do not use associated types placeholder for inlay hint

crates/hir_ty/src/display.rs

@@ -82,6 +82,20 @@ pub trait HirDisplay {
         };
         Ok(result)
     }
+
+    /// Returns a String representation of `self` for test purposes
+    fn display_test<'a>(&'a self, db: &'a dyn HirDatabase) -> HirDisplayWrapper<'a, Self>
+    where
+        Self: Sized,
+    {
+        HirDisplayWrapper {
+            db,
+            t: self,
+            max_size: None,
+            omit_verbose_types: false,
+            display_target: DisplayTarget::Test,
+        }
+    }
 }
 
 impl<'a> HirFormatter<'a> {
@@ -134,12 +148,17 @@ enum DisplayTarget {
     /// Display types for inserting them in source files.
     /// The generated code should compile, so paths need to be qualified.
     SourceCode { module_id: ModuleId },
+    /// Only for test purpose to keep real types
+    Test,
 }
 
 impl DisplayTarget {
     fn is_source_code(&self) -> bool {
         matches!(self, Self::SourceCode {..})
     }
+    fn is_test(&self) -> bool {
+        matches!(self, Self::Test)
+    }
 }
 
 #[derive(Debug)]
@@ -313,14 +332,18 @@ impl HirDisplay for ApplicationTy {
                     let ret_display = if f.omit_verbose_types() {
                         ret.display_truncated(f.db, f.max_size)
                     } else {
-                        ret.display(f.db)
+                        if f.display_target.is_test() {
+                            ret.display_test(f.db)
+                        } else {
+                            ret.display(f.db)
+                        }
                     };
                     write!(f, " -> {}", ret_display)?;
                 }
             }
             TypeCtor::Adt(def_id) => {
                 match f.display_target {
-                    DisplayTarget::Diagnostics => {
+                    DisplayTarget::Diagnostics | DisplayTarget::Test => {
                         let name = match def_id {
                             AdtId::StructId(it) => f.db.struct_data(it).name.clone(),
                             AdtId::UnionId(it) => f.db.union_data(it).name.clone(),
@@ -389,12 +412,23 @@ impl HirDisplay for ApplicationTy {
                     _ => panic!("not an associated type"),
                 };
                 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 {
-                    write!(f, "<")?;
-                    f.write_joined(&*self.parameters.0, ", ")?;
-                    write!(f, ">")?;
+                let type_alias_data = f.db.type_alias_data(type_alias);
+
+                // Use placeholder associated types when the target is test (https://rust-lang.github.io/chalk/book/clauses/type_equality.html#placeholder-associated-types)
+                if f.display_target.is_test() {
+                    write!(f, "{}::{}", trait_.name, type_alias_data.name)?;
+                    if self.parameters.len() > 0 {
+                        write!(f, "<")?;
+                        f.write_joined(&*self.parameters.0, ", ")?;
+                        write!(f, ">")?;
+                    }
+                } else {
+                    let projection_ty = ProjectionTy {
+                        associated_ty: type_alias,
+                        parameters: self.parameters.clone(),
+                    };
+
+                    projection_ty.hir_fmt(f)?;
                 }
             }
             TypeCtor::ForeignType(type_alias) => {
@@ -442,7 +476,11 @@ impl HirDisplay for ApplicationTy {
                     let ret_display = if f.omit_verbose_types() {
                         sig.ret().display_truncated(f.db, f.max_size)
                     } else {
-                        sig.ret().display(f.db)
+                        if f.display_target.is_test() {
+                            sig.ret().display_test(f.db)
+                        } else {
+                            sig.ret().display(f.db)
+                        }
                     };
                     write!(f, " -> {}", ret_display)?;
                 } else {

crates/hir_ty/src/tests.rs

@@ -74,7 +74,7 @@ fn check_types_impl(ra_fixture: &str, display_source: bool) {
                 let module = db.module_for_file(file_id);
                 ty.display_source_code(&db, module).unwrap()
             } else {
-                ty.display(&db).to_string()
+                ty.display_test(&db).to_string()
             };
             assert_eq!(expected, actual);
             checked_one = true;
@@ -163,7 +163,7 @@ fn infer_with_mismatches(content: &str, include_mismatches: bool) -> String {
                 macro_prefix,
                 range,
                 ellipsize(text, 15),
-                ty.display(&db)
+                ty.display_test(&db)
             );
         }
         if include_mismatches {
@@ -179,8 +179,8 @@ fn infer_with_mismatches(content: &str, include_mismatches: bool) -> String {
                     "{}{:?}: expected {}, got {}\n",
                     macro_prefix,
                     range,
-                    mismatch.expected.display(&db),
-                    mismatch.actual.display(&db),
+                    mismatch.expected.display_test(&db),
+                    mismatch.actual.display_test(&db),
                 );
             }
         }

crates/ide/src/inlay_hints.rs

@@ -1235,4 +1235,25 @@ fn main() {
 "#,
         );
     }
+
+    #[test]
+    fn infer_call_method_return_associated_types_with_generic() {
+        check(
+            r#"
+            pub trait Default {
+                fn default() -> Self;
+            }
+            pub trait Foo {
+                type Bar: Default;
+            }
+
+            pub fn quux<T: Foo>() -> T::Bar {
+                let y = Default::default();
+                  //^ <T as Foo>::Bar
+
+                y
+            }
+            "#,
+        );
+    }
 }