2 files changed, 34 insertions, 26 deletions

diff --git a/crates/ra_hir_ty/src/lower.rs b/crates/ra_hir_ty/src/lower.rs
index 3d2421223..39406d8ce 100644
--- a/crates/ra_hir_ty/src/lower.rs
+++ b/crates/ra_hir_ty/src/lower.rs
@@ -40,8 +40,17 @@ pub struct TyLoweringContext<'a, DB: HirDatabase> {
 
 #[derive(Clone, Debug)]
 pub enum ImplTraitLoweringMode {
+    /// `impl Trait` gets lowered into an opaque type that doesn't unify with
+    /// anything except itself. This is used in places where values flow 'out',
+    /// i.e. for arguments of the function we're currently checking, and return
+    /// types of functions we're calling.
     Opaque,
+    /// `impl Trait` gets lowered into a placeholder that can unify with some
+    /// type. This is used in places where values flow 'in', i.e. for arguments
+    /// of functions we're calling, and the return type of the function we're
+    /// currently checking.
     Placeholder,
+    /// `impl Trait` is disallowed and will be an error.
     Disallowed,
 }
 
@@ -87,12 +96,23 @@ impl Ty {
                 Ty::Dyn(predicates)
             }
             TypeRef::ImplTrait(bounds) => {
-                let self_ty = Ty::Bound(0);
-                let predicates = bounds
-                    .iter()
-                    .flat_map(|b| GenericPredicate::from_type_bound(ctx, b, self_ty.clone()))
-                    .collect();
-                Ty::Opaque(predicates)
+                match ctx.impl_trait_mode {
+                    ImplTraitLoweringMode::Opaque => {
+                        let self_ty = Ty::Bound(0);
+                        let predicates = bounds
+                            .iter()
+                            .flat_map(|b| GenericPredicate::from_type_bound(ctx, b, self_ty.clone()))
+                            .collect();
+                        Ty::Opaque(predicates)
+                    },
+                    ImplTraitLoweringMode::Placeholder => {
+                        todo!()
+                    },
+                    ImplTraitLoweringMode::Disallowed => {
+                        // FIXME: report error
+                        Ty::Unknown
+                    },
+                }
             }
             TypeRef::Error => Ty::Unknown,
         }
diff --git a/crates/ra_hir_ty/src/tests/traits.rs b/crates/ra_hir_ty/src/tests/traits.rs
index a6ac18f86..764ab2800 100644
--- a/crates/ra_hir_ty/src/tests/traits.rs
+++ b/crates/ra_hir_ty/src/tests/traits.rs
@@ -994,29 +994,17 @@ fn weird_bounds() {
     assert_snapshot!(
         infer(r#"
 trait Trait {}
-fn test() {
-    let a: impl Trait + 'lifetime = foo;
-    let b: impl 'lifetime = foo;
-    let b: impl (Trait) = foo;
-    let b: impl ('lifetime) = foo;
-    let d: impl ?Sized = foo;
-    let e: impl Trait + ?Sized = foo;
+fn test(a: impl Trait + 'lifetime, b: impl 'lifetime, c: impl (Trait), d: impl ('lifetime), e: impl ?Sized, f: impl Trait + ?Sized) {
 }
 "#),
         @r###"
-    [26; 237) '{     ...foo; }': ()
-    [36; 37) 'a': impl Trait + {error}
-    [64; 67) 'foo': impl Trait + {error}
-    [77; 78) 'b': impl {error}
-    [97; 100) 'foo': impl {error}
-    [110; 111) 'b': impl Trait
-    [128; 131) 'foo': impl Trait
-    [141; 142) 'b': impl {error}
-    [163; 166) 'foo': impl {error}
-    [176; 177) 'd': impl {error}
-    [193; 196) 'foo': impl {error}
-    [206; 207) 'e': impl Trait + {error}
-    [231; 234) 'foo': impl Trait + {error}
+    [24; 25) 'a': impl Trait + {error}
+    [51; 52) 'b': impl {error}
+    [70; 71) 'c': impl Trait
+    [87; 88) 'd': impl {error}
+    [108; 109) 'e': impl {error}
+    [124; 125) 'f': impl Trait + {error}
+    [148; 151) '{ }': ()
     "###
     );
 }