9 files changed, 124 insertions, 43 deletions

diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index 05a60e158..4ee08ef21 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -51,7 +51,7 @@ use hir_def::{
 };
 use hir_expand::{diagnostics::DiagnosticSink, name::name, MacroDefKind};
 use hir_ty::{
-    autoderef,
+    autoderef, could_unify,
     method_resolution::{self, TyFingerprint},
     primitive::UintTy,
     to_assoc_type_id,
@@ -2154,6 +2154,10 @@ impl Type {
 
         walk_type(db, self, &mut cb);
     }
+
+    pub fn could_unify_with(&self, other: &Type) -> bool {
+        could_unify(&self.ty, &other.ty)
+    }
 }
 
 // FIXME: closures
diff --git a/crates/hir_ty/src/infer.rs b/crates/hir_ty/src/infer.rs
index 8f9cf7480..e4407ff50 100644
--- a/crates/hir_ty/src/infer.rs
+++ b/crates/hir_ty/src/infer.rs
@@ -45,6 +45,11 @@ use crate::{
     to_assoc_type_id, to_chalk_trait_id, AliasEq, AliasTy, Interner, TyKind,
 };
 
+// This lint has a false positive here. See the link below for details.
+//
+// https://github.com/rust-lang/rust/issues/57411
+#[allow(unreachable_pub)]
+pub use unify::could_unify;
 pub(crate) use unify::unify;
 
 mod unify;
diff --git a/crates/hir_ty/src/infer/unify.rs b/crates/hir_ty/src/infer/unify.rs
index 75250a369..6e7b0f5a6 100644
--- a/crates/hir_ty/src/infer/unify.rs
+++ b/crates/hir_ty/src/infer/unify.rs
@@ -157,6 +157,10 @@ impl<T> Canonicalized<T> {
     }
 }
 
+pub fn could_unify(t1: &Ty, t2: &Ty) -> bool {
+    InferenceTable::new().unify(t1, t2)
+}
+
 pub(crate) fn unify(tys: &Canonical<(Ty, Ty)>) -> Option<Substitution> {
     let mut table = InferenceTable::new();
     let vars = Substitution(
diff --git a/crates/hir_ty/src/lib.rs b/crates/hir_ty/src/lib.rs
index 69265286f..6f9c698e6 100644
--- a/crates/hir_ty/src/lib.rs
+++ b/crates/hir_ty/src/lib.rs
@@ -41,7 +41,7 @@ use crate::{
 };
 
 pub use autoderef::autoderef;
-pub use infer::{InferenceResult, InferenceVar};
+pub use infer::{could_unify, InferenceResult, InferenceVar};
 pub use lower::{
     associated_type_shorthand_candidates, callable_item_sig, CallableDefId, ImplTraitLoweringMode,
     TyDefId, TyLoweringContext, ValueTyDefId,
diff --git a/crates/ide_completion/src/item.rs b/crates/ide_completion/src/item.rs
index 9a4b5217a..cc4ac9ea2 100644
--- a/crates/ide_completion/src/item.rs
+++ b/crates/ide_completion/src/item.rs
@@ -122,7 +122,7 @@ impl fmt::Debug for CompletionItem {
     }
 }
 
-#[derive(Debug, Clone, Copy, Ord, PartialOrd, Eq, PartialEq, Default)]
+#[derive(Debug, Clone, Copy, Eq, PartialEq, Default)]
 pub struct CompletionRelevance {
     /// This is set in cases like these:
     ///
@@ -134,31 +134,41 @@ pub struct CompletionRelevance {
     /// }
     /// ```
     pub exact_name_match: bool,
+    /// See CompletionRelevanceTypeMatch doc comments for cases where this is set.
+    pub type_match: Option<CompletionRelevanceTypeMatch>,
     /// This is set in cases like these:
     ///
     /// ```
-    /// fn f(spam: String) {}
-    /// fn main {
-    ///     let foo = String::new();
-    ///     f($0) // type of local matches the type of param
+    /// fn foo(a: u32) {
+    ///     let b = 0;
+    ///     $0 // `a` and `b` are local
     /// }
     /// ```
-    pub exact_type_match: bool,
+    pub is_local: bool,
+}
+
+#[derive(Debug, Clone, Copy, Eq, PartialEq)]
+pub enum CompletionRelevanceTypeMatch {
     /// This is set in cases like these:
     ///
     /// ```
-    /// fn foo(bar: u32) {
-    ///     $0 // `bar` is local
+    /// enum Option<T> { Some(T), None }
+    /// fn f(a: Option<u32>) {}
+    /// fn main {
+    ///     f(Option::N$0) // type `Option<T>` could unify with `Option<u32>`
     /// }
     /// ```
+    CouldUnify,
+    /// This is set in cases like these:
     ///
     /// ```
-    /// fn foo() {
-    ///     let bar = 0;
-    ///     $0 // `bar` is local
+    /// fn f(spam: String) {}
+    /// fn main {
+    ///     let foo = String::new();
+    ///     f($0) // type of local matches the type of param
     /// }
     /// ```
-    pub is_local: bool,
+    Exact,
 }
 
 impl CompletionRelevance {
@@ -177,9 +187,11 @@ impl CompletionRelevance {
         if self.exact_name_match {
             score += 1;
         }
-        if self.exact_type_match {
-            score += 3;
-        }
+        score += match self.type_match {
+            Some(CompletionRelevanceTypeMatch::Exact) => 4,
+            Some(CompletionRelevanceTypeMatch::CouldUnify) => 3,
+            None => 0,
+        };
         if self.is_local {
             score += 1;
         }
@@ -342,7 +354,7 @@ impl CompletionItem {
         // match, but with exact type match set because self.ref_match
         // is only set if there is an exact type match.
         let mut relevance = self.relevance;
-        relevance.exact_type_match = true;
+        relevance.type_match = Some(CompletionRelevanceTypeMatch::Exact);
 
         self.ref_match.map(|mutability| (mutability, relevance))
     }
@@ -523,7 +535,7 @@ mod tests {
     use itertools::Itertools;
     use test_utils::assert_eq_text;
 
-    use super::CompletionRelevance;
+    use super::{CompletionRelevance, CompletionRelevanceTypeMatch};
 
     /// Check that these are CompletionRelevance are sorted in ascending order
     /// by their relevance score.
@@ -576,15 +588,22 @@ mod tests {
                 is_local: true,
                 ..CompletionRelevance::default()
             }],
-            vec![CompletionRelevance { exact_type_match: true, ..CompletionRelevance::default() }],
+            vec![CompletionRelevance {
+                type_match: Some(CompletionRelevanceTypeMatch::CouldUnify),
+                ..CompletionRelevance::default()
+            }],
+            vec![CompletionRelevance {
+                type_match: Some(CompletionRelevanceTypeMatch::Exact),
+                ..CompletionRelevance::default()
+            }],
             vec![CompletionRelevance {
                 exact_name_match: true,
-                exact_type_match: true,
+                type_match: Some(CompletionRelevanceTypeMatch::Exact),
                 ..CompletionRelevance::default()
             }],
             vec![CompletionRelevance {
                 exact_name_match: true,
-                exact_type_match: true,
+                type_match: Some(CompletionRelevanceTypeMatch::Exact),
                 is_local: true,
             }],
         ];
diff --git a/crates/ide_completion/src/render.rs b/crates/ide_completion/src/render.rs
index 9ce49074f..1a762d3dc 100644
--- a/crates/ide_completion/src/render.rs
+++ b/crates/ide_completion/src/render.rs
@@ -20,8 +20,8 @@ use ide_db::{
 use syntax::TextRange;
 
 use crate::{
-    item::ImportEdit, CompletionContext, CompletionItem, CompletionItemKind, CompletionKind,
-    CompletionRelevance,
+    item::{CompletionRelevanceTypeMatch, ImportEdit},
+    CompletionContext, CompletionItem, CompletionItemKind, CompletionKind, CompletionRelevance,
 };
 
 use crate::render::{enum_variant::render_variant, function::render_fn, macro_::render_macro};
@@ -143,7 +143,7 @@ impl<'a> Render<'a> {
             .set_deprecated(is_deprecated);
 
         item.set_relevance(CompletionRelevance {
-            exact_type_match: compute_exact_type_match(self.ctx.completion, ty),
+            type_match: compute_type_match(self.ctx.completion, ty),
             exact_name_match: compute_exact_name_match(self.ctx.completion, name.to_string()),
             ..CompletionRelevance::default()
         });
@@ -245,7 +245,7 @@ impl<'a> Render<'a> {
             }
 
             item.set_relevance(CompletionRelevance {
-                exact_type_match: compute_exact_type_match(self.ctx.completion, &ty),
+                type_match: compute_type_match(self.ctx.completion, &ty),
                 exact_name_match: compute_exact_name_match(self.ctx.completion, &local_name),
                 is_local: true,
                 ..CompletionRelevance::default()
@@ -309,14 +309,24 @@ impl<'a> Render<'a> {
     }
 }
 
-fn compute_exact_type_match(ctx: &CompletionContext, completion_ty: &hir::Type) -> bool {
-    match ctx.expected_type.as_ref() {
-        Some(expected_type) => {
-            // We don't ever consider unit type to be an exact type match, since
-            // nearly always this is not meaningful to the user.
-            completion_ty == expected_type && !expected_type.is_unit()
-        }
-        None => false,
+fn compute_type_match(
+    ctx: &CompletionContext,
+    completion_ty: &hir::Type,
+) -> Option<CompletionRelevanceTypeMatch> {
+    let expected_type = ctx.expected_type.as_ref()?;
+
+    // We don't ever consider unit type to be an exact type match, since
+    // nearly always this is not meaningful to the user.
+    if expected_type.is_unit() {
+        return None;
+    }
+
+    if completion_ty == expected_type {
+        Some(CompletionRelevanceTypeMatch::Exact)
+    } else if expected_type.could_unify_with(completion_ty) {
+        Some(CompletionRelevanceTypeMatch::CouldUnify)
+    } else {
+        None
     }
 }
 
@@ -348,6 +358,7 @@ mod tests {
     use itertools::Itertools;
 
     use crate::{
+        item::CompletionRelevanceTypeMatch,
         test_utils::{check_edit, do_completion, get_all_items, TEST_CONFIG},
         CompletionKind, CompletionRelevance,
     };
@@ -360,7 +371,11 @@ mod tests {
     fn check_relevance(ra_fixture: &str, expect: Expect) {
         fn display_relevance(relevance: CompletionRelevance) -> String {
             let relevance_factors = vec![
-                (relevance.exact_type_match, "type"),
+                (relevance.type_match == Some(CompletionRelevanceTypeMatch::Exact), "type"),
+                (
+                    relevance.type_match == Some(CompletionRelevanceTypeMatch::CouldUnify),
+                    "type_could_unify",
+                ),
                 (relevance.exact_name_match, "name"),
                 (relevance.is_local, "local"),
             ]
@@ -533,7 +548,9 @@ fn main() { let _: m::Spam = S$0 }
                         detail: "(i32)",
                         relevance: CompletionRelevance {
                             exact_name_match: false,
-                            exact_type_match: true,
+                            type_match: Some(
+                                Exact,
+                            ),
                             is_local: false,
                         },
                         trigger_call_info: true,
@@ -559,7 +576,9 @@ fn main() { let _: m::Spam = S$0 }
                         detail: "()",
                         relevance: CompletionRelevance {
                             exact_name_match: false,
-                            exact_type_match: true,
+                            type_match: Some(
+                                Exact,
+                            ),
                             is_local: false,
                         },
                     },
@@ -1108,7 +1127,7 @@ fn main() {
                         detail: "S",
                         relevance: CompletionRelevance {
                             exact_name_match: true,
-                            exact_type_match: false,
+                            type_match: None,
                             is_local: true,
                         },
                         ref_match: "&mut ",
@@ -1353,4 +1372,34 @@ fn foo(f: Foo) { let _: &u32 = f.b$0 }
             "#]],
         );
     }
+
+    #[test]
+    fn generic_enum() {
+        check_relevance(
+            r#"
+enum Foo<T> { A(T), B }
+// bar() should not be an exact type match
+// because the generic parameters are different
+fn bar() -> Foo<u8> { Foo::B }
+// FIXME baz() should be an exact type match
+// because the types could unify, but it currently
+// is not. This is due to the T here being
+// TyKind::Placeholder rather than TyKind::Missing.
+fn baz<T>() -> Foo<T> { Foo::B }
+fn foo() {
+    let foo: Foo<u32> = Foo::B;
+    let _: Foo<u32> = f$0;
+}
+"#,
+            expect![[r#"
+                ev Foo::A(…) [type_could_unify]
+                ev Foo::B [type_could_unify]
+                lc foo [type+local]
+                en Foo []
+                fn baz() []
+                fn bar() []
+                fn foo() []
+            "#]],
+        );
+    }
 }
diff --git a/crates/ide_completion/src/render/enum_variant.rs b/crates/ide_completion/src/render/enum_variant.rs
index 374247b05..832f5ced1 100644
--- a/crates/ide_completion/src/render/enum_variant.rs
+++ b/crates/ide_completion/src/render/enum_variant.rs
@@ -6,7 +6,7 @@ use itertools::Itertools;
 
 use crate::{
     item::{CompletionItem, CompletionKind, ImportEdit},
-    render::{builder_ext::Params, compute_exact_type_match, compute_ref_match, RenderContext},
+    render::{builder_ext::Params, compute_ref_match, compute_type_match, RenderContext},
     CompletionRelevance,
 };
 
@@ -77,7 +77,7 @@ impl<'a> EnumRender<'a> {
 
         let ty = self.variant.parent_enum(self.ctx.completion.db).ty(self.ctx.completion.db);
         item.set_relevance(CompletionRelevance {
-            exact_type_match: compute_exact_type_match(self.ctx.completion, &ty),
+            type_match: compute_type_match(self.ctx.completion, &ty),
             ..CompletionRelevance::default()
         });
 
diff --git a/crates/ide_completion/src/render/function.rs b/crates/ide_completion/src/render/function.rs
index b1eba20e8..d681e2c91 100644
--- a/crates/ide_completion/src/render/function.rs
+++ b/crates/ide_completion/src/render/function.rs
@@ -8,7 +8,7 @@ use syntax::ast::Fn;
 use crate::{
     item::{CompletionItem, CompletionItemKind, CompletionKind, CompletionRelevance, ImportEdit},
     render::{
-        builder_ext::Params, compute_exact_name_match, compute_exact_type_match, compute_ref_match,
+        builder_ext::Params, compute_exact_name_match, compute_ref_match, compute_type_match,
         RenderContext,
     },
 };
@@ -73,7 +73,7 @@ impl<'a> FunctionRender<'a> {
 
         let ret_type = self.func.ret_type(self.ctx.db());
         item.set_relevance(CompletionRelevance {
-            exact_type_match: compute_exact_type_match(self.ctx.completion, &ret_type),
+            type_match: compute_type_match(self.ctx.completion, &ret_type),
             exact_name_match: compute_exact_name_match(self.ctx.completion, self.name.clone()),
             ..CompletionRelevance::default()
         });
diff --git a/crates/rust-analyzer/src/to_proto.rs b/crates/rust-analyzer/src/to_proto.rs
index 25169005f..530c8a5a4 100644
--- a/crates/rust-analyzer/src/to_proto.rs
+++ b/crates/rust-analyzer/src/to_proto.rs
@@ -1138,7 +1138,7 @@ mod tests {
                 (
                     "&arg",
                     Some(
-                        "fffffffa",
+                        "fffffff9",
                     ),
                 ),
                 (