_1 == c(r_1, .., r_a)`, i.e. we have a constructor pattern.
+//!           Then, the usefulness of `p_1` can be reduced to whether it is useful when
+//!           we ignore all the patterns in the first column of `P` that involve other constructors.
+//!           This is where `S(c, P)` comes in:
+//!           `U(P, p) := U(S(c, P), S(c, p))`
+//!           This special case is handled in `is_useful_specialized`.
+//!
+//!           For example, if `P` is:
+//!           [
+//!               [Some(true), _],
+//!               [None, 0],
+//!           ]
+//!           and `p` is [Some(false), 0], then we don't care about row 2 since we know `p` only
+//!           matches values that row 2 doesn't. For row 1 however, we need to dig into the
+//!           arguments of `Some` to know whether some new value is covered. So we compute
+//!           `U([[true, _]], [false, 0])`.
+//!
+//!         - If `p_1 == _`, then we look at the list of constructors that appear in the first
+//!               component of the rows of `P`:
+//!             + If there are some constructors that aren't present, then we might think that the
+//!               wildcard `_` is useful, since it covers those constructors that weren't covered
+//!               before.
+//!               That's almost correct, but only works if there were no wildcards in those first
+//!               components. So we need to check that `p` is useful with respect to the rows that
+//!               start with a wildcard, if there are any. This is where `D` comes in:
+//!               `U(P, p) := U(D(P), D(p))`
+//!
+//!               For example, if `P` is:
+//!               [
+//!                   [_, true, _],
+//!                   [None, false, 1],
+//!               ]
+//!               and `p` is [_, false, _], the `Some` constructor doesn't appear in `P`. So if we
+//!               only had row 2, we'd know that `p` is useful. However row 1 starts with a
+//!               wildcard, so we need to check whether `U([[true, _]], [false, 1])`.
+//!
+//!             + Otherwise, all possible constructors (for the relevant type) are present. In this
+//!               case we must check whether the wildcard pattern covers any unmatched value. For
+//!               that, we can think of the `_` pattern as a big OR-pattern that covers all
+//!               possible constructors. For `Option`, that would mean `_ = None | Some(_)` for
+//!               example. The wildcard pattern is useful in this case if it is useful when
+//!               specialized to one of the possible constructors. So we compute:
+//!               `U(P, p) := ∃(k ϵ constructors) U(S(k, P), S(k, p))`
+//!
+//!               For example, if `P` is:
+//!               [
+//!                   [Some(true), _],
+//!                   [None, false],
+//!               ]
+//!               and `p` is [_, false], both `None` and `Some` constructors appear in the first
+//!               components of `P`. We will therefore try popping both constructors in turn: we
+//!               compute U([[true, _]], [_, false]) for the `Some` constructor, and U([[false]],
+//!               [false]) for the `None` constructor. The first case returns true, so we know that
+//!               `p` is useful for `P`. Indeed, it matches `[Some(false), _]` that wasn't matched
+//!               before.
+//!
+//!         - If `p_1 == r_1 | r_2`, then the usefulness depends on each `r_i` separately:
+//!           `U(P, p) := U(P, (r_1, p_2, .., p_n))
+//!                    || U(P, (r_2, p_2, .., p_n))`
+use std::sync::Arc;
+
+use smallvec::{smallvec, SmallVec};
+
+use crate::{
+    db::HirDatabase,
+    expr::{Body, Expr, Literal, Pat, PatId},
+    InferenceResult,
+};
+use hir_def::{adt::VariantData, EnumVariantId, VariantId};
+
+#[derive(Debug, Clone, Copy)]
+/// Either a pattern from the source code being analyzed, represented as
+/// as `PatId`, or a `Wild` pattern which is created as an intermediate
+/// step in the match checking algorithm and thus is not backed by a
+/// real `PatId`.
+///
+/// Note that it is totally valid for the `PatId` variant to contain
+/// a `PatId` which resolves to a `Wild` pattern, if that wild pattern
+/// exists in the source code being analyzed.
+enum PatIdOrWild {
+    PatId(PatId),
+    Wild,
+}
+
+impl PatIdOrWild {
+    fn as_pat(self, cx: &MatchCheckCtx) -> Pat {
+        match self {
+            PatIdOrWild::PatId(id) => cx.body.pats[id].clone(),
+            PatIdOrWild::Wild => Pat::Wild,
+        }
+    }
+
+    fn as_id(self) -> Option<PatId> {
+        match self {
+            PatIdOrWild::PatId(id) => Some(id),
+            PatIdOrWild::Wild => None,
+        }
+    }
+}
+
+impl From<PatId> for PatIdOrWild {
+    fn from(pat_id: PatId) -> Self {
+        Self::PatId(pat_id)
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct MatchCheckNotImplemented;
+
+/// The return type of `is_useful` is either an indication of usefulness
+/// of the match arm, or an error in the case the match statement
+/// is made up of types for which exhaustiveness checking is currently
+/// not completely implemented.
+///
+/// The `std::result::Result` type is used here rather than a custom enum
+/// to allow the use of `?`.
+pub type MatchCheckResult<T> = Result<T, MatchCheckNotImplemented>;
+
+#[derive(Debug)]
+/// A row in a Matrix.
+///
+/// This type is modeled from the struct of the same name in `rustc`.
+pub(crate) struct PatStack(PatStackInner);
+type PatStackInner = SmallVec<[PatIdOrWild; 2]>;
+
+impl PatStack {
+    pub(crate) fn from_pattern(pat_id: PatId) -> PatStack {
+        Self(smallvec!(pat_id.into()))
+    }
+
+    pub(crate) fn from_wild() -> PatStack {
+        Self(smallvec!(PatIdOrWild::Wild))
+    }
+
+    fn from_slice(slice: &[PatIdOrWild]) -> PatStack {
+        Self(SmallVec::from_slice(slice))
+    }
+
+    fn from_vec(v: PatStackInner) -> PatStack {
+        Self(v)
+    }
+
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    fn head(&self) -> PatIdOrWild {
+        self.0[0]
+    }
+
+    fn get_head(&self) -> Option<PatIdOrWild> {
+        self.0.first().copied()
+    }
+
+    fn to_tail(&self) -> PatStack {
+        Self::from_slice(&self.0[1..])
+    }
+
+    fn replace_head_with(&self, pat_ids: &[PatId]) -> PatStack {
+        let mut patterns: PatStackInner = smallvec![];
+        for pat in pat_ids {
+            patterns.push((*pat).into());
+        }
+        for pat in &self.0[1..] {
+            patterns.push(*pat);
+        }
+        PatStack::from_vec(patterns)
+    }
+
+    /// Computes `D(self)`.
+    ///
+    /// See the module docs and the associated documentation in rustc for details.
+    fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Option<PatStack> {
+        if matches!(self.head().as_pat(cx), Pat::Wild) {
+            Some(self.to_tail())
+        } else {
+            None
+        }
+    }
+
+    /// Computes `S(constructor, self)`.
+    ///
+    /// See the module docs and the associated documentation in rustc for details.
+    fn specialize_constructor(
+        &self,
+        cx: &MatchCheckCtx,
+        constructor: &Constructor,
+    ) -> MatchCheckResult<Option<PatStack>> {
+        let result = match (self.head().as_pat(cx), constructor) {
+            (Pat::Tuple(ref pat_ids), Constructor::Tuple { arity }) => {
+                debug_assert_eq!(
+                    pat_ids.len(),
+                    *arity,
+                    "we type check before calling this code, so we should never hit this case",
+                );
+
+                Some(self.replace_head_with(pat_ids))
+            }
+            (Pat::Lit(lit_expr), Constructor::Bool(constructor_val)) => {
+                match cx.body.exprs[lit_expr] {
+                    Expr::Literal(Literal::Bool(pat_val)) if *constructor_val == pat_val => {
+                        Some(self.to_tail())
+                    }
+                    // it was a bool but the value doesn't match
+                    Expr::Literal(Literal::Bool(_)) => None,
+                    // perhaps this is actually unreachable given we have
+                    // already checked that these match arms have the appropriate type?
+                    _ => return Err(MatchCheckNotImplemented),
+                }
+            }
+            (Pat::Wild, constructor) => Some(self.expand_wildcard(cx, constructor)?),
+            (Pat::Path(_), Constructor::Enum(constructor)) => {
+                // enums with no associated data become `Pat::Path`
+                let pat_id = self.head().as_id().expect("we know this isn't a wild");
+                if !enum_variant_matches(cx, pat_id, *constructor) {
+                    None
+                } else {
+                    Some(self.to_tail())
+                }
+            }
+            (Pat::TupleStruct { args: ref pat_ids, .. }, Constructor::Enum(constructor)) => {
+                let pat_id = self.head().as_id().expect("we know this isn't a wild");
+                if !enum_variant_matches(cx, pat_id, *constructor) {
+                    None
+                } else {
+                    Some(self.replace_head_with(pat_ids))
+                }
+            }
+            (Pat::Or(_), _) => return Err(MatchCheckNotImplemented),
+            (_, _) => return Err(MatchCheckNotImplemented),
+        };
+
+        Ok(result)
+    }
+
+    /// A special case of `specialize_constructor` where the head of the pattern stack
+    /// is a Wild pattern.
+    ///
+    /// Replaces the Wild pattern at the head of the pattern stack with N Wild patterns
+    /// (N >= 0), where N is the arity of the given constructor.
+    fn expand_wildcard(
+        &self,
+        cx: &MatchCheckCtx,
+        constructor: &Constructor,
+    ) -> MatchCheckResult<PatStack> {
+        assert_eq!(
+            Pat::Wild,
+            self.head().as_pat(cx),
+            "expand_wildcard must only be called on PatStack with wild at head",
+        );
+
+        let mut patterns: PatStackInner = smallvec![];
+
+        for _ in 0..constructor.arity(cx)? {
+            patterns.push(PatIdOrWild::Wild);
+        }
+
+        for pat in &self.0[1..] {
+            patterns.push(*pat);
+        }
+
+        Ok(PatStack::from_vec(patterns))
+    }
+}
+
+#[derive(Debug)]
+/// A collection of PatStack.
+///
+/// This type is modeled from the struct of the same name in `rustc`.
+pub(crate) struct Matrix(Vec<PatStack>);
+
+impl Matrix {
+    pub(crate) fn empty() -> Self {
+        Self(vec![])
+    }
+
+    pub(crate) fn push(&mut self, cx: &MatchCheckCtx, row: PatStack) {
+        if let Some(Pat::Or(pat_ids)) = row.get_head().map(|pat_id| pat_id.as_pat(cx)) {
+            // Or patterns are expanded here
+            for pat_id in pat_ids {
+                self.0.push(PatStack::from_pattern(pat_id));
+            }
+        } else {
+            self.0.push(row);
+        }
+    }
+
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    fn heads(&self) -> Vec<PatIdOrWild> {
+        self.0.iter().map(|p| p.head()).collect()
+    }
+
+    /// Computes `D(self)` for each contained PatStack.
+    ///
+    /// See the module docs and the associated documentation in rustc for details.
+    fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Self {
+        Self::collect(cx, self.0.iter().filter_map(|r| r.specialize_wildcard(cx)))
+    }
+
+    /// Computes `S(constructor, self)` for each contained PatStack.
+    ///
+    /// See the module docs and the associated documentation in rustc for details.
+    fn specialize_constructor(
+        &self,
+        cx: &MatchCheckCtx,
+        constructor: &Constructor,
+    ) -> MatchCheckResult<Self> {
+        let mut new_matrix = Matrix::empty();
+        for pat in &self.0 {
+            if let Some(pat) = pat.specialize_constructor(cx, constructor)? {
+                new_matrix.push(cx, pat);
+            }
+        }
+
+        Ok(new_matrix)
+    }
+
+    fn collect<T: IntoIterator<Item = PatStack>>(cx: &MatchCheckCtx, iter: T) -> Self {
+        let mut matrix = Matrix::empty();
+
+        for pat in iter {
+            // using push ensures we expand or-patterns
+            matrix.push(cx, pat);
+        }
+
+        matrix
+    }
+}
+
+#[derive(Clone, Debug, PartialEq)]
+/// An indication of the usefulness of a given match arm, where
+/// usefulness is defined as matching some patterns which were
+/// not matched by an prior match arms.
+///
+/// We may eventually need an `Unknown` variant here.
+pub enum Usefulness {
+    Useful,
+    NotUseful,
+}
+
+pub struct MatchCheckCtx<'a> {
+    pub body: Arc<Body>,
+    pub infer: Arc<InferenceResult>,
+    pub db: &'a dyn HirDatabase,
+}
+
+/// Given a set of patterns `matrix`, and pattern to consider `v`, determines
+/// whether `v` is useful. A pattern is useful if it covers cases which were
+/// not previously covered.
+///
+/// When calling this function externally (that is, not the recursive calls) it
+/// expected that you have already type checked the match arms. All patterns in
+/// matrix should be the same type as v, as well as they should all be the same
+/// type as the match expression.
+pub(crate) fn is_useful(
+    cx: &MatchCheckCtx,
+    matrix: &Matrix,
+    v: &PatStack,
+) -> MatchCheckResult<Usefulness> {
+    if v.is_empty() {
+        let result = if matrix.is_empty() { Usefulness::Useful } else { Usefulness::NotUseful };
+
+        return Ok(result);
+    }
+
+    if let Pat::Or(pat_ids) = v.head().as_pat(cx) {
+        let mut found_unimplemented = false;
+        let any_useful = pat_ids.iter().any(|&pat_id| {
+            let v = PatStack::from_pattern(pat_id);
+
+            match is_useful(cx, matrix, &v) {
+                Ok(Usefulness::Useful) => true,
+                Ok(Usefulness::NotUseful) => false,
+                _ => {
+                    found_unimplemented = true;
+                    false
+                }
+            }
+        });
+
+        return if any_useful {
+            Ok(Usefulness::Useful)
+        } else if found_unimplemented {
+            Err(MatchCheckNotImplemented)
+        } else {
+            Ok(Usefulness::NotUseful)
+        };
+    }
+
+    if let Some(constructor) = pat_constructor(cx, v.head())? {
+        let matrix = matrix.specialize_constructor(&cx, &constructor)?;
+        let v = v
+            .specialize_constructor(&cx, &constructor)?
+            .expect("we know this can't fail because we get the constructor from `v.head()` above");
+
+        is_useful(&cx, &matrix, &v)
+    } else {
+        // expanding wildcard
+        let mut used_constructors: Vec<Constructor> = vec![];
+        for pat in matrix.heads() {
+            if let Some(constructor) = pat_constructor(cx, pat)? {
+                used_constructors.push(constructor);
+            }
+        }
+
+        // We assume here that the first constructor is the "correct" type. Since we
+        // only care about the "type" of the constructor (i.e. if it is a bool we
+        // don't care about the value), this assumption should be valid as long as
+        // the match statement is well formed. We currently uphold this invariant by
+        // filtering match arms before calling `is_useful`, only passing in match arms
+        // whose type matches the type of the match expression.
+        match &used_constructors.first() {
+            Some(constructor) if all_constructors_covered(&cx, constructor, &used_constructors) => {
+                // If all constructors are covered, then we need to consider whether
+                // any values are covered by this wildcard.
+                //
+                // For example, with matrix '[[Some(true)], [None]]', all
+                // constructors are covered (`Some`/`None`), so we need
+                // to perform specialization to see that our wildcard will cover
+                // the `Some(false)` case.
+                //
+                // Here we create a constructor for each variant and then check
+                // usefulness after specializing for that constructor.
+                let mut found_unimplemented = false;
+                for constructor in constructor.all_constructors(cx) {
+                    let matrix = matrix.specialize_constructor(&cx, &constructor)?;
+                    let v = v.expand_wildcard(&cx, &constructor)?;
+
+                    match is_useful(&cx, &matrix, &v) {
+                        Ok(Usefulness::Useful) => return Ok(Usefulness::Useful),
+                        Ok(Usefulness::NotUseful) => continue,
+                        _ => found_unimplemented = true,
+                    };
+                }
+
+                if found_unimplemented {
+                    Err(MatchCheckNotImplemented)
+                } else {
+                    Ok(Usefulness::NotUseful)
+                }
+            }
+            _ => {
+                // Either not all constructors are covered, or the only other arms
+                // are wildcards. Either way, this pattern is useful if it is useful
+                // when compared to those arms with wildcards.
+                let matrix = matrix.specialize_wildcard(&cx);
+                let v = v.to_tail();
+
+                is_useful(&cx, &matrix, &v)
+            }
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+/// Similar to TypeCtor, but includes additional information about the specific
+/// value being instantiated. For example, TypeCtor::Bool doesn't contain the
+/// boolean value.
+enum Constructor {
+    Bool(bool),
+    Tuple { arity: usize },
+    Enum(EnumVariantId),
+}
+
+impl Constructor {
+    fn arity(&self, cx: &MatchCheckCtx) -> MatchCheckResult<usize> {
+        let arity = match self {
+            Constructor::Bool(_) => 0,
+            Constructor::Tuple { arity } => *arity,
+            Constructor::Enum(e) => {
+                match cx.db.enum_data(e.parent).variants[e.local_id].variant_data.as_ref() {
+                    VariantData::Tuple(struct_field_data) => struct_field_data.len(),
+                    VariantData::Unit => 0,
+                    _ => return Err(MatchCheckNotImplemented),
+                }
+            }
+        };
+
+        Ok(arity)
+    }
+
+    fn all_constructors(&self, cx: &MatchCheckCtx) -> Vec<Constructor> {
+        match self {
+            Constructor::Bool(_) => vec![Constructor::Bool(true), Constructor::Bool(false)],
+            Constructor::Tuple { .. } => vec![*self],
+            Constructor::Enum(e) => cx
+                .db
+                .enum_data(e.parent)
+                .variants
+                .iter()
+                .map(|(local_id, _)| {
+                    Constructor::Enum(EnumVariantId { parent: e.parent, local_id })
+                })
+                .collect(),
+        }
+    }
+}
+
+/// Returns the constructor for the given pattern. Should only return None
+/// in the case of a Wild pattern.
+fn pat_constructor(cx: &MatchCheckCtx, pat: PatIdOrWild) -> MatchCheckResult<Option<Constructor>> {
+    let res = match pat.as_pat(cx) {
+        Pat::Wild => None,
+        Pat::Tuple(pats) => Some(Constructor::Tuple { arity: pats.len() }),
+        Pat::Lit(lit_expr) => match cx.body.exprs[lit_expr] {
+            Expr::Literal(Literal::Bool(val)) => Some(Constructor::Bool(val)),
+            _ => return Err(MatchCheckNotImplemented),
+        },
+        Pat::TupleStruct { .. } | Pat::Path(_) => {
+            let pat_id = pat.as_id().expect("we already know this pattern is not a wild");
+            let variant_id =
+                cx.infer.variant_resolution_for_pat(pat_id).ok_or(MatchCheckNotImplemented)?;
+            match variant_id {
+                VariantId::EnumVariantId(enum_variant_id) => {
+                    Some(Constructor::Enum(enum_variant_id))
+                }
+                _ => return Err(MatchCheckNotImplemented),
+            }
+        }
+        _ => return Err(MatchCheckNotImplemented),
+    };
+
+    Ok(res)
+}
+
+fn all_constructors_covered(
+    cx: &MatchCheckCtx,
+    constructor: &Constructor,
+    used_constructors: &[Constructor],
+) -> bool {
+    match constructor {
+        Constructor::Tuple { arity } => {
+            used_constructors.iter().any(|constructor| match constructor {
+                Constructor::Tuple { arity: used_arity } => arity == used_arity,
+                _ => false,
+            })
+        }
+        Constructor::Bool(_) => {
+            if used_constructors.is_empty() {
+                return false;
+            }
+
+            let covers_true =
+                used_constructors.iter().any(|c| matches!(c, Constructor::Bool(true)));
+            let covers_false =
+                used_constructors.iter().any(|c| matches!(c, Constructor::Bool(false)));
+
+            covers_true && covers_false
+        }
+        Constructor::Enum(e) => cx.db.enum_data(e.parent).variants.iter().all(|(id, _)| {
+            for constructor in used_constructors {
+                if let Constructor::Enum(e) = constructor {
+                    if id == e.local_id {
+                        return true;
+                    }
+                }
+            }
+
+            false
+        }),
+    }
+}
+
+fn enum_variant_matches(cx: &MatchCheckCtx, pat_id: PatId, enum_variant_id: EnumVariantId) -> bool {
+    Some(enum_variant_id.into()) == cx.infer.variant_resolution_for_pat(pat_id)
+}
+
+#[cfg(test)]
+mod tests {
+    pub(super) use insta::assert_snapshot;
+    pub(super) use ra_db::fixture::WithFixture;
+
+    pub(super) use crate::test_db::TestDB;
+
+    pub(super) fn check_diagnostic_message(content: &str) -> String {
+        TestDB::with_single_file(content).0.diagnostics().0
+    }
+
+    pub(super) fn check_diagnostic(content: &str) {
+        let diagnostic_count = TestDB::with_single_file(content).0.diagnostics().1;
+
+        assert_eq!(1, diagnostic_count, "no diagnostic reported");
+    }
+
+    pub(super) fn check_no_diagnostic(content: &str) {
+        let diagnostic_count = TestDB::with_single_file(content).0.diagnostics().1;
+
+        assert_eq!(0, diagnostic_count, "expected no diagnostic, found one");
+    }
+
+    #[test]
+    fn empty_tuple_no_arms_diagnostic_message() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                }
+            }
+        ";
+
+        assert_snapshot!(
+            check_diagnostic_message(content),
+            @"\"()\": Missing match arm\n"
+        );
+    }
+
+    #[test]
+    fn empty_tuple_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn empty_tuple_wild() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                    _ => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn empty_tuple_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                    () => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_empty_tuple_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match (()) {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_empty_tuple_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (()) {
+                    (()) => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_two_empty_tuple_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match ((), ()) {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_two_empty_tuple_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match ((), ()) {
+                    ((), ()) => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn bool_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match false {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn bool_missing_arm() {
+        let content = r"
+            fn test_fn() {
+                match false {
+                    true => {}
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn bool_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match false {
+                    true => {}
+                    false => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (true, true) => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_missing_arm() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (false, true) => {},
+                    (false, false) => {},
+                    (true, false) => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_with_wilds() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (false, _) => {},
+                    (true, false) => {},
+                    (_, true) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (true, true) => {},
+                    (true, false) => {},
+                    (false, true) => {},
+                    (false, false) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_binding_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (true, _x) => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_binding_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (true, _x) => {},
+                    (false, true) => {},
+                    (false, false) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, ((), true)) => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, ((), true)) => {},
+                    (true, ((), false)) => {},
+                    (false, ((), true)) => {},
+                    (false, ((), false)) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_wildcard_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, _) => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_wildcard_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, ((), true)) => {},
+                    (true, ((), false)) => {},
+                    (false, _) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_no_arms() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match Either::A {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_missing_arms() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A => {},
+                    Either::B => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_ref_missing_arms() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match &Either::B {
+                    Either::A => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_ref_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match &Either::B {
+                    Either::A => {},
+                    Either::B => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_no_arms() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_missing_arms() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A(true) => (),
+                    Either::B => (),
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A(true) => (),
+                    Either::A(false) => (),
+                    Either::B => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_with_wild_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::B => (),
+                    _ => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_with_wild_2_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A(_) => (),
+                    Either::B => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_different_sizes_missing_arms() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            fn test_fn() {
+                match Either::A(false) {
+                    Either::A(_) => (),
+                    Either::B(false, _) => (),
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_different_sizes_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            fn test_fn() {
+                match Either::A(false) {
+                    Either::A(_) => (),
+                    Either::B(true, _) => (),
+                    Either::B(false, _) => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn or_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            fn test_fn() {
+                match Either::A(false) {
+                    Either::A(true) | Either::A(false) => (),
+                    Either::B(true, _) => (),
+                    Either::B(false, _) => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_enum_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            enum Either2 {
+                C,
+                D,
+            }
+            fn test_fn() {
+                match (Either::A(false), Either2::C) {
+                    (Either::A(true), _) | (Either::A(false), _) => (),
+                    (Either::B(true, _), Either2::C) => (),
+                    (Either::B(false, _), Either2::C) => (),
+                    (Either::B(_, _), Either2::D) => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn mismatched_types() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            enum Either2 {
+                C,
+                D,
+            }
+            fn test_fn() {
+                match Either::A {
+                    Either2::C => (),
+                    Either2::D => (),
+                }
+            }
+        ";
+
+        // Match arms with the incorrect type are filtered out.
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn mismatched_types_with_different_arity() {
+        let content = r"
+            fn test_fn() {
+                match (true, false) {
+                    (true, false, true) => (),
+                    (true) => (),
+                }
+            }
+        ";
+
+        // Match arms with the incorrect type are filtered out.
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_not_in_scope() {
+        let content = r"
+            fn test_fn() {
+                match Foo::Bar {
+                    Foo::Baz => (),
+                }
+            }
+        ";
+
+        // The enum is not in scope so we don't perform exhaustiveness
+        // checking, but we want to be sure we don't panic here (and
+        // we don't create a diagnostic).
+        check_no_diagnostic(content);
+    }
+}
+
+#[cfg(test)]
+mod false_negatives {
+    //! The implementation of match checking here is a work in progress. As we roll this out, we
+    //! prefer false negatives to false positives (ideally there would be no false positives). This
+    //! test module should document known false negatives. Eventually we will have a complete
+    //! implementation of match checking and this module will be empty.
+    //!
+    //! The reasons for documenting known false negatives:
+    //!
+    //!   1. It acts as a backlog of work that can be done to improve the behavior of the system.
+    //!   2. It ensures the code doesn't panic when handling these cases.
+
+    use super::tests::*;
+
+    #[test]
+    fn integers() {
+        let content = r"
+            fn test_fn() {
+                match 5 {
+                    10 => (),
+                    11..20 => (),
+                }
+            }
+        ";
+
+        // This is a false negative.
+        // We don't currently check integer exhaustiveness.
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_record() {
+        let content = r"
+            enum Either {
+                A { foo: u32 },
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A { foo: 5 } => (),
+                }
+            }
+        ";
+
+        // This is a false negative.
+        // We don't currently handle enum record types.
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn internal_or() {
+        let content = r"
+            fn test_fn() {
+                enum Either {
+                    A(bool),
+                    B,
+                }
+                match Either::B {
+                    Either::A(true | false) => (),
+                }
+            }
+        ";
+
+        // This is a false negative.
+        // We do not currently handle patterns with internal `or`s.
+        check_no_diagnostic(content);
+    }
+}
diff --git a/crates/ra_hir_ty/src/autoderef.rs b/crates/ra_hir_ty/src/autoderef.rs
index 53e81e85d..d91c21e24 100644
--- a/crates/ra_hir_ty/src/autoderef.rs
+++ b/crates/ra_hir_ty/src/autoderef.rs
@@ -14,7 +14,7 @@ use crate::{
     db::HirDatabase,
     traits::{InEnvironment, Solution},
     utils::generics,
-    Canonical, Substs, Ty, TypeWalk,
+    BoundVar, Canonical, DebruijnIndex, Substs, Ty,
 };
 
 const AUTODEREF_RECURSION_LIMIT: usize = 10;
@@ -61,14 +61,13 @@ fn deref_by_trait(
         return None;
     }
 
-    // FIXME make the Canonical handling nicer
+    // FIXME make the Canonical / bound var handling nicer
 
-    let parameters = Substs::build_for_generics(&generic_params)
-        .push(ty.value.value.clone().shift_bound_vars(1))
-        .build();
+    let parameters =
+        Substs::build_for_generics(&generic_params).push(ty.value.value.clone()).build();
 
     let projection = super::traits::ProjectionPredicate {
-        ty: Ty::Bound(0),
+        ty: Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, ty.value.num_vars)),
         projection_ty: super::ProjectionTy { associated_ty: target, parameters },
     };
 
@@ -93,12 +92,16 @@ fn deref_by_trait(
             // we have `impl<T> Deref for Foo<T> { Target = T }`, that should be
             // the case.
             for i in 1..vars.0.num_vars {
-                if vars.0.value[i] != Ty::Bound((i - 1) as u32) {
+                if vars.0.value[i - 1] != Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, i - 1))
+                {
                     warn!("complex solution for derefing {:?}: {:?}, ignoring", ty.value, solution);
                     return None;
                 }
             }
-            Some(Canonical { value: vars.0.value[0].clone(), num_vars: vars.0.num_vars })
+            Some(Canonical {
+                value: vars.0.value[vars.0.value.len() - 1].clone(),
+                num_vars: vars.0.num_vars,
+            })
         }
         Solution::Ambig(_) => {
             info!("Ambiguous solution for derefing {:?}: {:?}", ty.value, solution);
diff --git a/crates/ra_hir_ty/src/diagnostics.rs b/crates/ra_hir_ty/src/diagnostics.rs
index 0f8522021..8cbce1168 100644
--- a/crates/ra_hir_ty/src/diagnostics.rs
+++ b/crates/ra_hir_ty/src/diagnostics.rs
@@ -6,7 +6,7 @@ use hir_expand::{db::AstDatabase, name::Name, HirFileId, InFile};
 use ra_syntax::{ast, AstNode, AstPtr, SyntaxNodePtr};
 use stdx::format_to;
 
-pub use hir_def::diagnostics::UnresolvedModule;
+pub use hir_def::{diagnostics::UnresolvedModule, expr::MatchArm};
 pub use hir_expand::diagnostics::{AstDiagnostic, Diagnostic, DiagnosticSink};
 
 #[derive(Debug)]
@@ -63,6 +63,25 @@ impl AstDiagnostic for MissingFields {
 }
 
 #[derive(Debug)]
+pub struct MissingMatchArms {
+    pub file: HirFileId,
+    pub match_expr: AstPtr<ast::Expr>,
+    pub arms: AstPtr<ast::MatchArmList>,
+}
+
+impl Diagnostic for MissingMatchArms {
+    fn message(&self) -> String {
+        String::from("Missing match arm")
+    }
+    fn source(&self) -> InFile<SyntaxNodePtr> {
+        InFile { file_id: self.file, value: self.match_expr.into() }
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
+
+#[derive(Debug)]
 pub struct MissingOkInTailExpr {
     pub file: HirFileId,
     pub expr: AstPtr<ast::Expr>,
diff --git a/crates/ra_hir_ty/src/display.rs b/crates/ra_hir_ty/src/display.rs
index 13ecd537a..0e9313aa1 100644
--- a/crates/ra_hir_ty/src/display.rs
+++ b/crates/ra_hir_ty/src/display.rs
@@ -155,7 +155,11 @@ impl HirDisplay for ApplicationTy {
                 let sig = FnSig::from_fn_ptr_substs(&self.parameters);
                 write!(f, "fn(")?;
                 f.write_joined(sig.params(), ", ")?;
-                write!(f, ") -> {}", sig.ret().display(f.db))?;
+                write!(f, ")")?;
+                let ret = sig.ret();
+                if *ret != Ty::unit() {
+                    write!(f, " -> {}", ret.display(f.db))?;
+                }
             }
             TypeCtor::FnDef(def) => {
                 let sig = f.db.callable_item_signature(def).subst(&self.parameters);
@@ -180,7 +184,11 @@ impl HirDisplay for ApplicationTy {
                 }
                 write!(f, "(")?;
                 f.write_joined(sig.params(), ", ")?;
-                write!(f, ") -> {}", sig.ret().display(f.db))?;
+                write!(f, ")")?;
+                let ret = sig.ret();
+                if *ret != Ty::unit() {
+                    write!(f, " -> {}", ret.display(f.db))?;
+                }
             }
             TypeCtor::Adt(def_id) => {
                 let name = match def_id {
@@ -242,16 +250,16 @@ impl HirDisplay for ApplicationTy {
                 let sig = self.parameters[0]
                     .callable_sig(f.db)
                     .expect("first closure parameter should contain signature");
-                let return_type_hint = sig.ret().display(f.db);
                 if sig.params().is_empty() {
-                    write!(f, "|| -> {}", return_type_hint)?;
+                    write!(f, "||")?;
                 } else if f.omit_verbose_types() {
-                    write!(f, "|{}| -> {}", TYPE_HINT_TRUNCATION, return_type_hint)?;
+                    write!(f, "|{}|", TYPE_HINT_TRUNCATION)?;
                 } else {
                     write!(f, "|")?;
                     f.write_joined(sig.params(), ", ")?;
-                    write!(f, "| -> {}", return_type_hint)?;
+                    write!(f, "|")?;
                 };
+                write!(f, " -> {}", sig.ret().display(f.db))?;
             }
         }
         Ok(())
@@ -303,7 +311,7 @@ impl HirDisplay for Ty {
                     }
                 }
             }
-            Ty::Bound(idx) => write!(f, "?{}", idx)?,
+            Ty::Bound(idx) => write!(f, "?{}.{}", idx.debruijn.depth(), idx.index)?,
             Ty::Dyn(predicates) | Ty::Opaque(predicates) => {
                 match self {
                     Ty::Dyn(_) => write!(f, "dyn ")?,
diff --git a/crates/ra_hir_ty/src/expr.rs b/crates/ra_hir_ty/src/expr.rs
index b7b476b4c..b4592fbf5 100644
--- a/crates/ra_hir_ty/src/expr.rs
+++ b/crates/ra_hir_ty/src/expr.rs
@@ -2,21 +2,18 @@
 
 use std::sync::Arc;
 
-use hir_def::{
-    path::{path, Path},
-    resolver::HasResolver,
-    AdtId, FunctionId,
-};
-use hir_expand::{diagnostics::DiagnosticSink, name::Name};
+use hir_def::{path::path, resolver::HasResolver, AdtId, FunctionId};
+use hir_expand::diagnostics::DiagnosticSink;
 use ra_syntax::ast;
 use ra_syntax::AstPtr;
 use rustc_hash::FxHashSet;
 
 use crate::{
     db::HirDatabase,
-    diagnostics::{MissingFields, MissingOkInTailExpr},
+    diagnostics::{MissingFields, MissingMatchArms, MissingOkInTailExpr},
     utils::variant_data,
     ApplicationTy, InferenceResult, Ty, TypeCtor,
+    _match::{is_useful, MatchCheckCtx, Matrix, PatStack, Usefulness},
 };
 
 pub use hir_def::{
@@ -28,7 +25,7 @@ pub use hir_def::{
         ArithOp, Array, BinaryOp, BindingAnnotation, CmpOp, Expr, ExprId, Literal, LogicOp,
         MatchArm, Ordering, Pat, PatId, RecordFieldPat, RecordLitField, Statement, UnaryOp,
     },
-    VariantId,
+    LocalStructFieldId, VariantId,
 };
 
 pub struct ExprValidator<'a, 'b: 'a> {
@@ -49,66 +46,118 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
     pub fn validate_body(&mut self, db: &dyn HirDatabase) {
         let body = db.body(self.func.into());
 
-        for e in body.exprs.iter() {
-            if let (id, Expr::RecordLit { path, fields, spread }) = e {
-                self.validate_record_literal(id, path, fields, *spread, db);
+        for (id, expr) in body.exprs.iter() {
+            if let Some((variant_def, missed_fields, true)) =
+                record_literal_missing_fields(db, &self.infer, id, expr)
+            {
+                // XXX: only look at source_map if we do have missing fields
+                let (_, source_map) = db.body_with_source_map(self.func.into());
+
+                if let Ok(source_ptr) = source_map.expr_syntax(id) {
+                    if let Some(expr) = source_ptr.value.left() {
+                        let root = source_ptr.file_syntax(db.upcast());
+                        if let ast::Expr::RecordLit(record_lit) = expr.to_node(&root) {
+                            if let Some(field_list) = record_lit.record_field_list() {
+                                let variant_data = variant_data(db.upcast(), variant_def);
+                                let missed_fields = missed_fields
+                                    .into_iter()
+                                    .map(|idx| variant_data.fields()[idx].name.clone())
+                                    .collect();
+                                self.sink.push(MissingFields {
+                                    file: source_ptr.file_id,
+                                    field_list: AstPtr::new(&field_list),
+                                    missed_fields,
+                                })
+                            }
+                        }
+                    }
+                }
+            }
+            if let Expr::Match { expr, arms } = expr {
+                self.validate_match(id, *expr, arms, db, self.infer.clone());
             }
         }
-
         let body_expr = &body[body.body_expr];
-        if let Expr::Block { statements: _, tail: Some(t) } = body_expr {
+        if let Expr::Block { tail: Some(t), .. } = body_expr {
             self.validate_results_in_tail_expr(body.body_expr, *t, db);
         }
     }
 
-    fn validate_record_literal(
+    fn validate_match(
         &mut self,
         id: ExprId,
-        _path: &Option<Path>,
-        fields: &[RecordLitField],
-        spread: Option<ExprId>,
+        match_expr: ExprId,
+        arms: &[MatchArm],
         db: &dyn HirDatabase,
+        infer: Arc<InferenceResult>,
     ) {
-        if spread.is_some() {
-            return;
-        };
-        let variant_def: VariantId = match self.infer.variant_resolution_for_expr(id) {
-            Some(VariantId::UnionId(_)) | None => return,
-            Some(it) => it,
+        let (body, source_map): (Arc<Body>, Arc<BodySourceMap>) =
+            db.body_with_source_map(self.func.into());
+
+        let match_expr_ty = match infer.type_of_expr.get(match_expr) {
+            Some(ty) => ty,
+            // If we can't resolve the type of the match expression
+            // we cannot perform exhaustiveness checks.
+            None => return,
         };
-        if let VariantId::UnionId(_) = variant_def {
-            return;
-        }
 
-        let variant_data = variant_data(db.upcast(), variant_def);
-
-        let lit_fields: FxHashSet<_> = fields.iter().map(|f| &f.name).collect();
-        let missed_fields: Vec<Name> = variant_data
-            .fields()
-            .iter()
-            .filter_map(|(_f, d)| {
-                let name = d.name.clone();
-                if lit_fields.contains(&name) {
-                    None
-                } else {
-                    Some(name)
+        let cx = MatchCheckCtx { body, infer: infer.clone(), db };
+        let pats = arms.iter().map(|arm| arm.pat);
+
+        let mut seen = Matrix::empty();
+        for pat in pats {
+            // We skip any patterns whose type we cannot resolve.
+            //
+            // This could lead to false positives in this diagnostic, so
+            // it might be better to skip the entire diagnostic if we either
+            // cannot resolve a match arm or determine that the match arm has
+            // the wrong type.
+            if let Some(pat_ty) = infer.type_of_pat.get(pat) {
+                // We only include patterns whose type matches the type
+                // of the match expression. If we had a InvalidMatchArmPattern
+                // diagnostic or similar we could raise that in an else
+                // block here.
+                //
+                // When comparing the types, we also have to consider that rustc
+                // will automatically de-reference the match expression type if
+                // necessary.
+                //
+                // FIXME we should use the type checker for this.
+                if pat_ty == match_expr_ty
+                    || match_expr_ty
+                        .as_reference()
+                        .map(|(match_expr_ty, _)| match_expr_ty == pat_ty)
+                        .unwrap_or(false)
+                {
+                    // If we had a NotUsefulMatchArm diagnostic, we could
+                    // check the usefulness of each pattern as we added it
+                    // to the matrix here.
+                    let v = PatStack::from_pattern(pat);
+                    seen.push(&cx, v);
                 }
-            })
-            .collect();
-        if missed_fields.is_empty() {
-            return;
+            }
+        }
+
+        match is_useful(&cx, &seen, &PatStack::from_wild()) {
+            Ok(Usefulness::Useful) => (),
+            // if a wildcard pattern is not useful, then all patterns are covered
+            Ok(Usefulness::NotUseful) => return,
+            // this path is for unimplemented checks, so we err on the side of not
+            // reporting any errors
+            _ => return,
         }
-        let (_, source_map) = db.body_with_source_map(self.func.into());
 
         if let Ok(source_ptr) = source_map.expr_syntax(id) {
             if let Some(expr) = source_ptr.value.left() {
                 let root = source_ptr.file_syntax(db.upcast());
-                if let ast::Expr::RecordLit(record_lit) = expr.to_node(&root) {
-                    if let Some(field_list) = record_lit.record_field_list() {
-                        self.sink.push(MissingFields {
+                if let ast::Expr::MatchExpr(match_expr) = expr.to_node(&root) {
+                    if let (Some(match_expr), Some(arms)) =
+                        (match_expr.expr(), match_expr.match_arm_list())
+                    {
+                        self.sink.push(MissingMatchArms {
                             file: source_ptr.file_id,
-                            field_list: AstPtr::new(&field_list),
-                            missed_fields,
+                            match_expr: AstPtr::new(&match_expr),
+                            arms: AstPtr::new(&arms),
                         })
                     }
                 }
@@ -148,3 +197,63 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
         }
     }
 }
+
+pub fn record_literal_missing_fields(
+    db: &dyn HirDatabase,
+    infer: &InferenceResult,
+    id: ExprId,
+    expr: &Expr,
+) -> Option<(VariantId, Vec<LocalStructFieldId>, /*exhaustive*/ bool)> {
+    let (fields, exhausitve) = match expr {
+        Expr::RecordLit { path: _, fields, spread } => (fields, spread.is_none()),
+        _ => return None,
+    };
+
+    let variant_def = infer.variant_resolution_for_expr(id)?;
+    if let VariantId::UnionId(_) = variant_def {
+        return None;
+    }
+
+    let variant_data = variant_data(db.upcast(), variant_def);
+
+    let specified_fields: FxHashSet<_> = fields.iter().map(|f| &f.name).collect();
+    let missed_fields: Vec<LocalStructFieldId> = variant_data
+        .fields()
+        .iter()
+        .filter_map(|(f, d)| if specified_fields.contains(&d.name) { None } else { Some(f) })
+        .collect();
+    if missed_fields.is_empty() {
+        return None;
+    }
+    Some((variant_def, missed_fields, exhausitve))
+}
+
+pub fn record_pattern_missing_fields(
+    db: &dyn HirDatabase,
+    infer: &InferenceResult,
+    id: PatId,
+    pat: &Pat,
+) -> Option<(VariantId, Vec<LocalStructFieldId>)> {
+    let fields = match pat {
+        Pat::Record { path: _, args } => args,
+        _ => return None,
+    };
+
+    let variant_def = infer.variant_resolution_for_pat(id)?;
+    if let VariantId::UnionId(_) = variant_def {
+        return None;
+    }
+
+    let variant_data = variant_data(db.upcast(), variant_def);
+
+    let specified_fields: FxHashSet<_> = fields.iter().map(|f| &f.name).collect();
+    let missed_fields: Vec<LocalStructFieldId> = variant_data
+        .fields()
+        .iter()
+        .filter_map(|(f, d)| if specified_fields.contains(&d.name) { None } else { Some(f) })
+        .collect();
+    if missed_fields.is_empty() {
+        return None;
+    }
+    Some((variant_def, missed_fields))
+}
diff --git a/crates/ra_hir_ty/src/infer/pat.rs b/crates/ra_hir_ty/src/infer/pat.rs
index baed6225b..69bbb4307 100644
--- a/crates/ra_hir_ty/src/infer/pat.rs
+++ b/crates/ra_hir_ty/src/infer/pat.rs
@@ -11,7 +11,7 @@ use hir_def::{
 use hir_expand::name::Name;
 use test_utils::tested_by;
 
-use super::{BindingMode, InferenceContext};
+use super::{BindingMode, Expectation, InferenceContext};
 use crate::{utils::variant_data, Substs, Ty, TypeCtor};
 
 impl<'a> InferenceContext<'a> {
@@ -21,9 +21,13 @@ impl<'a> InferenceContext<'a> {
         subpats: &[PatId],
         expected: &Ty,
         default_bm: BindingMode,
+        id: PatId,
     ) -> Ty {
         let (ty, def) = self.resolve_variant(path);
         let var_data = def.map(|it| variant_data(self.db.upcast(), it));
+        if let Some(variant) = def {
+            self.write_variant_resolution(id.into(), variant);
+        }
         self.unify(&ty, expected);
 
         let substs = ty.substs().unwrap_or_else(Substs::empty);
@@ -152,7 +156,7 @@ impl<'a> InferenceContext<'a> {
                 Ty::apply_one(TypeCtor::Ref(*mutability), subty)
             }
             Pat::TupleStruct { path: p, args: subpats } => {
-                self.infer_tuple_struct_pat(p.as_ref(), subpats, expected, default_bm)
+                self.infer_tuple_struct_pat(p.as_ref(), subpats, expected, default_bm, pat)
             }
             Pat::Record { path: p, args: fields } => {
                 self.infer_record_pat(p.as_ref(), fields, expected, default_bm, pat)
@@ -198,7 +202,14 @@ impl<'a> InferenceContext<'a> {
 
                 Ty::apply_one(container_ty, elem_ty)
             }
-            _ => Ty::Unknown,
+            Pat::Wild => expected.clone(),
+            Pat::Range { start, end } => {
+                let start_ty = self.infer_expr(*start, &Expectation::has_type(expected.clone()));
+                let end_ty = self.infer_expr(*end, &Expectation::has_type(start_ty));
+                end_ty
+            }
+            Pat::Lit(expr) => self.infer_expr(*expr, &Expectation::has_type(expected.clone())),
+            Pat::Missing => Ty::Unknown,
         };
         // use a new type variable if we got Ty::Unknown here
         let ty = self.insert_type_vars_shallow(ty);
diff --git a/crates/ra_hir_ty/src/infer/path.rs b/crates/ra_hir_ty/src/infer/path.rs
index 318652c61..2b6bc0f79 100644
--- a/crates/ra_hir_ty/src/infer/path.rs
+++ b/crates/ra_hir_ty/src/infer/path.rs
@@ -67,8 +67,16 @@ impl<'a> InferenceContext<'a> {
             ValueNs::FunctionId(it) => it.into(),
             ValueNs::ConstId(it) => it.into(),
             ValueNs::StaticId(it) => it.into(),
-            ValueNs::StructId(it) => it.into(),
-            ValueNs::EnumVariantId(it) => it.into(),
+            ValueNs::StructId(it) => {
+                self.write_variant_resolution(id, it.into());
+
+                it.into()
+            }
+            ValueNs::EnumVariantId(it) => {
+                self.write_variant_resolution(id, it.into());
+
+                it.into()
+            }
         };
 
         let ty = self.db.value_ty(typable);
diff --git a/crates/ra_hir_ty/src/infer/unify.rs b/crates/ra_hir_ty/src/infer/unify.rs
index 0bf8fbd63..ac25f8a80 100644
--- a/crates/ra_hir_ty/src/infer/unify.rs
+++ b/crates/ra_hir_ty/src/infer/unify.rs
@@ -7,7 +7,9 @@ use ena::unify::{InPlaceUnificationTable, NoError, UnifyKey, UnifyValue};
 use test_utils::tested_by;
 
 use super::{InferenceContext, Obligation};
-use crate::{Canonical, InEnvironment, InferTy, Substs, Ty, TypeCtor, TypeWalk};
+use crate::{
+    BoundVar, Canonical, DebruijnIndex, InEnvironment, InferTy, Substs, Ty, TypeCtor, TypeWalk,
+};
 
 impl<'a> InferenceContext<'a> {
     pub(super) fn canonicalizer<'b>(&'b mut self) -> Canonicalizer<'a, 'b>
@@ -47,7 +49,7 @@ where
         })
     }
 
-    fn do_canonicalize<T: TypeWalk>(&mut self, t: T, binders: usize) -> T {
+    fn do_canonicalize<T: TypeWalk>(&mut self, t: T, binders: DebruijnIndex) -> T {
         t.fold_binders(
             &mut |ty, binders| match ty {
                 Ty::Infer(tv) => {
@@ -72,7 +74,7 @@ where
                             InferTy::MaybeNeverTypeVar(_) => InferTy::MaybeNeverTypeVar(root),
                         };
                         let position = self.add(free_var);
-                        Ty::Bound((position + binders) as u32)
+                        Ty::Bound(BoundVar::new(binders, position))
                     }
                 }
                 _ => ty,
@@ -89,7 +91,7 @@ where
     }
 
     pub(crate) fn canonicalize_ty(mut self, ty: Ty) -> Canonicalized<Ty> {
-        let result = self.do_canonicalize(ty, 0);
+        let result = self.do_canonicalize(ty, DebruijnIndex::INNERMOST);
         self.into_canonicalized(result)
     }
 
@@ -98,8 +100,12 @@ where
         obligation: InEnvironment<Obligation>,
     ) -> Canonicalized<InEnvironment<Obligation>> {
         let result = match obligation.value {
-            Obligation::Trait(tr) => Obligation::Trait(self.do_canonicalize(tr, 0)),
-            Obligation::Projection(pr) => Obligation::Projection(self.do_canonicalize(pr, 0)),
+            Obligation::Trait(tr) => {
+                Obligation::Trait(self.do_canonicalize(tr, DebruijnIndex::INNERMOST))
+            }
+            Obligation::Projection(pr) => {
+                Obligation::Projection(self.do_canonicalize(pr, DebruijnIndex::INNERMOST))
+            }
         };
         self.into_canonicalized(InEnvironment {
             value: result,
@@ -112,13 +118,13 @@ impl<T> Canonicalized<T> {
     pub fn decanonicalize_ty(&self, mut ty: Ty) -> Ty {
         ty.walk_mut_binders(
             &mut |ty, binders| {
-                if let &mut Ty::Bound(idx) = ty {
-                    if idx as usize >= binders && (idx as usize - binders) < self.free_vars.len() {
-                        *ty = Ty::Infer(self.free_vars[idx as usize - binders]);
+                if let &mut Ty::Bound(bound) = ty {
+                    if bound.debruijn >= binders {
+                        *ty = Ty::Infer(self.free_vars[bound.index]);
                     }
                 }
             },
-            0,
+            DebruijnIndex::INNERMOST,
         );
         ty
     }
@@ -150,7 +156,7 @@ pub fn unify(ty1: &Canonical<Ty>, ty2: &Canonical<Ty>) -> Option<Substs> {
     // (kind of hacky)
     for (i, var) in vars.iter().enumerate() {
         if &*table.resolve_ty_shallow(var) == var {
-            table.unify(var, &Ty::Bound(i as u32));
+            table.unify(var, &Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, i)));
         }
     }
     Some(
diff --git a/crates/ra_hir_ty/src/lib.rs b/crates/ra_hir_ty/src/lib.rs
index 6c5469ecd..18f74d3b1 100644
--- a/crates/ra_hir_ty/src/lib.rs
+++ b/crates/ra_hir_ty/src/lib.rs
@@ -1,6 +1,11 @@
 //! The type system. We currently use this to infer types for completion, hover
 //! information and various assists.
 
+#[allow(unused)]
+macro_rules! eprintln {
+    ($($tt:tt)*) => { stdx::eprintln!($($tt)*) };
+}
+
 macro_rules! impl_froms {
     ($e:ident: $($v:ident $(($($sv:ident),*))?),*) => {
         $(
@@ -38,6 +43,7 @@ mod tests;
 #[cfg(test)]
 mod test_db;
 mod marks;
+mod _match;
 
 use std::ops::Deref;
 use std::sync::Arc;
@@ -64,6 +70,8 @@ pub use lower::{
 };
 pub use traits::{InEnvironment, Obligation, ProjectionPredicate, TraitEnvironment};
 
+pub use chalk_ir::{BoundVar, DebruijnIndex};
+
 /// A type constructor or type name: this might be something like the primitive
 /// type `bool`, a struct like `Vec`, or things like function pointers or
 /// tuples.
@@ -265,7 +273,11 @@ impl TypeWalk for ProjectionTy {
         self.parameters.walk(f);
     }
 
-    fn walk_mut_binders(&mut self, f: &mut impl FnMut(&mut Ty, usize), binders: usize) {
+    fn walk_mut_binders(
+        &mut self,
+        f: &mut impl FnMut(&mut Ty, DebruijnIndex),
+        binders: DebruijnIndex,
+    ) {
         self.parameters.walk_mut_binders(f, binders);
     }
 }
@@ -299,7 +311,7 @@ pub enum Ty {
     /// parameters get turned into variables; during trait resolution, inference
     /// variables get turned into bound variables and back; and in `Dyn` the
     /// `Self` type is represented with a bound variable as well.
-    Bound(u32),
+    Bound(BoundVar),
 
     /// A type variable used during type checking.
     Infer(InferTy),
@@ -337,7 +349,11 @@ impl TypeWalk for Substs {
         }
     }
 
-    fn walk_mut_binders(&mut self, f: &mut impl FnMut(&mut Ty, usize), binders: usize) {
+    fn walk_mut_binders(
+        &mut self,
+        f: &mut impl FnMut(&mut Ty, DebruijnIndex),
+        binders: DebruijnIndex,
+    ) {
         for t in make_mut_slice(&mut self.0) {
             t.walk_mut_binders(f, binders);
         }
@@ -381,7 +397,13 @@ impl Substs {
 
     /// Return Substs that replace each parameter by a bound variable.
     pub(crate) fn bound_vars(generic_params: &Generics) -> Substs {
-        Substs(generic_params.iter().enumerate().map(|(idx, _)| Ty::Bound(idx as u32)).collect())
+        Substs(
+            generic_params
+                .iter()
+                .enumerate()
+                .map(|(idx, _)| Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, idx)))
+                .collect(),
+        )
     }
 
     pub fn build_for_def(db: &dyn HirDatabase, def: impl Into<GenericDefId>) -> SubstsBuilder {
@@ -425,8 +447,8 @@ impl SubstsBuilder {
         self.param_count - self.vec.len()
     }
 
-    pub fn fill_with_bound_vars(self, starting_from: u32) -> Self {
-        self.fill((starting_from..).map(Ty::Bound))
+    pub fn fill_with_bound_vars(self, debruijn: DebruijnIndex, starting_from: usize) -> Self {
+        self.fill((starting_from..).map(|idx| Ty::Bound(BoundVar::new(debruijn, idx))))
     }
 
     pub fn fill_with_unknown(self) -> Self {
@@ -507,7 +529,11 @@ impl TypeWalk for TraitRef {
         self.substs.walk(f);
     }
 
-    fn walk_mut_binders(&mut self, f: &mut impl FnMut(&mut Ty, usize), binders: usize) {
+    fn walk_mut_binders(
+        &mut self,
+        f: &mut impl FnMut(&mut Ty, DebruijnIndex),
+        binders: DebruijnIndex,
+    ) {
         self.substs.walk_mut_binders(f, binders);
     }
 }
@@ -558,7 +584,11 @@ impl TypeWalk for GenericPredicate {
         }
     }
 
-    fn walk_mut_binders(&mut self, f: &mut impl FnMut(&mut Ty, usize), binders: usize) {
+    fn walk_mut_binders(
+        &mut self,
+        f: &mut impl FnMut(&mut Ty, DebruijnIndex),
+        binders: DebruijnIndex,
+    ) {
         match self {
             GenericPredicate::Implemented(trait_ref) => trait_ref.walk_mut_binders(f, binders),
             GenericPredicate::Projection(projection_pred) => {
@@ -616,7 +646,11 @@ impl TypeWalk for FnSig {
         }
     }
 
-    fn walk_mut_binders(&mut self, f: &mut impl FnMut(&mut Ty, usize), binders: usize) {
+    fn walk_mut_binders(
+        &mut self,
+        f: &mut impl FnMut(&mut Ty, DebruijnIndex),
+        binders: DebruijnIndex,
+    ) {
         for t in make_mut_slice(&mut self.params_and_return) {
             t.walk_mut_binders(f, binders);
         }
@@ -755,7 +789,7 @@ impl Ty {
 pub trait TypeWalk {
     fn walk(&self, f: &mut impl FnMut(&Ty));
     fn walk_mut(&mut self, f: &mut impl FnMut(&mut Ty)) {
-        self.walk_mut_binders(&mut |ty, _binders| f(ty), 0);
+        self.walk_mut_binders(&mut |ty, _binders| f(ty), DebruijnIndex::INNERMOST);
     }
     /// Walk the type, counting entered binders.
     ///
@@ -767,9 +801,17 @@ pub trait TypeWalk {
     /// that. Currently, the only thing that introduces bound variables on our
     /// side are `Ty::Dyn` and `Ty::Opaque`, which each introduce a bound
     /// variable for the self type.
-    fn walk_mut_binders(&mut self, f: &mut impl FnMut(&mut Ty, usize), binders: usize);
-
-    fn fold_binders(mut self, f: &mut impl FnMut(Ty, usize) -> Ty, binders: usize) -> Self
+    fn walk_mut_binders(
+        &mut self,
+        f: &mut impl FnMut(&mut Ty, DebruijnIndex),
+        binders: DebruijnIndex,
+    );
+
+    fn fold_binders(
+        mut self,
+        f: &mut impl FnMut(Ty, DebruijnIndex) -> Ty,
+        binders: DebruijnIndex,
+    ) -> Self
     where
         Self: Sized,
     {
@@ -795,40 +837,44 @@ pub trait TypeWalk {
     }
 
     /// Substitutes `Ty::Bound` vars with the given substitution.
-    fn subst_bound_vars(mut self, substs: &Substs) -> Self
+    fn subst_bound_vars(self, substs: &Substs) -> Self
+    where
+        Self: Sized,
+    {
+        self.subst_bound_vars_at_depth(substs, DebruijnIndex::INNERMOST)
+    }
+
+    /// Substitutes `Ty::Bound` vars with the given substitution.
+    fn subst_bound_vars_at_depth(mut self, substs: &Substs, depth: DebruijnIndex) -> Self
     where
         Self: Sized,
     {
         self.walk_mut_binders(
             &mut |ty, binders| {
-                if let &mut Ty::Bound(idx) = ty {
-                    if idx as usize >= binders && (idx as usize - binders) < substs.len() {
-                        *ty = substs.0[idx as usize - binders].clone();
-                    } else if idx as usize >= binders + substs.len() {
-                        // shift free binders
-                        *ty = Ty::Bound(idx - substs.len() as u32);
+                if let &mut Ty::Bound(bound) = ty {
+                    if bound.debruijn >= binders {
+                        *ty = substs.0[bound.index].clone();
                     }
                 }
             },
-            0,
+            depth,
         );
         self
     }
 
-    /// Shifts up `Ty::Bound` vars by `n`.
-    fn shift_bound_vars(self, n: i32) -> Self
+    /// Shifts up debruijn indices of `Ty::Bound` vars by `n`.
+    fn shift_bound_vars(self, n: DebruijnIndex) -> Self
     where
         Self: Sized,
     {
         self.fold_binders(
             &mut |ty, binders| match ty {
-                Ty::Bound(idx) if idx as usize >= binders => {
-                    assert!(idx as i32 >= -n);
-                    Ty::Bound((idx as i32 + n) as u32)
+                Ty::Bound(bound) if bound.debruijn >= binders => {
+                    Ty::Bound(bound.shifted_in_from(n))
                 }
                 ty => ty,
             },
-            0,
+            DebruijnIndex::INNERMOST,
         )
     }
 }
@@ -856,7 +902,11 @@ impl TypeWalk for Ty {
         f(self);
     }
 
-    fn walk_mut_binders(&mut self, f: &mut impl FnMut(&mut Ty, usize), binders: usize) {
+    fn walk_mut_binders(
+        &mut self,
+        f: &mut impl FnMut(&mut Ty, DebruijnIndex),
+        binders: DebruijnIndex,
+    ) {
         match self {
             Ty::Apply(a_ty) => {
                 a_ty.parameters.walk_mut_binders(f, binders);
@@ -866,7 +916,7 @@ impl TypeWalk for Ty {
             }
             Ty::Dyn(predicates) | Ty::Opaque(predicates) => {
                 for p in make_mut_slice(predicates) {
-                    p.walk_mut_binders(f, binders + 1);
+                    p.walk_mut_binders(f, binders.shifted_in());
                 }
             }
             Ty::Placeholder { .. } | Ty::Bound(_) | Ty::Infer(_) | Ty::Unknown => {}
diff --git a/crates/ra_hir_ty/src/lower.rs b/crates/ra_hir_ty/src/lower.rs
index d7f250783..6c7bbc448 100644
--- a/crates/ra_hir_ty/src/lower.rs
+++ b/crates/ra_hir_ty/src/lower.rs
@@ -29,8 +29,8 @@ use crate::{
         all_super_traits, associated_type_by_name_including_super_traits, generics, make_mut_slice,
         variant_data,
     },
-    Binders, FnSig, GenericPredicate, PolyFnSig, ProjectionPredicate, ProjectionTy, Substs,
-    TraitEnvironment, TraitRef, Ty, TypeCtor,
+    Binders, BoundVar, DebruijnIndex, FnSig, GenericPredicate, PolyFnSig, ProjectionPredicate,
+    ProjectionTy, Substs, TraitEnvironment, TraitRef, Ty, TypeCtor,
 };
 
 #[derive(Debug)]
@@ -131,7 +131,7 @@ impl Ty {
                 Ty::apply(TypeCtor::FnPtr { num_args: sig.len() as u16 - 1 }, sig)
             }
             TypeRef::DynTrait(bounds) => {
-                let self_ty = Ty::Bound(0);
+                let self_ty = Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, 0));
                 let predicates = bounds
                     .iter()
                     .flat_map(|b| GenericPredicate::from_type_bound(ctx, b, self_ty.clone()))
@@ -141,7 +141,7 @@ impl Ty {
             TypeRef::ImplTrait(bounds) => {
                 match ctx.impl_trait_mode {
                     ImplTraitLoweringMode::Opaque => {
-                        let self_ty = Ty::Bound(0);
+                        let self_ty = Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, 0));
                         let predicates = bounds
                             .iter()
                             .flat_map(|b| {
@@ -177,12 +177,10 @@ impl Ty {
                             } else {
                                 (0, 0, 0, 0)
                             };
-                        Ty::Bound(
-                            idx as u32
-                                + parent_params as u32
-                                + self_params as u32
-                                + list_params as u32,
-                        )
+                        Ty::Bound(BoundVar::new(
+                            DebruijnIndex::INNERMOST,
+                            idx as usize + parent_params + self_params + list_params,
+                        ))
                     }
                     ImplTraitLoweringMode::Disallowed => {
                         // FIXME: report error
@@ -249,7 +247,11 @@ impl Ty {
         let ty = match resolution {
             TypeNs::TraitId(trait_) => {
                 // if this is a bare dyn Trait, we'll directly put the required ^0 for the self type in there
-                let self_ty = if remaining_segments.len() == 0 { Some(Ty::Bound(0)) } else { None };
+                let self_ty = if remaining_segments.len() == 0 {
+                    Some(Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, 0)))
+                } else {
+                    None
+                };
                 let trait_ref =
                     TraitRef::from_resolved_path(ctx, trait_, resolved_segment, self_ty);
                 let ty = if remaining_segments.len() == 1 {
@@ -289,7 +291,7 @@ impl Ty {
                     TypeParamLoweringMode::Placeholder => Ty::Placeholder(param_id),
                     TypeParamLoweringMode::Variable => {
                         let idx = generics.param_idx(param_id).expect("matching generics");
-                        Ty::Bound(idx)
+                        Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, idx))
                     }
                 }
             }
@@ -558,7 +560,7 @@ impl GenericPredicate {
                     TypeParamLoweringMode::Placeholder => Ty::Placeholder(param_id),
                     TypeParamLoweringMode::Variable => {
                         let idx = generics.param_idx(param_id).expect("matching generics");
-                        Ty::Bound(idx)
+                        Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, idx))
                     }
                 }
             }
diff --git a/crates/ra_hir_ty/src/method_resolution.rs b/crates/ra_hir_ty/src/method_resolution.rs
index 533c6ccfb..74a0bc7db 100644
--- a/crates/ra_hir_ty/src/method_resolution.rs
+++ b/crates/ra_hir_ty/src/method_resolution.rs
@@ -20,7 +20,8 @@ use crate::{
     db::HirDatabase,
     primitive::{FloatBitness, Uncertain},
     utils::all_super_traits,
-    ApplicationTy, Canonical, InEnvironment, TraitEnvironment, TraitRef, Ty, TypeCtor, TypeWalk,
+    ApplicationTy, Canonical, DebruijnIndex, InEnvironment, TraitEnvironment, TraitRef, Ty,
+    TypeCtor, TypeWalk,
 };
 
 /// This is used as a key for indexing impls.
@@ -507,8 +508,9 @@ pub(crate) fn inherent_impl_substs(
 ) -> Option<Substs> {
     // we create a var for each type parameter of the impl; we need to keep in
     // mind here that `self_ty` might have vars of its own
-    let vars =
-        Substs::build_for_def(db, impl_id).fill_with_bound_vars(self_ty.num_vars as u32).build();
+    let vars = Substs::build_for_def(db, impl_id)
+        .fill_with_bound_vars(DebruijnIndex::INNERMOST, self_ty.num_vars)
+        .build();
     let self_ty_with_vars = db.impl_self_ty(impl_id).subst(&vars);
     let self_ty_with_vars =
         Canonical { num_vars: vars.len() + self_ty.num_vars, value: self_ty_with_vars };
@@ -526,8 +528,8 @@ pub(crate) fn inherent_impl_substs(
 fn fallback_bound_vars(s: Substs, num_vars_to_keep: usize) -> Substs {
     s.fold_binders(
         &mut |ty, binders| {
-            if let Ty::Bound(idx) = &ty {
-                if *idx >= binders as u32 {
+            if let Ty::Bound(bound) = &ty {
+                if bound.index >= num_vars_to_keep && bound.debruijn >= binders {
                     Ty::Unknown
                 } else {
                     ty
@@ -536,7 +538,7 @@ fn fallback_bound_vars(s: Substs, num_vars_to_keep: usize) -> Substs {
                 ty
             }
         },
-        num_vars_to_keep,
+        DebruijnIndex::INNERMOST,
     )
 }
 
@@ -586,7 +588,7 @@ fn generic_implements_goal(
     let num_vars = self_ty.num_vars;
     let substs = super::Substs::build_for_def(db, trait_)
         .push(self_ty.value)
-        .fill_with_bound_vars(num_vars as u32)
+        .fill_with_bound_vars(DebruijnIndex::INNERMOST, num_vars)
         .build();
     let num_vars = substs.len() - 1 + self_ty.num_vars;
     let trait_ref = TraitRef { trait_, substs };
diff --git a/crates/ra_hir_ty/src/test_db.rs b/crates/ra_hir_ty/src/test_db.rs
index 208096aab..3a4d58bf9 100644
--- a/crates/ra_hir_ty/src/test_db.rs
+++ b/crates/ra_hir_ty/src/test_db.rs
@@ -105,8 +105,9 @@ impl TestDB {
     }
 
     // FIXME: don't duplicate this
-    pub fn diagnostics(&self) -> String {
+    pub fn diagnostics(&self) -> (String, u32) {
         let mut buf = String::new();
+        let mut count = 0;
         let crate_graph = self.crate_graph();
         for krate in crate_graph.iter() {
             let crate_def_map = self.crate_def_map(krate);
@@ -133,13 +134,14 @@ impl TestDB {
                 let infer = self.infer(f.into());
                 let mut sink = DiagnosticSink::new(|d| {
                     format_to!(buf, "{:?}: {}\n", d.syntax_node(self).text(), d.message());
+                    count += 1;
                 });
                 infer.add_diagnostics(self, f, &mut sink);
                 let mut validator = ExprValidator::new(f, infer, &mut sink);
                 validator.validate_body(self);
             }
         }
-        buf
+        (buf, count)
     }
 }
 
diff --git a/crates/ra_hir_ty/src/tests.rs b/crates/ra_hir_ty/src/tests.rs
index 027e5a8f8..e4a103d1b 100644
--- a/crates/ra_hir_ty/src/tests.rs
+++ b/crates/ra_hir_ty/src/tests.rs
@@ -309,7 +309,8 @@ fn no_such_field_diagnostics() {
         }
         ",
     )
-    .diagnostics();
+    .diagnostics()
+    .0;
 
     assert_snapshot!(diagnostics, @r###"
     "baz: 62": no such field
diff --git a/crates/ra_hir_ty/src/tests/coercion.rs b/crates/ra_hir_ty/src/tests/coercion.rs
index 1e303f5ce..b2c971f02 100644
--- a/crates/ra_hir_ty/src/tests/coercion.rs
+++ b/crates/ra_hir_ty/src/tests/coercion.rs
@@ -275,12 +275,14 @@ fn test(i: i32) {
     [70; 147) 'match ...     }': &[i32]
     [76; 77) 'i': i32
     [88; 89) '2': i32
+    [88; 89) '2': i32
     [93; 96) 'foo': fn foo<i32>(&[i32]) -> &[i32]
     [93; 102) 'foo(&[2])': &[i32]
     [97; 101) '&[2]': &[i32; _]
     [98; 101) '[2]': [i32; _]
     [99; 100) '2': i32
     [112; 113) '1': i32
+    [112; 113) '1': i32
     [117; 121) '&[1]': &[i32; _]
     [118; 121) '[1]': [i32; _]
     [119; 120) '1': i32
@@ -316,10 +318,12 @@ fn test(i: i32) {
     [70; 147) 'match ...     }': &[i32]
     [76; 77) 'i': i32
     [88; 89) '1': i32
+    [88; 89) '1': i32
     [93; 97) '&[1]': &[i32; _]
     [94; 97) '[1]': [i32; _]
     [95; 96) '1': i32
     [107; 108) '2': i32
+    [107; 108) '2': i32
     [112; 115) 'foo': fn foo<i32>(&[i32]) -> &[i32]
     [112; 121) 'foo(&[2])': &[i32]
     [116; 120) '&[2]': &[i32; _]
@@ -357,9 +361,11 @@ fn test() {
     [45; 142) 'match ...     }': *const i32
     [51; 52) '1': i32
     [63; 64) '1': i32
+    [63; 64) '1': i32
     [68; 69) 't': &mut i32
     [68; 81) 't as *mut i32': *mut i32
     [91; 92) '2': i32
+    [91; 92) '2': i32
     [96; 97) 't': &mut i32
     [96; 105) 't as &i32': &i32
     [115; 116) '_': i32
@@ -401,16 +407,16 @@ fn test() {
     [30; 31) 'x': &Foo
     [39; 41) '{}': ()
     [52; 133) '{     ...oo); }': ()
-    [58; 71) 'takes_ref_foo': fn takes_ref_foo(&Foo) -> ()
+    [58; 71) 'takes_ref_foo': fn takes_ref_foo(&Foo)
     [58; 77) 'takes_...(&Foo)': ()
     [72; 76) '&Foo': &Foo
     [73; 76) 'Foo': Foo
-    [83; 96) 'takes_ref_foo': fn takes_ref_foo(&Foo) -> ()
+    [83; 96) 'takes_ref_foo': fn takes_ref_foo(&Foo)
     [83; 103) 'takes_...&&Foo)': ()
     [97; 102) '&&Foo': &&Foo
     [98; 102) '&Foo': &Foo
     [99; 102) 'Foo': Foo
-    [109; 122) 'takes_ref_foo': fn takes_ref_foo(&Foo) -> ()
+    [109; 122) 'takes_ref_foo': fn takes_ref_foo(&Foo)
     [109; 130) 'takes_...&&Foo)': ()
     [123; 129) '&&&Foo': &&&Foo
     [124; 129) '&&Foo': &&Foo
@@ -478,7 +484,7 @@ fn test() {
     [171; 178) 'loop {}': !
     [176; 178) '{}': ()
     [191; 236) '{     ... }); }': ()
-    [197; 210) 'takes_ref_str': fn takes_ref_str(&str) -> ()
+    [197; 210) 'takes_ref_str': fn takes_ref_str(&str)
     [197; 233) 'takes_...g() })': ()
     [211; 232) '&{ ret...ng() }': &String
     [212; 232) '{ retu...ng() }': String
diff --git a/crates/ra_hir_ty/src/tests/macros.rs b/crates/ra_hir_ty/src/tests/macros.rs
index eb97288f1..ff4599b71 100644
--- a/crates/ra_hir_ty/src/tests/macros.rs
+++ b/crates/ra_hir_ty/src/tests/macros.rs
@@ -1,8 +1,10 @@
-use super::{infer, type_at, type_at_pos};
-use crate::test_db::TestDB;
 use insta::assert_snapshot;
 use ra_db::fixture::WithFixture;
 
+use super::{infer, type_at, type_at_pos};
+
+use crate::test_db::TestDB;
+
 #[test]
 fn cfg_impl_def() {
     let (db, pos) = TestDB::with_position(
@@ -658,3 +660,28 @@ fn test() {
     );
     assert_eq!("S", type_at_pos(&db, pos));
 }
+
+#[test]
+fn macro_in_arm() {
+    assert_snapshot!(
+        infer(r#"
+macro_rules! unit {
+    () => { () };
+}
+
+fn main() {
+    let x = match () {
+        unit!() => 92u32,
+    };
+}
+"#),
+        @r###"
+    [52; 111) '{     ...  }; }': ()
+    [62; 63) 'x': u32
+    [66; 108) 'match ...     }': u32
+    [72; 74) '()': ()
+    [85; 92) 'unit!()': ()
+    [96; 101) '92u32': u32
+    "###
+    );
+}
diff --git a/crates/ra_hir_ty/src/tests/patterns.rs b/crates/ra_hir_ty/src/tests/patterns.rs
index 76aa32024..6e5d2247c 100644
--- a/crates/ra_hir_ty/src/tests/patterns.rs
+++ b/crates/ra_hir_ty/src/tests/patterns.rs
@@ -82,6 +82,90 @@ fn test(x: &i32) {
 }
 
 #[test]
+fn infer_literal_pattern() {
+    assert_snapshot!(
+        infer_with_mismatches(r#"
+fn any<T>() -> T { loop {} }
+fn test(x: &i32) {
+    if let "foo" = any() {}
+    if let 1 = any() {}
+    if let 1u32 = any() {}
+    if let 1f32 = any() {}
+    if let 1.0 = any() {}
+    if let true = any() {}
+}
+"#, true),
+        @r###"
+    [18; 29) '{ loop {} }': T
+    [20; 27) 'loop {}': !
+    [25; 27) '{}': ()
+    [38; 39) 'x': &i32
+    [47; 209) '{     ...) {} }': ()
+    [53; 76) 'if let...y() {}': ()
+    [60; 65) '"foo"': &str
+    [60; 65) '"foo"': &str
+    [68; 71) 'any': fn any<&str>() -> &str
+    [68; 73) 'any()': &str
+    [74; 76) '{}': ()
+    [81; 100) 'if let...y() {}': ()
+    [88; 89) '1': i32
+    [88; 89) '1': i32
+    [92; 95) 'any': fn any<i32>() -> i32
+    [92; 97) 'any()': i32
+    [98; 100) '{}': ()
+    [105; 127) 'if let...y() {}': ()
+    [112; 116) '1u32': u32
+    [112; 116) '1u32': u32
+    [119; 122) 'any': fn any<u32>() -> u32
+    [119; 124) 'any()': u32
+    [125; 127) '{}': ()
+    [132; 154) 'if let...y() {}': ()
+    [139; 143) '1f32': f32
+    [139; 143) '1f32': f32
+    [146; 149) 'any': fn any<f32>() -> f32
+    [146; 151) 'any()': f32
+    [152; 154) '{}': ()
+    [159; 180) 'if let...y() {}': ()
+    [166; 169) '1.0': f64
+    [166; 169) '1.0': f64
+    [172; 175) 'any': fn any<f64>() -> f64
+    [172; 177) 'any()': f64
+    [178; 180) '{}': ()
+    [185; 207) 'if let...y() {}': ()
+    [192; 196) 'true': bool
+    [192; 196) 'true': bool
+    [199; 202) 'any': fn any<bool>() -> bool
+    [199; 204) 'any()': bool
+    [205; 207) '{}': ()
+    "###
+    );
+}
+
+#[test]
+fn infer_range_pattern() {
+    assert_snapshot!(
+        infer_with_mismatches(r#"
+fn test(x: &i32) {
+    if let 1..76 = 2u32 {}
+    if let 1..=76 = 2u32 {}
+}
+"#, true),
+        @r###"
+    [9; 10) 'x': &i32
+    [18; 76) '{     ...2 {} }': ()
+    [24; 46) 'if let...u32 {}': ()
+    [31; 36) '1..76': u32
+    [39; 43) '2u32': u32
+    [44; 46) '{}': ()
+    [51; 74) 'if let...u32 {}': ()
+    [58; 64) '1..=76': u32
+    [67; 71) '2u32': u32
+    [72; 74) '{}': ()
+    "###
+    );
+}
+
+#[test]
 fn infer_pattern_match_ergonomics() {
     assert_snapshot!(
         infer(r#"
@@ -212,6 +296,7 @@ fn test() {
     [59; 62) 'arr': [f64; _]
     [73; 81) '[1.0, a]': [f64; _]
     [74; 77) '1.0': f64
+    [74; 77) '1.0': f64
     [79; 80) 'a': f64
     [85; 111) '{     ...     }': ()
     [99; 100) 'a': f64
diff --git a/crates/ra_hir_ty/src/tests/regression.rs b/crates/ra_hir_ty/src/tests/regression.rs
index a02e3ee05..3402e0cb5 100644
--- a/crates/ra_hir_ty/src/tests/regression.rs
+++ b/crates/ra_hir_ty/src/tests/regression.rs
@@ -206,7 +206,8 @@ pub fn compute() {
     [24; 106) 'match ...     }': ()
     [30; 37) 'nope!()': {unknown}
     [48; 94) 'SizeSk...tail }': {unknown}
-    [82; 86) 'true': {unknown}
+    [82; 86) 'true': bool
+    [82; 86) 'true': bool
     [88; 92) 'tail': {unknown}
     [98; 100) '{}': ()
     "###
@@ -375,7 +376,7 @@ fn issue_2669() {
         ),
         @r###"
     [147; 262) '{     ...     }': ()
-    [161; 164) 'end': fn end<{unknown}>() -> ()
+    [161; 164) 'end': fn end<{unknown}>()
     [161; 166) 'end()': ()
     [199; 252) '{     ...     }': ()
     [221; 223) '_x': !
diff --git a/crates/ra_hir_ty/src/tests/simple.rs b/crates/ra_hir_ty/src/tests/simple.rs
index c140bd513..c2f1ded2f 100644
--- a/crates/ra_hir_ty/src/tests/simple.rs
+++ b/crates/ra_hir_ty/src/tests/simple.rs
@@ -521,7 +521,7 @@ fn test() -> &mut &f64 {
     [88; 89) 'a': u32
     [92; 108) 'unknow...nction': {unknown}
     [92; 110) 'unknow...tion()': u32
-    [116; 125) 'takes_u32': fn takes_u32(u32) -> ()
+    [116; 125) 'takes_u32': fn takes_u32(u32)
     [116; 128) 'takes_u32(a)': ()
     [126; 127) 'a': u32
     [138; 139) 'b': i32
@@ -948,6 +948,7 @@ fn foo() {
     [165; 247) 'match ...     }': i32
     [171; 175) 'true': bool
     [186; 190) 'true': bool
+    [186; 190) 'true': bool
     [194; 195) '3': i32
     [205; 206) '_': bool
     [210; 241) '{     ...     }': !
@@ -956,6 +957,7 @@ fn foo() {
     [263; 320) 'match ...     }': i32
     [269; 273) 'true': bool
     [284; 288) 'true': bool
+    [284; 288) 'true': bool
     [292; 293) '4': i32
     [303; 304) '_': bool
     [308; 314) 'return': !
@@ -1727,3 +1729,29 @@ fn foo() -> u32 {
     "###
     );
 }
+
+#[test]
+fn fn_pointer_return() {
+    assert_snapshot!(
+        infer(r#"
+struct Vtable {
+    method: fn(),
+}
+
+fn main() {
+    let vtable = Vtable { method: || {} };
+    let m = vtable.method;
+}
+"#),
+        @r###"
+    [48; 121) '{     ...hod; }': ()
+    [58; 64) 'vtable': Vtable
+    [67; 91) 'Vtable...| {} }': Vtable
+    [84; 89) '|| {}': || -> ()
+    [87; 89) '{}': ()
+    [101; 102) 'm': fn()
+    [105; 111) 'vtable': Vtable
+    [105; 118) 'vtable.method': fn()
+    "###
+    );
+}
diff --git a/crates/ra_hir_ty/src/tests/traits.rs b/crates/ra_hir_ty/src/tests/traits.rs
index f009a708c..22ae6ca90 100644
--- a/crates/ra_hir_ty/src/tests/traits.rs
+++ b/crates/ra_hir_ty/src/tests/traits.rs
@@ -263,7 +263,7 @@ fn test() {
     [119; 120) 'S': S<u32>(u32) -> S<u32>
     [119; 129) 'S(unknown)': S<u32>
     [121; 128) 'unknown': u32
-    [135; 138) 'foo': fn foo<S<u32>>(S<u32>) -> ()
+    [135; 138) 'foo': fn foo<S<u32>>(S<u32>)
     [135; 141) 'foo(s)': ()
     [139; 140) 's': S<u32>
     "###
@@ -962,7 +962,7 @@ fn test(x: impl Trait<u64>, y: &impl Trait<u32>) {
     [224; 225) 'S': S<u16>(u16) -> S<u16>
     [224; 228) 'S(1)': S<u16>
     [226; 227) '1': u16
-    [234; 237) 'bar': fn bar(S<u16>) -> ()
+    [234; 237) 'bar': fn bar(S<u16>)
     [234; 240) 'bar(z)': ()
     [238; 239) 'z': S<u16>
     [246; 247) 'x': impl Trait<u64>
@@ -1108,8 +1108,8 @@ fn test() {
     [40; 47) 'loop {}': !
     [45; 47) '{}': ()
     [91; 124) '{     ...foo; }': ()
-    [101; 102) 'f': fn(S) -> ()
-    [118; 121) 'foo': fn foo(S) -> ()
+    [101; 102) 'f': fn(S)
+    [118; 121) 'foo': fn foo(S)
     "###
     );
 }
@@ -1962,7 +1962,7 @@ fn test() -> impl Trait<i32> {
     [229; 241) 'S(default())': S<u32>
     [231; 238) 'default': fn default<u32>() -> u32
     [231; 240) 'default()': u32
-    [247; 250) 'foo': fn foo(S<u32>) -> ()
+    [247; 250) 'foo': fn foo(S<u32>)
     [247; 254) 'foo(s1)': ()
     [251; 253) 's1': S<u32>
     [264; 265) 'x': i32
@@ -2012,7 +2012,7 @@ fn main() {
     [147; 149) '_v': F
     [192; 195) '{ }': ()
     [207; 238) '{     ... }); }': ()
-    [213; 223) 'f::<(), _>': fn f<(), |&()| -> ()>(|&()| -> ()) -> ()
+    [213; 223) 'f::<(), _>': fn f<(), |&()| -> ()>(|&()| -> ())
     [213; 235) 'f::<()... z; })': ()
     [224; 234) '|z| { z; }': |&()| -> ()
     [225; 226) 'z': &()
@@ -2021,3 +2021,28 @@ fn main() {
     "###
     );
 }
+
+#[test]
+fn dyn_trait_through_chalk() {
+    let t = type_at(
+        r#"
+//- /main.rs
+struct Box<T> {}
+#[lang = "deref"]
+trait Deref {
+    type Target;
+}
+impl<T> Deref for Box<T> {
+    type Target = T;
+}
+trait Trait {
+    fn foo(&self);
+}
+
+fn test(x: Box<dyn Trait>) {
+    x.foo()<|>;
+}
+"#,
+    );
+    assert_eq!(t, "()");
+}
diff --git a/crates/ra_hir_ty/src/traits.rs b/crates/ra_hir_ty/src/traits.rs
index a1ca33c98..5a1e12ce9 100644
--- a/crates/ra_hir_ty/src/traits.rs
+++ b/crates/ra_hir_ty/src/traits.rs
@@ -7,7 +7,7 @@ use ra_db::{impl_intern_key, salsa, CrateId};
 use ra_prof::profile;
 use rustc_hash::FxHashSet;
 
-use crate::db::HirDatabase;
+use crate::{db::HirDatabase, DebruijnIndex};
 
 use super::{Canonical, GenericPredicate, HirDisplay, ProjectionTy, TraitRef, Ty, TypeWalk};
 
@@ -128,7 +128,11 @@ impl TypeWalk for ProjectionPredicate {
         self.ty.walk(f);
     }
 
-    fn walk_mut_binders(&mut self, f: &mut impl FnMut(&mut Ty, usize), binders: usize) {
+    fn walk_mut_binders(
+        &mut self,
+        f: &mut impl FnMut(&mut Ty, DebruijnIndex),
+        binders: DebruijnIndex,
+    ) {
         self.projection_ty.walk_mut_binders(f, binders);
         self.ty.walk_mut_binders(f, binders);
     }
@@ -144,7 +148,7 @@ pub(crate) fn trait_solve_query(
         Obligation::Trait(it) => db.trait_data(it.trait_).name.to_string(),
         Obligation::Projection(_) => "projection".to_string(),
     });
-    log::debug!("trait_solve_query({})", goal.value.value.display(db));
+    log::info!("trait_solve_query({})", goal.value.value.display(db));
 
     if let Obligation::Projection(pred) = &goal.value.value {
         if let Ty::Bound(_) = &pred.projection_ty.parameters[0] {
@@ -153,7 +157,7 @@ pub(crate) fn trait_solve_query(
         }
     }
 
-    let canonical = goal.to_chalk(db).cast();
+    let canonical = goal.to_chalk(db).cast(&Interner);
 
     // We currently don't deal with universes (I think / hope they're not yet
     // relevant for our use cases?)
@@ -194,8 +198,8 @@ fn solution_from_chalk(
     let convert_subst = |subst: chalk_ir::Canonical<chalk_ir::Substitution<Interner>>| {
         let value = subst
             .value
-            .into_iter()
-            .map(|p| match p.ty() {
+            .iter(&Interner)
+            .map(|p| match p.ty(&Interner) {
                 Some(ty) => from_chalk(db, ty.clone()),
                 None => unimplemented!(),
             })
diff --git a/crates/ra_hir_ty/src/traits/builtin.rs b/crates/ra_hir_ty/src/traits/builtin.rs
index 73e3c5c78..ccab246bf 100644
--- a/crates/ra_hir_ty/src/traits/builtin.rs
+++ b/crates/ra_hir_ty/src/traits/builtin.rs
@@ -8,7 +8,8 @@ use super::{AssocTyValue, Impl, UnsizeToSuperTraitObjectData};
 use crate::{
     db::HirDatabase,
     utils::{all_super_traits, generics},
-    ApplicationTy, Binders, GenericPredicate, Substs, TraitRef, Ty, TypeCtor,
+    ApplicationTy, Binders, BoundVar, DebruijnIndex, GenericPredicate, Substs, TraitRef, Ty,
+    TypeCtor, TypeWalk,
 };
 
 pub(super) struct BuiltinImplData {
@@ -164,11 +165,15 @@ fn closure_fn_trait_impl_datum(
 
     let arg_ty = Ty::apply(
         TypeCtor::Tuple { cardinality: num_args },
-        Substs::builder(num_args as usize).fill_with_bound_vars(0).build(),
+        Substs::builder(num_args as usize)
+            .fill_with_bound_vars(DebruijnIndex::INNERMOST, 0)
+            .build(),
     );
     let sig_ty = Ty::apply(
         TypeCtor::FnPtr { num_args },
-        Substs::builder(num_args as usize + 1).fill_with_bound_vars(0).build(),
+        Substs::builder(num_args as usize + 1)
+            .fill_with_bound_vars(DebruijnIndex::INNERMOST, 0)
+            .build(),
     );
 
     let self_ty = Ty::apply_one(TypeCtor::Closure { def: data.def, expr: data.expr }, sig_ty);
@@ -203,7 +208,7 @@ fn closure_fn_trait_output_assoc_ty_value(
         }
     };