386 files changed, 7376 insertions, 934 deletions
diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index fb7afe9fd..02a3b6228 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -10,7 +10,7 @@ on:
 env:
  CARGO_INCREMENTAL: 0
  CARGO_NET_RETRY: 10
-  RUN_SLOW_TESTS: 1
+  CI: 1
  RUST_BACKTRACE: short
  RUSTFLAGS: -D warnings
  RUSTUP_MAX_RETRIES: 10
@@ -45,7 +45,7 @@ jobs:
    name: Rust
    runs-on: ${{ matrix.os }}
    env:
-      CC: deny_c
+     CC: deny_c
    strategy:
      fail-fast: false
diff --git a/.github/workflows/release.yaml b/.github/workflows/release.yaml
index fd184e8f1..2c1192f07 100644
--- a/.github/workflows/release.yaml
+++ b/.github/workflows/release.yaml
@@ -50,16 +50,20 @@ jobs:
    - name: Dist
      if: matrix.os == 'ubuntu-latest' && github.ref == 'refs/heads/release'
-      run: cargo xtask dist --client --version 0.2.$GITHUB_RUN_NUMBER         --tag $(date --iso --utc)
+      run: cargo xtask dist --client 0.2.$GITHUB_RUN_NUMBER
    - name: Dist
      if: matrix.os == 'ubuntu-latest' && github.ref != 'refs/heads/release'
-      run: cargo xtask dist --client --version 0.3.$GITHUB_RUN_NUMBER-nightly --tag nightly
+      run: cargo xtask dist --nightly --client 0.3.$GITHUB_RUN_NUMBER-nightly
    - name: Dist
      if: matrix.os != 'ubuntu-latest'
      run: cargo xtask dist
+    - name: Nightly analysis-stats check
+      if: matrix.os == 'ubuntu-latest' && github.ref != 'refs/heads/release'
+      run: ./dist/rust-analyzer-linux analysis-stats .
    - name: Upload artifacts
      uses: actions/upload-artifact@v1
      with:
diff --git a/Cargo.lock b/Cargo.lock
index c07a9614a..eb9824218 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -114,7 +114,7 @@ checksum = "4785bdd1c96b2a846b2bd7cc02e86b6b3dbf14e7e53446c4f54c92a361040822"
 [[package]]
 name = "chalk-derive"
 version = "0.1.0"
-source = "git+https://github.com/rust-lang/chalk.git?rev=177d71340acc7a7204a33115fc63075d86452179#177d71340acc7a7204a33115fc63075d86452179"
+source = "git+https://github.com/rust-lang/chalk.git?rev=039fc904a05f8cb3d0c682c9a57a63dda7a35356#039fc904a05f8cb3d0c682c9a57a63dda7a35356"
 dependencies = [
 "proc-macro2",
 "quote",
@@ -124,7 +124,7 @@ dependencies = [
 [[package]]
 name = "chalk-engine"
 version = "0.9.0"
-source = "git+https://github.com/rust-lang/chalk.git?rev=177d71340acc7a7204a33115fc63075d86452179#177d71340acc7a7204a33115fc63075d86452179"
+source = "git+https://github.com/rust-lang/chalk.git?rev=039fc904a05f8cb3d0c682c9a57a63dda7a35356#039fc904a05f8cb3d0c682c9a57a63dda7a35356"
 dependencies = [
 "chalk-macros",
 "rustc-hash",
@@ -133,7 +133,7 @@ dependencies = [
 [[package]]
 name = "chalk-ir"
 version = "0.1.0"
-source = "git+https://github.com/rust-lang/chalk.git?rev=177d71340acc7a7204a33115fc63075d86452179#177d71340acc7a7204a33115fc63075d86452179"
+source = "git+https://github.com/rust-lang/chalk.git?rev=039fc904a05f8cb3d0c682c9a57a63dda7a35356#039fc904a05f8cb3d0c682c9a57a63dda7a35356"
 dependencies = [
 "chalk-derive",
 "chalk-engine",
@@ -143,7 +143,7 @@ dependencies = [
 [[package]]
 name = "chalk-macros"
 version = "0.1.1"
-source = "git+https://github.com/rust-lang/chalk.git?rev=177d71340acc7a7204a33115fc63075d86452179#177d71340acc7a7204a33115fc63075d86452179"
+source = "git+https://github.com/rust-lang/chalk.git?rev=039fc904a05f8cb3d0c682c9a57a63dda7a35356#039fc904a05f8cb3d0c682c9a57a63dda7a35356"
 dependencies = [
 "lazy_static",
 ]
@@ -151,7 +151,7 @@ dependencies = [
 [[package]]
 name = "chalk-rust-ir"
 version = "0.1.0"
-source = "git+https://github.com/rust-lang/chalk.git?rev=177d71340acc7a7204a33115fc63075d86452179#177d71340acc7a7204a33115fc63075d86452179"
+source = "git+https://github.com/rust-lang/chalk.git?rev=039fc904a05f8cb3d0c682c9a57a63dda7a35356#039fc904a05f8cb3d0c682c9a57a63dda7a35356"
 dependencies = [
 "chalk-derive",
 "chalk-engine",
@@ -162,7 +162,7 @@ dependencies = [
 [[package]]
 name = "chalk-solve"
 version = "0.1.0"
-source = "git+https://github.com/rust-lang/chalk.git?rev=177d71340acc7a7204a33115fc63075d86452179#177d71340acc7a7204a33115fc63075d86452179"
+source = "git+https://github.com/rust-lang/chalk.git?rev=039fc904a05f8cb3d0c682c9a57a63dda7a35356#039fc904a05f8cb3d0c682c9a57a63dda7a35356"
 dependencies = [
 "chalk-derive",
 "chalk-engine",
@@ -170,7 +170,7 @@ dependencies = [
 "chalk-macros",
 "chalk-rust-ir",
 "ena",
- "itertools 0.8.2",
+ "itertools",
 "petgraph",
 "rustc-hash",
 ]
@@ -380,9 +380,9 @@ dependencies = [
 [[package]]
 name = "fst"
-version = "0.4.0"
+version = "0.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3f7c13470d799474d44e2b9c6a0925807def7af4d120cd4de761433be76f7579"
+checksum = "4eaf9ea41cc964d742f7fc7861db75d2d6e83a3ce0d897d5c6f8b621f015ddc8"
 [[package]]
 name = "fuchsia-zircon"
@@ -507,15 +507,6 @@ dependencies = [
 [[package]]
 name = "itertools"
-version = "0.8.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f56a2d0bc861f9165be4eb3442afd3c236d8a98afd426f65d92324ae1091a484"
-dependencies = [
- "either",
-]
-[[package]]
-name = "itertools"
 version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "284f18f85651fe11e8a991b2adb42cb078325c996ed026d994719efcfca1d54b"
@@ -792,9 +783,9 @@ dependencies = [
 [[package]]
 name = "paste"
-version = "0.1.9"
+version = "0.1.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "092d791bf7847f70bbd49085489fba25fc2c193571752bff9e36e74e72403932"
+checksum = "ab4fb1930692d1b6a9cfabdde3d06ea0a7d186518e2f4d67660d8970e2fa647a"
 dependencies = [
 "paste-impl",
 "proc-macro-hack",
@@ -802,9 +793,9 @@ dependencies = [
 [[package]]
 name = "paste-impl"
-version = "0.1.9"
+version = "0.1.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "406c23fb4c45cc6f68a9bbabb8ec7bd6f8cfcbd17e9e8f72c2460282f8325729"
+checksum = "a62486e111e571b1e93b710b61e8f493c0013be39629b714cb166bdb06aa5a8a"
 dependencies = [
 "proc-macro-hack",
 "proc-macro2",
@@ -873,7 +864,7 @@ name = "ra_assists"
 version = "0.1.0"
 dependencies = [
 "either",
- "itertools 0.9.0",
+ "itertools",
 "ra_db",
 "ra_fmt",
 "ra_hir",
@@ -927,7 +918,7 @@ dependencies = [
 name = "ra_fmt"
 version = "0.1.0"
 dependencies = [
- "itertools 0.9.0",
+ "itertools",
 "ra_syntax",
 ]
@@ -937,7 +928,7 @@ version = "0.1.0"
 dependencies = [
 "arrayvec",
 "either",
- "itertools 0.9.0",
+ "itertools",
 "log",
 "ra_db",
 "ra_hir_def",
@@ -1004,6 +995,7 @@ dependencies = [
 "ra_prof",
 "ra_syntax",
 "rustc-hash",
+ "smallvec",
 "stdx",
 "test_utils",
 ]
@@ -1015,7 +1007,7 @@ dependencies = [
 "either",
 "indexmap",
 "insta",
- "itertools 0.9.0",
+ "itertools",
 "log",
 "ra_assists",
 "ra_cfg",
@@ -1084,6 +1076,17 @@ dependencies = [
 ]
 [[package]]
+name = "ra_proc_macro_srv"
+version = "0.1.0"
+dependencies = [
+ "cargo_metadata",
+ "difference",
+ "ra_proc_macro",
+ "ra_tt",
+ "serde_derive",
+]
+[[package]]
 name = "ra_prof"
 version = "0.1.0"
 dependencies = [
@@ -1114,13 +1117,13 @@ name = "ra_syntax"
 version = "0.1.0"
 dependencies = [
 "arrayvec",
- "itertools 0.9.0",
+ "itertools",
 "once_cell",
 "ra_parser",
 "ra_text_edit",
 "rowan",
+ "rustc-ap-rustc_lexer",
 "rustc-hash",
- "rustc_lexer",
 "serde",
 "smol_str",
 "stdx",
@@ -1292,7 +1295,7 @@ dependencies = [
 "crossbeam-channel",
 "env_logger",
 "globset",
- "itertools 0.9.0",
+ "itertools",
 "jod-thread",
 "log",
 "lsp-server",
@@ -1323,6 +1326,15 @@ dependencies = [
 ]
 [[package]]
+name = "rustc-ap-rustc_lexer"
+version = "652.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3a6a43c4d0889218c5e2ae68ffea239f303fc05ab1078c73f74e63feb87f7889"
+dependencies = [
+ "unicode-xid",
+]
+[[package]]
 name = "rustc-demangle"
 version = "0.1.16"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -1335,15 +1347,6 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "08d43f7aa6b08d49f382cde6a7982047c3426db949b1424bc4b7ec9ae12c6ce2"
 [[package]]
-name = "rustc_lexer"
-version = "0.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c86aae0c77166108c01305ee1a36a1e77289d7dc6ca0a3cd91ff4992de2d16a5"
-dependencies = [
- "unicode-xid",
-]
-[[package]]
 name = "ryu"
 version = "1.0.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -1410,18 +1413,18 @@ checksum = "388a1df253eca08550bef6c72392cfe7c30914bf41df5269b68cbd6ff8f570a3"
 [[package]]
 name = "serde"
-version = "1.0.105"
+version = "1.0.104"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e707fbbf255b8fc8c3b99abb91e7257a622caeb20a9818cbadbeeede4e0932ff"
+checksum = "414115f25f818d7dfccec8ee535d76949ae78584fc4f79a6f45a904bf8ab4449"
 dependencies = [
 "serde_derive",
 ]
 [[package]]
 name = "serde_derive"
-version = "1.0.105"
+version = "1.0.104"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ac5d00fc561ba2724df6758a17de23df5914f20e41cb00f94d5b7ae42fffaff8"
+checksum = "128f9e303a5a29922045a830221b8f78ec74a5f544944f3d5984f8ec3895ef64"
 dependencies = [
 "proc-macro2",
 "quote",
@@ -1430,9 +1433,9 @@ dependencies = [
 [[package]]
 name = "serde_json"
-version = "1.0.50"
+version = "1.0.51"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "78a7a12c167809363ec3bd7329fc0a3369056996de43c4b37ef3cd54a6ce4867"
+checksum = "da07b57ee2623368351e9a0488bb0b261322a15a6e0ae53e243cbdc0f4208da9"
 dependencies = [
 "itoa",
 "ryu",
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::FnDef(it) => self.fn_to_def(container.with_value(it))?.into(),
-                    ast::StructDef(it) => { self.struct_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::EnumDef(it) => self.enum_to_def(container.with_value(it))?.into(),
-                    ast::TraitDef(it) => { self.trait_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::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::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::ConstDef(it) => self.const_to_def(container.with_value(it))?.into(),
-                    ast::StaticDef(it) => { self.static_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::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::BoxPat(_) | ast::Pat::RangePat(_) | ast::Pat::MacroPat(_) => Pat::Missing,
-            ast::Pat::RangePat(_) => 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::StructDef(it) => (it.name(), it.type_param_list()),
-            ast::EnumDef(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::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-solve =   { git = "https://github.com/rust-lang/chalk.git", rev = "039fc904a05f8cb3d0c682c9a57a63dda7a35356" }
-chalk-rust-ir = { git = "https://github.com/rust-lang/chalk.git", rev = "177d71340acc7a7204a33115fc63075d86452179" }
+chalk-rust-ir = { git = "https://github.com/rust-lang/chalk.git", rev = "039fc904a05f8cb3d0c682c9a57a63dda7a35356" }
-chalk-ir =      { git = "https://github.com/rust-lang/chalk.git", rev = "177d71340acc7a7204a33115fc63075d86452179" }
+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)
+    let parameters =
-        .push(ty.value.value.clone().shift_bound_vars(1))
+        Substs::build_for_generics(&generic_params).push(ty.value.value.clone()).build();
-        .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> {