do not use associated types placeholder for inlay hint #6191

Signed-off-by: Benjamin Coenen <5719034+bnjjj@users.noreply.github.com>

crates/hir_ty/src/display.rs

@@ -390,11 +390,27 @@ impl HirDisplay for ApplicationTy {
                 };
                 let trait_ = f.db.trait_data(trait_);
                 let type_alias = f.db.type_alias_data(type_alias);
-                write!(f, "{}::{}", trait_.name, type_alias.name)?;
+
-                if self.parameters.len() > 0 {
+                // Use placeholder associated types when the target is source code (https://rust-lang.github.io/chalk/book/clauses/type_equality.html#placeholder-associated-types)
-                    write!(f, "<")?;
+                if f.display_target.is_source_code() || self.parameters.len() > 1 {
-                    f.write_joined(&*self.parameters.0, ", ")?;
+                    write!(f, "{}::{}", trait_.name, type_alias.name)?;
-                    write!(f, ">")?;
+                    if self.parameters.len() > 0 {
+                        write!(f, "<")?;
+                        f.write_joined(&*self.parameters.0, ", ")?;
+                        write!(f, ">")?;
+                    }
+                } else {
+                    if self.parameters.len() == 1 {
+                        write!(
+                            f,
+                            "<{} as {}>::{}",
+                            self.parameters.as_single().display(f.db),
+                            trait_.name,
+                            type_alias.name
+                        )?;
+                    } else {
+                        write!(f, "{}::{}", trait_.name, type_alias.name)?;
+                    }
                 }
             }
             TypeCtor::ForeignType(type_alias) => {

crates/hir_ty/src/tests/regression.rs

@@ -831,11 +831,11 @@ fn issue_4966() {
             356..362 'repeat': Repeat<Map<|&f64| -> f64>>
             365..390 'Repeat...nner }': Repeat<Map<|&f64| -> f64>>
             383..388 'inner': Map<|&f64| -> f64>
-            401..404 'vec': Vec<IntoIterator::Item<Repeat<Map<|&f64| -> f64>>>>
+            401..404 'vec': Vec<<Repeat<Map<|&f64| -> f64>> as IntoIterator>::Item>
-            407..416 'from_iter': fn from_iter<IntoIterator::Item<Repeat<Map<|&f64| -> f64>>>, Repeat<Map<|&f64| -> f64>>>(Repeat<Map<|&f64| -> f64>>) -> Vec<IntoIterator::Item<Repeat<Map<|&f64| -> f64>>>>
+            407..416 'from_iter': fn from_iter<<Repeat<Map<|&f64| -> f64>> as IntoIterator>::Item, Repeat<Map<|&f64| -> f64>>>(Repeat<Map<|&f64| -> f64>>) -> Vec<<Repeat<Map<|&f64| -> f64>> as IntoIterator>::Item>
-            407..424 'from_i...epeat)': Vec<IntoIterator::Item<Repeat<Map<|&f64| -> f64>>>>
+            407..424 'from_i...epeat)': Vec<<Repeat<Map<|&f64| -> f64>> as IntoIterator>::Item>
             417..423 'repeat': Repeat<Map<|&f64| -> f64>>
-            431..434 'vec': Vec<IntoIterator::Item<Repeat<Map<|&f64| -> f64>>>>
+            431..434 'vec': Vec<<Repeat<Map<|&f64| -> f64>> as IntoIterator>::Item>
             431..444 'vec.foo_bar()': {unknown}
         "#]],
     );

crates/hir_ty/src/tests/traits.rs

@@ -384,12 +384,12 @@ fn infer_project_associated_type() {
             108..261 '{     ...ter; }': ()
             118..119 'x': u32
             145..146 '1': u32
-            156..157 'y': Iterable::Item<T>
+            156..157 'y': <T as Iterable>::Item
-            183..192 'no_matter': Iterable::Item<T>
+            183..192 'no_matter': <T as Iterable>::Item
-            202..203 'z': Iterable::Item<T>
+            202..203 'z': <T as Iterable>::Item
-            215..224 'no_matter': Iterable::Item<T>
+            215..224 'no_matter': <T as Iterable>::Item
-            234..235 'a': Iterable::Item<T>
+            234..235 'a': <T as Iterable>::Item
-            249..258 'no_matter': Iterable::Item<T>
+            249..258 'no_matter': <T as Iterable>::Item
         "#]],
     );
 }
@@ -908,7 +908,6 @@ fn test<T: Trait>(t: T) { (*t); }
 
 #[test]
 fn associated_type_placeholder() {
-    // inside the generic function, the associated type gets normalized to a placeholder `ApplL::Out<T>` [https://rust-lang.github.io/rustc-guide/traits/associated-types.html#placeholder-associated-types].
     check_types(
         r#"
 pub trait ApplyL {
@@ -924,7 +923,7 @@ impl<T> ApplyL for RefMutL<T> {
 fn test<T: ApplyL>() {
     let y: <RefMutL<T> as ApplyL>::Out = no_matter;
     y;
-} //^ ApplyL::Out<T>
+} //^ <T as ApplyL>::Out
 "#,
     );
 }
@@ -941,7 +940,7 @@ fn foo<T: ApplyL>(t: T) -> <T as ApplyL>::Out;
 fn test<T: ApplyL>(t: T) {
     let y = foo(t);
     y;
-} //^ ApplyL::Out<T>
+} //^ <T as ApplyL>::Out
 "#,
     );
 }
@@ -2120,7 +2119,7 @@ fn unselected_projection_on_impl_self() {
         "#,
         expect![[r#"
             40..44 'self': &Self
-            46..47 'x': Trait::Item<Self>
+            46..47 'x': <Self as Trait>::Item
             126..130 'self': &S
             132..133 'x': u32
             147..161 '{ let y = x; }': ()
@@ -3151,3 +3150,30 @@ fn test() {
     "#,
     );
 }
+
+#[test]
+fn infer_call_method_return_associated_types_with_generic() {
+    check_infer(
+        r#"
+        pub trait Default {
+            fn default() -> Self;
+        }
+        pub trait Foo {
+            type Bar: Default;
+        }
+
+        pub fn quux<T: Foo>() -> T::Bar {
+            let y = Default::default();
+
+            y
+        }
+        "#,
+        expect![[r#"
+            122..164 '{     ...   y }': <T as Foo>::Bar
+            132..133 'y': <T as Foo>::Bar
+            136..152 'Defaul...efault': fn default<<T as Foo>::Bar>() -> <T as Foo>::Bar
+            136..154 'Defaul...ault()': <T as Foo>::Bar
+            161..162 'y': <T as Foo>::Bar
+        "#]],
+    );
+}