23 files changed, 1549 insertions, 149 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 8dcea6ea4..f1847e769 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -168,9 +168,9 @@ checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
 
 [[package]]
 name = "chalk-derive"
-version = "0.37.0"
+version = "0.43.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "564b529b0d620da43dc7ea46fa95b5c602e783e1870aeb07e8cbb6d7ff71bee6"
+checksum = "e2d9e0c8adcced1ab0fea5cb8a38647922893d5b495e363e1814299fd380469b"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -180,9 +180,9 @@ dependencies = [
 
 [[package]]
 name = "chalk-ir"
-version = "0.37.0"
+version = "0.43.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e67d29482387f4cbed6d8f1b1f7d24f00ff10612c700c65fe4af220df11e4d24"
+checksum = "c5218266a5709bc4943de997e64d3fab41c9e9f68efd54a898de53135e987bd3"
 dependencies = [
  "chalk-derive",
  "lazy_static",
@@ -190,9 +190,9 @@ dependencies = [
 
 [[package]]
 name = "chalk-recursive"
-version = "0.37.0"
+version = "0.43.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5c52032be6fbdf91b6a7df3cafba3a6683fdabeff88e7ab73eea96e28657d973"
+checksum = "ed8f34f13fd4f30251f9f6f1dc56f80363201390ecbcac2fdfc8e33036cd9c4a"
 dependencies = [
  "chalk-derive",
  "chalk-ir",
@@ -203,9 +203,9 @@ dependencies = [
 
 [[package]]
 name = "chalk-solve"
-version = "0.37.0"
+version = "0.43.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e0378bdfe1547b6fd545f518373b08c1e0c14920f7555a62d049021250a2b89b"
+checksum = "379c9f584488346044709d4c638c38d61a06fe593d4de2ac5f15fd2b0ba4cd9d"
 dependencies = [
  "chalk-derive",
  "chalk-ir",
diff --git a/crates/assists/src/handlers/early_return.rs b/crates/assists/src/handlers/early_return.rs
index 7fd78e9d4..7bcc318a9 100644
--- a/crates/assists/src/handlers/early_return.rs
+++ b/crates/assists/src/handlers/early_return.rs
@@ -112,7 +112,7 @@ pub(crate) fn convert_to_guarded_return(acc: &mut Assists, ctx: &AssistContext)
                         let then_branch =
                             make::block_expr(once(make::expr_stmt(early_expression).into()), None);
                         let cond = invert_boolean_expression(cond_expr);
-                        make::expr_if(make::condition(cond, None), then_branch)
+                        make::expr_if(make::condition(cond, None), then_branch, None)
                             .indent(if_indent_level)
                     };
                     replace(new_expr.syntax(), &then_block, &parent_block, &if_expr)
diff --git a/crates/assists/src/handlers/move_guard.rs b/crates/assists/src/handlers/move_guard.rs
index e1855b63d..eaffd80ce 100644
--- a/crates/assists/src/handlers/move_guard.rs
+++ b/crates/assists/src/handlers/move_guard.rs
@@ -42,6 +42,7 @@ pub(crate) fn move_guard_to_arm_body(acc: &mut Assists, ctx: &AssistContext) ->
     let if_expr = make::expr_if(
         make::condition(guard_condition, None),
         make::block_expr(None, Some(arm_expr.clone())),
+        None,
     )
     .indent(arm_expr.indent_level());
 
diff --git a/crates/assists/src/handlers/replace_if_let_with_match.rs b/crates/assists/src/handlers/replace_if_let_with_match.rs
index 9a49c48c1..4a355c66f 100644
--- a/crates/assists/src/handlers/replace_if_let_with_match.rs
+++ b/crates/assists/src/handlers/replace_if_let_with_match.rs
@@ -1,3 +1,6 @@
+use std::iter;
+
+use ide_db::{ty_filter::TryEnum, RootDatabase};
 use syntax::{
     ast::{
         self,
@@ -8,7 +11,6 @@ use syntax::{
 };
 
 use crate::{utils::unwrap_trivial_block, AssistContext, AssistId, AssistKind, Assists};
-use ide_db::ty_filter::TryEnum;
 
 // Assist: replace_if_let_with_match
 //
@@ -79,6 +81,91 @@ pub(crate) fn replace_if_let_with_match(acc: &mut Assists, ctx: &AssistContext)
     )
 }
 
+// Assist: replace_match_with_if_let
+//
+// Replaces a binary `match` with a wildcard pattern and no guards with an `if let` expression.
+//
+// ```
+// enum Action { Move { distance: u32 }, Stop }
+//
+// fn handle(action: Action) {
+//     <|>match action {
+//         Action::Move { distance } => foo(distance),
+//         _ => bar(),
+//     }
+// }
+// ```
+// ->
+// ```
+// enum Action { Move { distance: u32 }, Stop }
+//
+// fn handle(action: Action) {
+//     if let Action::Move { distance } = action {
+//         foo(distance)
+//     } else {
+//         bar()
+//     }
+// }
+// ```
+pub(crate) fn replace_match_with_if_let(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let match_expr: ast::MatchExpr = ctx.find_node_at_offset()?;
+    let mut arms = match_expr.match_arm_list()?.arms();
+    let first_arm = arms.next()?;
+    let second_arm = arms.next()?;
+    if arms.next().is_some() || first_arm.guard().is_some() || second_arm.guard().is_some() {
+        return None;
+    }
+    let condition_expr = match_expr.expr()?;
+    let (if_let_pat, then_expr, else_expr) = if is_pat_wildcard_or_sad(&ctx.sema, &first_arm.pat()?)
+    {
+        (second_arm.pat()?, second_arm.expr()?, first_arm.expr()?)
+    } else if is_pat_wildcard_or_sad(&ctx.sema, &second_arm.pat()?) {
+        (first_arm.pat()?, first_arm.expr()?, second_arm.expr()?)
+    } else {
+        return None;
+    };
+
+    let target = match_expr.syntax().text_range();
+    acc.add(
+        AssistId("replace_match_with_if_let", AssistKind::RefactorRewrite),
+        "Replace with if let",
+        target,
+        move |edit| {
+            let condition = make::condition(condition_expr, Some(if_let_pat));
+            let then_block = match then_expr.reset_indent() {
+                ast::Expr::BlockExpr(block) => block,
+                expr => make::block_expr(iter::empty(), Some(expr)),
+            };
+            let else_expr = match else_expr {
+                ast::Expr::BlockExpr(block)
+                    if block.statements().count() == 0 && block.expr().is_none() =>
+                {
+                    None
+                }
+                ast::Expr::TupleExpr(tuple) if tuple.fields().count() == 0 => None,
+                expr => Some(expr),
+            };
+            let if_let_expr = make::expr_if(
+                condition,
+                then_block,
+                else_expr.map(|else_expr| {
+                    ast::ElseBranch::Block(make::block_expr(iter::empty(), Some(else_expr)))
+                }),
+            )
+            .indent(IndentLevel::from_node(match_expr.syntax()));
+
+            edit.replace_ast::<ast::Expr>(match_expr.into(), if_let_expr);
+        },
+    )
+}
+
+fn is_pat_wildcard_or_sad(sema: &hir::Semantics<RootDatabase>, pat: &ast::Pat) -> bool {
+    sema.type_of_pat(&pat)
+        .and_then(|ty| TryEnum::from_ty(sema, &ty))
+        .map(|it| it.sad_pattern().syntax().text() == pat.syntax().text())
+        .unwrap_or_else(|| matches!(pat, ast::Pat::WildcardPat(_)))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -252,4 +339,192 @@ fn main() {
 "#,
         )
     }
+
+    #[test]
+    fn test_replace_match_with_if_let_unwraps_simple_expressions() {
+        check_assist(
+            replace_match_with_if_let,
+            r#"
+impl VariantData {
+    pub fn is_struct(&self) -> bool {
+        <|>match *self {
+            VariantData::Struct(..) => true,
+            _ => false,
+        }
+    }
+}           "#,
+            r#"
+impl VariantData {
+    pub fn is_struct(&self) -> bool {
+        if let VariantData::Struct(..) = *self {
+            true
+        } else {
+            false
+        }
+    }
+}           "#,
+        )
+    }
+
+    #[test]
+    fn test_replace_match_with_if_let_doesnt_unwrap_multiline_expressions() {
+        check_assist(
+            replace_match_with_if_let,
+            r#"
+fn foo() {
+    <|>match a {
+        VariantData::Struct(..) => {
+            bar(
+                123
+            )
+        }
+        _ => false,
+    }
+}           "#,
+            r#"
+fn foo() {
+    if let VariantData::Struct(..) = a {
+        bar(
+            123
+        )
+    } else {
+        false
+    }
+}           "#,
+        )
+    }
+
+    #[test]
+    fn replace_match_with_if_let_target() {
+        check_assist_target(
+            replace_match_with_if_let,
+            r#"
+impl VariantData {
+    pub fn is_struct(&self) -> bool {
+        <|>match *self {
+            VariantData::Struct(..) => true,
+            _ => false,
+        }
+    }
+}           "#,
+            r#"match *self {
+            VariantData::Struct(..) => true,
+            _ => false,
+        }"#,
+        );
+    }
+
+    #[test]
+    fn special_case_option_match_to_if_let() {
+        check_assist(
+            replace_match_with_if_let,
+            r#"
+enum Option<T> { Some(T), None }
+use Option::*;
+
+fn foo(x: Option<i32>) {
+    <|>match x {
+        Some(x) => println!("{}", x),
+        None => println!("none"),
+    }
+}
+           "#,
+            r#"
+enum Option<T> { Some(T), None }
+use Option::*;
+
+fn foo(x: Option<i32>) {
+    if let Some(x) = x {
+        println!("{}", x)
+    } else {
+        println!("none")
+    }
+}
+           "#,
+        );
+    }
+
+    #[test]
+    fn special_case_result_match_to_if_let() {
+        check_assist(
+            replace_match_with_if_let,
+            r#"
+enum Result<T, E> { Ok(T), Err(E) }
+use Result::*;
+
+fn foo(x: Result<i32, ()>) {
+    <|>match x {
+        Ok(x) => println!("{}", x),
+        Err(_) => println!("none"),
+    }
+}
+           "#,
+            r#"
+enum Result<T, E> { Ok(T), Err(E) }
+use Result::*;
+
+fn foo(x: Result<i32, ()>) {
+    if let Ok(x) = x {
+        println!("{}", x)
+    } else {
+        println!("none")
+    }
+}
+           "#,
+        );
+    }
+
+    #[test]
+    fn nested_indent_match_to_if_let() {
+        check_assist(
+            replace_match_with_if_let,
+            r#"
+fn main() {
+    if true {
+        <|>match path.strip_prefix(root_path) {
+            Ok(rel_path) => {
+                let rel_path = RelativePathBuf::from_path(rel_path).ok()?;
+                Some((*id, rel_path))
+            }
+            _ => None,
+        }
+    }
+}
+"#,
+            r#"
+fn main() {
+    if true {
+        if let Ok(rel_path) = path.strip_prefix(root_path) {
+            let rel_path = RelativePathBuf::from_path(rel_path).ok()?;
+            Some((*id, rel_path))
+        } else {
+            None
+        }
+    }
+}
+"#,
+        )
+    }
+
+    #[test]
+    fn replace_match_with_if_let_empty_wildcard_expr() {
+        check_assist(
+            replace_match_with_if_let,
+            r#"
+fn main() {
+    <|>match path.strip_prefix(root_path) {
+        Ok(rel_path) => println!("{}", rel_path),
+        _ => (),
+    }
+}
+"#,
+            r#"
+fn main() {
+    if let Ok(rel_path) = path.strip_prefix(root_path) {
+        println!("{}", rel_path)
+    }
+}
+"#,
+        )
+    }
 }
diff --git a/crates/assists/src/handlers/replace_let_with_if_let.rs b/crates/assists/src/handlers/replace_let_with_if_let.rs
index 69d3b08d3..5970e283c 100644
--- a/crates/assists/src/handlers/replace_let_with_if_let.rs
+++ b/crates/assists/src/handlers/replace_let_with_if_let.rs
@@ -60,7 +60,7 @@ pub(crate) fn replace_let_with_if_let(acc: &mut Assists, ctx: &AssistContext) ->
             };
             let block =
                 make::block_expr(None, None).indent(IndentLevel::from_node(let_stmt.syntax()));
-            let if_ = make::expr_if(make::condition(init, Some(with_placeholder)), block);
+            let if_ = make::expr_if(make::condition(init, Some(with_placeholder)), block, None);
             let stmt = make::expr_stmt(if_);
 
             let placeholder = stmt.syntax().descendants().find_map(ast::WildcardPat::cast).unwrap();
diff --git a/crates/assists/src/lib.rs b/crates/assists/src/lib.rs
index dfe6c2729..b8ce7418d 100644
--- a/crates/assists/src/lib.rs
+++ b/crates/assists/src/lib.rs
@@ -209,6 +209,7 @@ mod handlers {
             reorder_fields::reorder_fields,
             replace_derive_with_manual_impl::replace_derive_with_manual_impl,
             replace_if_let_with_match::replace_if_let_with_match,
+            replace_if_let_with_match::replace_match_with_if_let,
             replace_impl_trait_with_generic::replace_impl_trait_with_generic,
             replace_let_with_if_let::replace_let_with_if_let,
             replace_qualified_name_with_use::replace_qualified_name_with_use,
diff --git a/crates/assists/src/tests/generated.rs b/crates/assists/src/tests/generated.rs
index 8d50c8791..853bde09c 100644
--- a/crates/assists/src/tests/generated.rs
+++ b/crates/assists/src/tests/generated.rs
@@ -890,6 +890,34 @@ fn compute() -> Option<i32> { None }
 }
 
 #[test]
+fn doctest_replace_match_with_if_let() {
+    check_doc_test(
+        "replace_match_with_if_let",
+        r#####"
+enum Action { Move { distance: u32 }, Stop }
+
+fn handle(action: Action) {
+    <|>match action {
+        Action::Move { distance } => foo(distance),
+        _ => bar(),
+    }
+}
+"#####,
+        r#####"
+enum Action { Move { distance: u32 }, Stop }
+
+fn handle(action: Action) {
+    if let Action::Move { distance } = action {
+        foo(distance)
+    } else {
+        bar()
+    }
+}
+"#####,
+    )
+}
+
+#[test]
 fn doctest_replace_qualified_name_with_use() {
     check_doc_test(
         "replace_qualified_name_with_use",
diff --git a/crates/hir_ty/Cargo.toml b/crates/hir_ty/Cargo.toml
index cf5c38a23..289e812fe 100644
--- a/crates/hir_ty/Cargo.toml
+++ b/crates/hir_ty/Cargo.toml
@@ -17,9 +17,9 @@ ena = "0.14.0"
 log = "0.4.8"
 rustc-hash = "1.1.0"
 scoped-tls = "1"
-chalk-solve = { version = "0.37", default-features = false }
-chalk-ir = "0.37"
-chalk-recursive = "0.37"
+chalk-solve = { version = "0.43", default-features = false }
+chalk-ir = "0.43"
+chalk-recursive = "0.43"
 
 stdx = { path = "../stdx", version = "0.0.0" }
 hir_def = { path = "../hir_def", version = "0.0.0" }
diff --git a/crates/hir_ty/src/db.rs b/crates/hir_ty/src/db.rs
index 25cf9eb7f..66bdb8e88 100644
--- a/crates/hir_ty/src/db.rs
+++ b/crates/hir_ty/src/db.rs
@@ -99,6 +99,12 @@ pub trait HirDatabase: DefDatabase + Upcast<dyn DefDatabase> {
     #[salsa::invoke(crate::traits::chalk::fn_def_datum_query)]
     fn fn_def_datum(&self, krate: CrateId, fn_def_id: chalk::FnDefId) -> Arc<chalk::FnDefDatum>;
 
+    #[salsa::invoke(crate::traits::chalk::fn_def_variance_query)]
+    fn fn_def_variance(&self, krate: CrateId, fn_def_id: chalk::FnDefId) -> chalk::Variances;
+
+    #[salsa::invoke(crate::traits::chalk::adt_variance_query)]
+    fn adt_variance(&self, krate: CrateId, adt_id: chalk::AdtId) -> chalk::Variances;
+
     #[salsa::invoke(crate::traits::chalk::associated_ty_value_query)]
     fn associated_ty_value(
         &self,
diff --git a/crates/hir_ty/src/tests/regression.rs b/crates/hir_ty/src/tests/regression.rs
index 94d86b0d1..8cf4e7012 100644
--- a/crates/hir_ty/src/tests/regression.rs
+++ b/crates/hir_ty/src/tests/regression.rs
@@ -840,3 +840,46 @@ fn issue_4966() {
         "#]],
     );
 }
+
+#[test]
+fn issue_6628() {
+    check_infer(
+        r#"
+        #[lang = "fn_once"]
+        pub trait FnOnce<Args> {
+            type Output;
+        }
+
+        struct S<T>();
+        impl<T> S<T> {
+            fn f(&self, _t: T) {}
+            fn g<F: FnOnce(&T)>(&self, _f: F) {}
+        }
+        fn main() {
+            let s = S();
+            s.g(|_x| {});
+            s.f(10);
+        }
+        "#,
+        expect![[r#"
+            105..109 'self': &S<T>
+            111..113 '_t': T
+            118..120 '{}': ()
+            146..150 'self': &S<T>
+            152..154 '_f': F
+            159..161 '{}': ()
+            174..225 '{     ...10); }': ()
+            184..185 's': S<i32>
+            188..189 'S': S<i32>() -> S<i32>
+            188..191 'S()': S<i32>
+            197..198 's': S<i32>
+            197..209 's.g(|_x| {})': ()
+            201..208 '|_x| {}': |&i32| -> ()
+            202..204 '_x': &i32
+            206..208 '{}': ()
+            215..216 's': S<i32>
+            215..222 's.f(10)': ()
+            219..221 '10': i32
+        "#]],
+    );
+}
diff --git a/crates/hir_ty/src/traits.rs b/crates/hir_ty/src/traits.rs
index ce1174cbe..dfa51896b 100644
--- a/crates/hir_ty/src/traits.rs
+++ b/crates/hir_ty/src/traits.rs
@@ -1,4 +1,5 @@
 //! Trait solving using Chalk.
+use std::env::var;
 use std::sync::Arc;
 
 use base_db::CrateId;
@@ -15,12 +16,6 @@ use self::chalk::{from_chalk, Interner, ToChalk};
 
 pub(crate) mod chalk;
 
-// This controls the maximum size of types Chalk considers. If we set this too
-// high, we can run into slow edge cases; if we set it too low, Chalk won't
-// find some solutions.
-// FIXME this is currently hardcoded in the recursive solver
-// const CHALK_SOLVER_MAX_SIZE: usize = 10;
-
 /// This controls how much 'time' we give the Chalk solver before giving up.
 const CHALK_SOLVER_FUEL: i32 = 100;
 
@@ -31,9 +26,11 @@ struct ChalkContext<'a> {
 }
 
 fn create_chalk_solver() -> chalk_recursive::RecursiveSolver<Interner> {
-    let overflow_depth = 100;
+    let overflow_depth =
+        var("CHALK_OVERFLOW_DEPTH").ok().and_then(|s| s.parse().ok()).unwrap_or(100);
     let caching_enabled = true;
-    chalk_recursive::RecursiveSolver::new(overflow_depth, caching_enabled)
+    let max_size = var("CHALK_SOLVER_MAX_SIZE").ok().and_then(|s| s.parse().ok()).unwrap_or(30);
+    chalk_recursive::RecursiveSolver::new(overflow_depth, max_size, caching_enabled)
 }
 
 /// A set of clauses that we assume to be true. E.g. if we are inside this function:
diff --git a/crates/hir_ty/src/traits/chalk.rs b/crates/hir_ty/src/traits/chalk.rs
index 55e2c3a3e..69eae6f79 100644
--- a/crates/hir_ty/src/traits/chalk.rs
+++ b/crates/hir_ty/src/traits/chalk.rs
@@ -104,7 +104,7 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
         };
 
         // Note: Since we're using impls_for_trait, only impls where the trait
-        // can be resolved should ever reach Chalk. `impl_datum` relies on that
+        // can be resolved should ever reach Chalk. Symbol’s value as variable is void: impl_datum relies on that
         // and will panic if the trait can't be resolved.
         let in_deps = self.db.trait_impls_in_deps(self.krate);
         let in_self = self.db.trait_impls_in_crate(self.krate);
@@ -206,7 +206,7 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
                         Some((trait_, alias))
                     })
                 {
-                    // Making up `AsyncBlock<T>: Future<Output = T>`
+                    // Making up Symbol’s value as variable is void: AsyncBlock<T>:
                     //
                     // |--------------------OpaqueTyDatum-------------------|
                     //        |-------------OpaqueTyDatumBound--------------|
@@ -242,7 +242,7 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
                     // The opaque type has 1 parameter.
                     make_binders(bound, 1)
                 } else {
-                    // If failed to find `Future::Output`, return empty bounds as fallback.
+                    // If failed to find Symbol’s value as variable is void: Future::Output, return empty bounds as fallback.
                     let bound = OpaqueTyDatumBound {
                         bounds: make_binders(vec![], 0),
                         where_clauses: make_binders(vec![], 0),
@@ -343,6 +343,23 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
         // FIXME
         unimplemented!()
     }
+
+    fn unification_database(&self) -> &dyn chalk_ir::UnificationDatabase<Interner> {
+        self
+    }
+}
+
+impl<'a> chalk_ir::UnificationDatabase<Interner> for ChalkContext<'a> {
+    fn fn_def_variance(
+        &self,
+        fn_def_id: chalk_ir::FnDefId<Interner>,
+    ) -> chalk_ir::Variances<Interner> {
+        self.db.fn_def_variance(self.krate, fn_def_id)
+    }
+
+    fn adt_variance(&self, adt_id: chalk_ir::AdtId<Interner>) -> chalk_ir::Variances<Interner> {
+        self.db.adt_variance(self.krate, adt_id)
+    }
 }
 
 pub(crate) fn program_clauses_for_chalk_env_query(
@@ -644,6 +661,32 @@ pub(crate) fn fn_def_datum_query(
     Arc::new(datum)
 }
 
+pub(crate) fn fn_def_variance_query(
+    db: &dyn HirDatabase,
+    _krate: CrateId,
+    fn_def_id: FnDefId,
+) -> Variances {
+    let callable_def: CallableDefId = from_chalk(db, fn_def_id);
+    let generic_params = generics(db.upcast(), callable_def.into());
+    Variances::from(
+        &Interner,
+        std::iter::repeat(chalk_ir::Variance::Invariant).take(generic_params.len()),
+    )
+}
+
+pub(crate) fn adt_variance_query(
+    db: &dyn HirDatabase,
+    _krate: CrateId,
+    adt_id: AdtId,
+) -> Variances {
+    let adt: crate::AdtId = from_chalk(db, adt_id);
+    let generic_params = generics(db.upcast(), adt.into());
+    Variances::from(
+        &Interner,
+        std::iter::repeat(chalk_ir::Variance::Invariant).take(generic_params.len()),
+    )
+}
+
 impl From<FnDefId> for crate::db::InternedCallableDefId {
     fn from(fn_def_id: FnDefId) -> Self {
         InternKey::from_intern_id(fn_def_id.0)
diff --git a/crates/hir_ty/src/traits/chalk/interner.rs b/crates/hir_ty/src/traits/chalk/interner.rs
index 39569e690..6a4aa8333 100644
--- a/crates/hir_ty/src/traits/chalk/interner.rs
+++ b/crates/hir_ty/src/traits/chalk/interner.rs
@@ -25,6 +25,7 @@ pub(crate) type FnDefId = chalk_ir::FnDefId<Interner>;
 pub(crate) type FnDefDatum = chalk_solve::rust_ir::FnDefDatum<Interner>;
 pub(crate) type OpaqueTyId = chalk_ir::OpaqueTyId<Interner>;
 pub(crate) type OpaqueTyDatum = chalk_solve::rust_ir::OpaqueTyDatum<Interner>;
+pub(crate) type Variances = chalk_ir::Variances<Interner>;
 
 impl chalk_ir::interner::Interner for Interner {
     type InternedType = Arc<chalk_ir::TyData<Self>>;
@@ -41,6 +42,7 @@ impl chalk_ir::interner::Interner for Interner {
     type InternedVariableKinds = Vec<chalk_ir::VariableKind<Self>>;
     type InternedCanonicalVarKinds = Vec<chalk_ir::CanonicalVarKind<Self>>;
     type InternedConstraints = Vec<chalk_ir::InEnvironment<chalk_ir::Constraint<Self>>>;
+    type InternedVariances = Arc<[chalk_ir::Variance]>;
     type DefId = InternId;
     type InternedAdtId = hir_def::AdtId;
     type Identifier = TypeAliasId;
@@ -370,6 +372,20 @@ impl chalk_ir::interner::Interner for Interner {
     ) -> Option<fmt::Result> {
         None
     }
+
+    fn intern_variances<E>(
+        &self,
+        data: impl IntoIterator<Item = Result<chalk_ir::Variance, E>>,
+    ) -> Result<Self::InternedVariances, E> {
+        data.into_iter().collect()
+    }
+
+    fn variances_data<'a>(
+        &self,
+        variances: &'a Self::InternedVariances,
+    ) -> &'a [chalk_ir::Variance] {
+        &variances
+    }
 }
 
 impl chalk_ir::interner::HasInterner for Interner {
diff --git a/crates/hir_ty/src/traits/chalk/mapping.rs b/crates/hir_ty/src/traits/chalk/mapping.rs
index 86cbc4c7e..8700d664e 100644
--- a/crates/hir_ty/src/traits/chalk/mapping.rs
+++ b/crates/hir_ty/src/traits/chalk/mapping.rs
@@ -31,7 +31,8 @@ impl ToChalk for Ty {
                 TypeCtor::Ref(m) => ref_to_chalk(db, m, apply_ty.parameters),
                 TypeCtor::Array => array_to_chalk(db, apply_ty.parameters),
                 TypeCtor::FnPtr { num_args: _, is_varargs } => {
-                    let substitution = apply_ty.parameters.to_chalk(db).shifted_in(&Interner);
+                    let substitution =
+                        chalk_ir::FnSubst(apply_ty.parameters.to_chalk(db).shifted_in(&Interner));
                     chalk_ir::TyKind::Function(chalk_ir::FnPointer {
                         num_binders: 0,
                         sig: chalk_ir::FnSig {
@@ -183,7 +184,7 @@ impl ToChalk for Ty {
                 assert_eq!(num_binders, 0);
                 let parameters: Substs = from_chalk(
                     db,
-                    substitution.shifted_out(&Interner).expect("fn ptr should have no binders"),
+                    substitution.0.shifted_out(&Interner).expect("fn ptr should have no binders"),
                 );
                 Ty::Apply(ApplicationTy {
                     ctor: TypeCtor::FnPtr {
@@ -536,6 +537,7 @@ impl ToChalk for GenericPredicate {
         // we don't produce any where clauses with binders and can't currently deal with them
         match where_clause
             .skip_binders()
+            .clone()
             .shifted_out(&Interner)
             .expect("unexpected bound vars in where clause")
         {
@@ -661,7 +663,12 @@ where
                     chalk_ir::TyVariableKind::Integer => TyKind::Integer,
                     chalk_ir::TyVariableKind::Float => TyKind::Float,
                 },
-                chalk_ir::VariableKind::Lifetime => panic!("unexpected lifetime from Chalk"),
+                // HACK: Chalk can sometimes return new lifetime variables. We
+                // want to just skip them, but to not mess up the indices of
+                // other variables, we'll just create a new type variable in
+                // their place instead. This should not matter (we never see the
+                // actual *uses* of the lifetime variable).
+                chalk_ir::VariableKind::Lifetime => TyKind::General,
                 chalk_ir::VariableKind::Const(_) => panic!("unexpected const from Chalk"),
             })
             .collect();
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/clippy_pass_by_ref.txt b/crates/rust-analyzer/src/diagnostics/test_data/clippy_pass_by_ref.txt
index 72f6c5725..7576097b3 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/clippy_pass_by_ref.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/clippy_pass_by_ref.txt
@@ -104,4 +104,168 @@
         },
         fixes: [],
     },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/compiler/lib.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 0,
+                    character: 8,
+                },
+                end: Position {
+                    line: 0,
+                    character: 19,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "trivially_copy_pass_by_ref",
+                ),
+            ),
+            code_description: Some(
+                CodeDescription {
+                    href: Url {
+                        scheme: "https",
+                        host: Some(
+                            Domain(
+                                "rust-lang.github.io",
+                            ),
+                        ),
+                        port: None,
+                        path: "/rust-clippy/master/index.html",
+                        query: None,
+                        fragment: Some(
+                            "trivially_copy_pass_by_ref",
+                        ),
+                    },
+                },
+            ),
+            source: Some(
+                "clippy",
+            ),
+            message: "lint level defined here",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/compiler/mir/tagset.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 41,
+                                    character: 23,
+                                },
+                                end: Position {
+                                    line: 41,
+                                    character: 28,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [],
+    },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/compiler/mir/tagset.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 41,
+                    character: 23,
+                },
+                end: Position {
+                    line: 41,
+                    character: 28,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "trivially_copy_pass_by_ref",
+                ),
+            ),
+            code_description: Some(
+                CodeDescription {
+                    href: Url {
+                        scheme: "https",
+                        host: Some(
+                            Domain(
+                                "rust-lang.github.io",
+                            ),
+                        ),
+                        port: None,
+                        path: "/rust-clippy/master/index.html",
+                        query: None,
+                        fragment: Some(
+                            "trivially_copy_pass_by_ref",
+                        ),
+                    },
+                },
+            ),
+            source: Some(
+                "clippy",
+            ),
+            message: "consider passing by value instead",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/compiler/mir/tagset.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 41,
+                                    character: 23,
+                                },
+                                end: Position {
+                                    line: 41,
+                                    character: 28,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [],
+    },
 ]
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/macro_compiler_error.txt b/crates/rust-analyzer/src/diagnostics/test_data/macro_compiler_error.txt
index bbec6a796..bdcf2a38f 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/macro_compiler_error.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/macro_compiler_error.txt
@@ -4,19 +4,19 @@
             scheme: "file",
             host: None,
             port: None,
-            path: "/test/crates/hir_def/src/data.rs",
+            path: "/test/crates/hir_def/src/path.rs",
             query: None,
             fragment: None,
         },
         diagnostic: Diagnostic {
             range: Range {
                 start: Position {
-                    line: 79,
-                    character: 15,
+                    line: 264,
+                    character: 8,
                 },
                 end: Position {
-                    line: 79,
-                    character: 41,
+                    line: 264,
+                    character: 76,
                 },
             },
             severity: Some(
@@ -36,22 +36,22 @@
                                 scheme: "file",
                                 host: None,
                                 port: None,
-                                path: "/test/crates/hir_def/src/path.rs",
+                                path: "/test/crates/hir_def/src/data.rs",
                                 query: None,
                                 fragment: None,
                             },
                             range: Range {
                                 start: Position {
-                                    line: 264,
-                                    character: 8,
+                                    line: 79,
+                                    character: 15,
                                 },
                                 end: Position {
-                                    line: 264,
-                                    character: 76,
+                                    line: 79,
+                                    character: 41,
                                 },
                             },
                         },
-                        message: "Error originated from macro here",
+                        message: "Exact error occurred here",
                     },
                 ],
             ),
@@ -65,19 +65,19 @@
             scheme: "file",
             host: None,
             port: None,
-            path: "/test/crates/hir_def/src/path.rs",
+            path: "/test/crates/hir_def/src/data.rs",
             query: None,
             fragment: None,
         },
         diagnostic: Diagnostic {
             range: Range {
                 start: Position {
-                    line: 264,
-                    character: 8,
+                    line: 79,
+                    character: 15,
                 },
                 end: Position {
-                    line: 264,
-                    character: 76,
+                    line: 79,
+                    character: 41,
                 },
             },
             severity: Some(
@@ -89,33 +89,7 @@
                 "rustc",
             ),
             message: "Please register your known path in the path module",
-            related_information: Some(
-                [
-                    DiagnosticRelatedInformation {
-                        location: Location {
-                            uri: Url {
-                                scheme: "file",
-                                host: None,
-                                port: None,
-                                path: "/test/crates/hir_def/src/data.rs",
-                                query: None,
-                                fragment: None,
-                            },
-                            range: Range {
-                                start: Position {
-                                    line: 79,
-                                    character: 15,
-                                },
-                                end: Position {
-                                    line: 79,
-                                    character: 41,
-                                },
-                            },
-                        },
-                        message: "Exact error occured here",
-                    },
-                ],
-            ),
+            related_information: None,
             tags: None,
             data: None,
         },
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable.txt b/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable.txt
index c709de95f..23d42b4d0 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable.txt
@@ -32,7 +32,33 @@
                 "rustc",
             ),
             message: "unused variable: `foo`\n#[warn(unused_variables)] on by default",
-            related_information: None,
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/driver/subcommand/repl.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 290,
+                                    character: 8,
+                                },
+                                end: Position {
+                                    line: 290,
+                                    character: 11,
+                                },
+                            },
+                        },
+                        message: "consider prefixing with an underscore",
+                    },
+                ],
+            ),
             tags: Some(
                 [
                     Unnecessary,
@@ -87,4 +113,114 @@
             },
         ],
     },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/driver/subcommand/repl.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 290,
+                    character: 8,
+                },
+                end: Position {
+                    line: 290,
+                    character: 11,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "unused_variables",
+                ),
+            ),
+            code_description: None,
+            source: Some(
+                "rustc",
+            ),
+            message: "consider prefixing with an underscore",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/driver/subcommand/repl.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 290,
+                                    character: 8,
+                                },
+                                end: Position {
+                                    line: 290,
+                                    character: 11,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [
+            CodeAction {
+                title: "consider prefixing with an underscore",
+                group: None,
+                kind: Some(
+                    CodeActionKind(
+                        "quickfix",
+                    ),
+                ),
+                edit: Some(
+                    SnippetWorkspaceEdit {
+                        changes: Some(
+                            {
+                                Url {
+                                    scheme: "file",
+                                    host: None,
+                                    port: None,
+                                    path: "/test/driver/subcommand/repl.rs",
+                                    query: None,
+                                    fragment: None,
+                                }: [
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 290,
+                                                character: 8,
+                                            },
+                                            end: Position {
+                                                line: 290,
+                                                character: 11,
+                                            },
+                                        },
+                                        new_text: "_foo",
+                                    },
+                                ],
+                            },
+                        ),
+                        document_changes: None,
+                    },
+                ),
+                is_preferred: Some(
+                    true,
+                ),
+                data: None,
+            },
+        ],
+    },
 ]
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_hint.txt b/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_hint.txt
index 632f438d7..4e428bedc 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_hint.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_hint.txt
@@ -32,7 +32,33 @@
                 "rustc",
             ),
             message: "unused variable: `foo`\n#[warn(unused_variables)] on by default",
-            related_information: None,
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/driver/subcommand/repl.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 290,
+                                    character: 8,
+                                },
+                                end: Position {
+                                    line: 290,
+                                    character: 11,
+                                },
+                            },
+                        },
+                        message: "consider prefixing with an underscore",
+                    },
+                ],
+            ),
             tags: Some(
                 [
                     Unnecessary,
@@ -87,4 +113,114 @@
             },
         ],
     },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/driver/subcommand/repl.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 290,
+                    character: 8,
+                },
+                end: Position {
+                    line: 290,
+                    character: 11,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "unused_variables",
+                ),
+            ),
+            code_description: None,
+            source: Some(
+                "rustc",
+            ),
+            message: "consider prefixing with an underscore",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/driver/subcommand/repl.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 290,
+                                    character: 8,
+                                },
+                                end: Position {
+                                    line: 290,
+                                    character: 11,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [
+            CodeAction {
+                title: "consider prefixing with an underscore",
+                group: None,
+                kind: Some(
+                    CodeActionKind(
+                        "quickfix",
+                    ),
+                ),
+                edit: Some(
+                    SnippetWorkspaceEdit {
+                        changes: Some(
+                            {
+                                Url {
+                                    scheme: "file",
+                                    host: None,
+                                    port: None,
+                                    path: "/test/driver/subcommand/repl.rs",
+                                    query: None,
+                                    fragment: None,
+                                }: [
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 290,
+                                                character: 8,
+                                            },
+                                            end: Position {
+                                                line: 290,
+                                                character: 11,
+                                            },
+                                        },
+                                        new_text: "_foo",
+                                    },
+                                ],
+                            },
+                        ),
+                        document_changes: None,
+                    },
+                ),
+                is_preferred: Some(
+                    true,
+                ),
+                data: None,
+            },
+        ],
+    },
 ]
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_info.txt b/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_info.txt
index c0b79428d..4ddd7efae 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_info.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_info.txt
@@ -32,7 +32,33 @@
                 "rustc",
             ),
             message: "unused variable: `foo`\n#[warn(unused_variables)] on by default",
-            related_information: None,
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/driver/subcommand/repl.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 290,
+                                    character: 8,
+                                },
+                                end: Position {
+                                    line: 290,
+                                    character: 11,
+                                },
+                            },
+                        },
+                        message: "consider prefixing with an underscore",
+                    },
+                ],
+            ),
             tags: Some(
                 [
                     Unnecessary,
@@ -87,4 +113,114 @@
             },
         ],
     },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/driver/subcommand/repl.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 290,
+                    character: 8,
+                },
+                end: Position {
+                    line: 290,
+                    character: 11,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "unused_variables",
+                ),
+            ),
+            code_description: None,
+            source: Some(
+                "rustc",
+            ),
+            message: "consider prefixing with an underscore",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/driver/subcommand/repl.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 290,
+                                    character: 8,
+                                },
+                                end: Position {
+                                    line: 290,
+                                    character: 11,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [
+            CodeAction {
+                title: "consider prefixing with an underscore",
+                group: None,
+                kind: Some(
+                    CodeActionKind(
+                        "quickfix",
+                    ),
+                ),
+                edit: Some(
+                    SnippetWorkspaceEdit {
+                        changes: Some(
+                            {
+                                Url {
+                                    scheme: "file",
+                                    host: None,
+                                    port: None,
+                                    path: "/test/driver/subcommand/repl.rs",
+                                    query: None,
+                                    fragment: None,
+                                }: [
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 290,
+                                                character: 8,
+                                            },
+                                            end: Position {
+                                                line: 290,
+                                                character: 11,
+                                            },
+                                        },
+                                        new_text: "_foo",
+                                    },
+                                ],
+                            },
+                        ),
+                        document_changes: None,
+                    },
+                ),
+                is_preferred: Some(
+                    true,
+                ),
+                data: None,
+            },
+        ],
+    },
 ]
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/rustc_wrong_number_of_parameters.txt b/crates/rust-analyzer/src/diagnostics/test_data/rustc_wrong_number_of_parameters.txt
index b9650f3e4..f455cf25e 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/rustc_wrong_number_of_parameters.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/rustc_wrong_number_of_parameters.txt
@@ -81,4 +81,86 @@
         },
         fixes: [],
     },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/compiler/ty/select.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 218,
+                    character: 4,
+                },
+                end: Position {
+                    line: 230,
+                    character: 5,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "E0061",
+                ),
+            ),
+            code_description: Some(
+                CodeDescription {
+                    href: Url {
+                        scheme: "https",
+                        host: Some(
+                            Domain(
+                                "doc.rust-lang.org",
+                            ),
+                        ),
+                        port: None,
+                        path: "/error-index.html",
+                        query: None,
+                        fragment: Some(
+                            "E0061",
+                        ),
+                    },
+                },
+            ),
+            source: Some(
+                "rustc",
+            ),
+            message: "defined here",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/compiler/ty/select.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 103,
+                                    character: 17,
+                                },
+                                end: Position {
+                                    line: 103,
+                                    character: 29,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [],
+    },
 ]
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/snap_multi_line_fix.txt b/crates/rust-analyzer/src/diagnostics/test_data/snap_multi_line_fix.txt
index c45f68a91..4cbdb3b92 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/snap_multi_line_fix.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/snap_multi_line_fix.txt
@@ -74,6 +74,309 @@
                         },
                         message: "unnecessary let binding",
                     },
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/src/main.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 2,
+                                    character: 4,
+                                },
+                                end: Position {
+                                    line: 2,
+                                    character: 30,
+                                },
+                            },
+                        },
+                        message: "return the expression directly",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [
+            CodeAction {
+                title: "return the expression directly",
+                group: None,
+                kind: Some(
+                    CodeActionKind(
+                        "quickfix",
+                    ),
+                ),
+                edit: Some(
+                    SnippetWorkspaceEdit {
+                        changes: Some(
+                            {
+                                Url {
+                                    scheme: "file",
+                                    host: None,
+                                    port: None,
+                                    path: "/test/src/main.rs",
+                                    query: None,
+                                    fragment: None,
+                                }: [
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 2,
+                                                character: 4,
+                                            },
+                                            end: Position {
+                                                line: 2,
+                                                character: 30,
+                                            },
+                                        },
+                                        new_text: "",
+                                    },
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 3,
+                                                character: 4,
+                                            },
+                                            end: Position {
+                                                line: 3,
+                                                character: 5,
+                                            },
+                                        },
+                                        new_text: "(0..10).collect()",
+                                    },
+                                ],
+                            },
+                        ),
+                        document_changes: None,
+                    },
+                ),
+                is_preferred: Some(
+                    true,
+                ),
+                data: None,
+            },
+        ],
+    },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/src/main.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 2,
+                    character: 4,
+                },
+                end: Position {
+                    line: 2,
+                    character: 30,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "let_and_return",
+                ),
+            ),
+            code_description: Some(
+                CodeDescription {
+                    href: Url {
+                        scheme: "https",
+                        host: Some(
+                            Domain(
+                                "rust-lang.github.io",
+                            ),
+                        ),
+                        port: None,
+                        path: "/rust-clippy/master/index.html",
+                        query: None,
+                        fragment: Some(
+                            "let_and_return",
+                        ),
+                    },
+                },
+            ),
+            source: Some(
+                "clippy",
+            ),
+            message: "unnecessary let binding",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/src/main.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 3,
+                                    character: 4,
+                                },
+                                end: Position {
+                                    line: 3,
+                                    character: 5,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [
+            CodeAction {
+                title: "return the expression directly",
+                group: None,
+                kind: Some(
+                    CodeActionKind(
+                        "quickfix",
+                    ),
+                ),
+                edit: Some(
+                    SnippetWorkspaceEdit {
+                        changes: Some(
+                            {
+                                Url {
+                                    scheme: "file",
+                                    host: None,
+                                    port: None,
+                                    path: "/test/src/main.rs",
+                                    query: None,
+                                    fragment: None,
+                                }: [
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 2,
+                                                character: 4,
+                                            },
+                                            end: Position {
+                                                line: 2,
+                                                character: 30,
+                                            },
+                                        },
+                                        new_text: "",
+                                    },
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 3,
+                                                character: 4,
+                                            },
+                                            end: Position {
+                                                line: 3,
+                                                character: 5,
+                                            },
+                                        },
+                                        new_text: "(0..10).collect()",
+                                    },
+                                ],
+                            },
+                        ),
+                        document_changes: None,
+                    },
+                ),
+                is_preferred: Some(
+                    true,
+                ),
+                data: None,
+            },
+        ],
+    },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/src/main.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 2,
+                    character: 4,
+                },
+                end: Position {
+                    line: 2,
+                    character: 30,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "let_and_return",
+                ),
+            ),
+            code_description: Some(
+                CodeDescription {
+                    href: Url {
+                        scheme: "https",
+                        host: Some(
+                            Domain(
+                                "rust-lang.github.io",
+                            ),
+                        ),
+                        port: None,
+                        path: "/rust-clippy/master/index.html",
+                        query: None,
+                        fragment: Some(
+                            "let_and_return",
+                        ),
+                    },
+                },
+            ),
+            source: Some(
+                "clippy",
+            ),
+            message: "return the expression directly",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/src/main.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 3,
+                                    character: 4,
+                                },
+                                end: Position {
+                                    line: 3,
+                                    character: 5,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
                 ],
             ),
             tags: None,
diff --git a/crates/rust-analyzer/src/diagnostics/to_proto.rs b/crates/rust-analyzer/src/diagnostics/to_proto.rs
index 324019614..f16f97131 100644
--- a/crates/rust-analyzer/src/diagnostics/to_proto.rs
+++ b/crates/rust-analyzer/src/diagnostics/to_proto.rs
@@ -75,8 +75,10 @@ fn diagnostic_related_information(
 }
 
 enum MappedRustChildDiagnostic {
-    Related(lsp_types::DiagnosticRelatedInformation),
-    SuggestedFix(lsp_ext::CodeAction),
+    Related {
+        related: lsp_types::DiagnosticRelatedInformation,
+        suggested_fix: Option<lsp_ext::CodeAction>,
+    },
     MessageLine(String),
 }
 
@@ -103,23 +105,32 @@ fn map_rust_child_diagnostic(
     }
 
     if edit_map.is_empty() {
-        MappedRustChildDiagnostic::Related(lsp_types::DiagnosticRelatedInformation {
-            location: location(workspace_root, spans[0]),
-            message: rd.message.clone(),
-        })
+        MappedRustChildDiagnostic::Related {
+            related: lsp_types::DiagnosticRelatedInformation {
+                location: location(workspace_root, spans[0]),
+                message: rd.message.clone(),
+            },
+            suggested_fix: None,
+        }
     } else {
-        MappedRustChildDiagnostic::SuggestedFix(lsp_ext::CodeAction {
-            title: rd.message.clone(),
-            group: None,
-            kind: Some(lsp_types::CodeActionKind::QUICKFIX),
-            edit: Some(lsp_ext::SnippetWorkspaceEdit {
-                // FIXME: there's no good reason to use edit_map here....
-                changes: Some(edit_map),
-                document_changes: None,
+        MappedRustChildDiagnostic::Related {
+            related: lsp_types::DiagnosticRelatedInformation {
+                location: location(workspace_root, spans[0]),
+                message: rd.message.clone(),
+            },
+            suggested_fix: Some(lsp_ext::CodeAction {
+                title: rd.message.clone(),
+                group: None,
+                kind: Some(lsp_types::CodeActionKind::QUICKFIX),
+                edit: Some(lsp_ext::SnippetWorkspaceEdit {
+                    // FIXME: there's no good reason to use edit_map here....
+                    changes: Some(edit_map),
+                    document_changes: None,
+                }),
+                is_preferred: Some(true),
+                data: None,
             }),
-            is_preferred: Some(true),
-            data: None,
-        })
+        }
     }
 }
 
@@ -179,8 +190,12 @@ pub(crate) fn map_rust_diagnostic_to_lsp(
     for child in &rd.children {
         let child = map_rust_child_diagnostic(workspace_root, &child);
         match child {
-            MappedRustChildDiagnostic::Related(related) => related_information.push(related),
-            MappedRustChildDiagnostic::SuggestedFix(code_action) => fixes.push(code_action),
+            MappedRustChildDiagnostic::Related { related, suggested_fix } => {
+                related_information.push(related);
+                if let Some(code_action) = suggested_fix {
+                    fixes.push(code_action);
+                }
+            }
             MappedRustChildDiagnostic::MessageLine(message_line) => {
                 format_to!(message, "\n{}", message_line);
 
@@ -219,7 +234,7 @@ pub(crate) fn map_rust_diagnostic_to_lsp(
 
     primary_spans
         .iter()
-        .map(|primary_span| {
+        .flat_map(|primary_span| {
             let location = location(workspace_root, &primary_span);
 
             let mut message = message.clone();
@@ -229,72 +244,100 @@ pub(crate) fn map_rust_diagnostic_to_lsp(
                 }
             }
 
+            // Each primary diagnostic span may result in multiple LSP diagnostics.
+            let mut diagnostics = Vec::new();
+
+            let mut related_macro_info = None;
+
             // If error occurs from macro expansion, add related info pointing to
             // where the error originated
             // Also, we would generate an additional diagnostic, so that exact place of macro
             // will be highlighted in the error origin place.
-            let additional_diagnostic =
-                if !is_from_macro(&primary_span.file_name) && primary_span.expansion.is_some() {
-                    let in_macro_location = location_naive(workspace_root, &primary_span);
+            if !is_from_macro(&primary_span.file_name) && primary_span.expansion.is_some() {
+                let in_macro_location = location_naive(workspace_root, &primary_span);
 
-                    // Add related information for the main disagnostic.
-                    related_information.push(lsp_types::DiagnosticRelatedInformation {
-                        location: in_macro_location.clone(),
-                        message: "Error originated from macro here".to_string(),
-                    });
+                // Add related information for the main disagnostic.
+                related_macro_info = Some(lsp_types::DiagnosticRelatedInformation {
+                    location: in_macro_location.clone(),
+                    message: "Error originated from macro here".to_string(),
+                });
 
-                    // For the additional in-macro diagnostic we add the inverse message pointing to the error location in code.
-                    let information_for_additional_diagnostic =
-                        vec![lsp_types::DiagnosticRelatedInformation {
-                            location: location.clone(),
-                            message: "Exact error occured here".to_string(),
-                        }];
+                // For the additional in-macro diagnostic we add the inverse message pointing to the error location in code.
+                let information_for_additional_diagnostic =
+                    vec![lsp_types::DiagnosticRelatedInformation {
+                        location: location.clone(),
+                        message: "Exact error occurred here".to_string(),
+                    }];
 
-                    let diagnostic = lsp_types::Diagnostic {
-                        range: in_macro_location.range,
-                        severity,
-                        code: code.clone().map(lsp_types::NumberOrString::String),
-                        code_description: code_description.clone(),
-                        source: Some(source.clone()),
-                        message: message.clone(),
-                        related_information: Some(information_for_additional_diagnostic),
-                        tags: if tags.is_empty() { None } else { Some(tags.clone()) },
-                        data: None,
-                    };
-
-                    Some(MappedRustDiagnostic {
-                        url: in_macro_location.uri,
-                        diagnostic,
-                        fixes: fixes.clone(),
-                    })
-                } else {
-                    None
+                let diagnostic = lsp_types::Diagnostic {
+                    range: in_macro_location.range,
+                    severity,
+                    code: code.clone().map(lsp_types::NumberOrString::String),
+                    code_description: code_description.clone(),
+                    source: Some(source.clone()),
+                    message: message.clone(),
+                    related_information: Some(information_for_additional_diagnostic),
+                    tags: if tags.is_empty() { None } else { Some(tags.clone()) },
+                    data: None,
                 };
 
-            let diagnostic = lsp_types::Diagnostic {
-                range: location.range,
-                severity,
-                code: code.clone().map(lsp_types::NumberOrString::String),
-                code_description: code_description.clone(),
-                source: Some(source.clone()),
-                message,
-                related_information: if related_information.is_empty() {
-                    None
-                } else {
-                    Some(related_information.clone())
+                diagnostics.push(MappedRustDiagnostic {
+                    url: in_macro_location.uri,
+                    diagnostic,
+                    fixes: fixes.clone(),
+                });
+            }
+
+            // Emit the primary diagnostic.
+            diagnostics.push(MappedRustDiagnostic {
+                url: location.uri.clone(),
+                diagnostic: lsp_types::Diagnostic {
+                    range: location.range,
+                    severity,
+                    code: code.clone().map(lsp_types::NumberOrString::String),
+                    code_description: code_description.clone(),
+                    source: Some(source.clone()),
+                    message,
+                    related_information: if related_information.is_empty() {
+                        None
+                    } else {
+                        let mut related = related_information.clone();
+                        related.extend(related_macro_info);
+                        Some(related)
+                    },
+                    tags: if tags.is_empty() { None } else { Some(tags.clone()) },
+                    data: None,
                 },
-                tags: if tags.is_empty() { None } else { Some(tags.clone()) },
-                data: None,
-            };
+                fixes: fixes.clone(),
+            });
 
-            let main_diagnostic =
-                MappedRustDiagnostic { url: location.uri, diagnostic, fixes: fixes.clone() };
-            match additional_diagnostic {
-                None => vec![main_diagnostic],
-                Some(additional_diagnostic) => vec![main_diagnostic, additional_diagnostic],
+            // Emit hint-level diagnostics for all `related_information` entries such as "help"s.
+            // This is useful because they will show up in the user's editor, unlike
+            // `related_information`, which just produces hard-to-read links, at least in VS Code.
+            let back_ref = lsp_types::DiagnosticRelatedInformation {
+                location,
+                message: "original diagnostic".to_string(),
+            };
+            for info in &related_information {
+                diagnostics.push(MappedRustDiagnostic {
+                    url: info.location.uri.clone(),
+                    fixes: fixes.clone(), // share fixes to make them easier to apply
+                    diagnostic: lsp_types::Diagnostic {
+                        range: info.location.range,
+                        severity: Some(lsp_types::DiagnosticSeverity::Hint),
+                        code: code.clone().map(lsp_types::NumberOrString::String),
+                        code_description: code_description.clone(),
+                        source: Some(source.clone()),
+                        message: info.message.clone(),
+                        related_information: Some(vec![back_ref.clone()]),
+                        tags: None, // don't apply modifiers again
+                        data: None,
+                    },
+                });
             }
+
+            diagnostics
         })
-        .flatten()
         .collect()
 }
 
diff --git a/crates/syntax/src/ast/make.rs b/crates/syntax/src/ast/make.rs
index 876659a2b..cc09b77a5 100644
--- a/crates/syntax/src/ast/make.rs
+++ b/crates/syntax/src/ast/make.rs
@@ -171,8 +171,17 @@ pub fn expr_return() -> ast::Expr {
 pub fn expr_match(expr: ast::Expr, match_arm_list: ast::MatchArmList) -> ast::Expr {
     expr_from_text(&format!("match {} {}", expr, match_arm_list))
 }
-pub fn expr_if(condition: ast::Condition, then_branch: ast::BlockExpr) -> ast::Expr {
-    expr_from_text(&format!("if {} {}", condition, then_branch))
+pub fn expr_if(
+    condition: ast::Condition,
+    then_branch: ast::BlockExpr,
+    else_branch: Option<ast::ElseBranch>,
+) -> ast::Expr {
+    let else_branch = match else_branch {
+        Some(ast::ElseBranch::Block(block)) => format!("else {}", block),
+        Some(ast::ElseBranch::IfExpr(if_expr)) => format!("else {}", if_expr),
+        None => String::new(),
+    };
+    expr_from_text(&format!("if {} {} {}", condition, then_branch, else_branch))
 }
 pub fn expr_prefix(op: SyntaxKind, expr: ast::Expr) -> ast::Expr {
     let token = token(op);