Suggest more likely code when encountering an incorrect assoc item referencing the current trait

Author: ohadravid

src/librustc_typeck/astconv.rs

@@ -1773,11 +1773,21 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         } else {
             let path_str = tcx.def_path_str(trait_def_id);
 
+            let def_id = self.item_def_id();
+
+            debug!("qpath_to_ty: self.item_def_id()={:?}", def_id);
+
+            let parent_def_id = def_id.and_then(|def_id| tcx.hir().as_local_hir_id(def_id))
+                .map(|hir_id| tcx.hir().get_parent_did(hir_id));
+
+            debug!("qpath_to_ty: parent_def_id={:?}", parent_def_id);
+
             // If the trait in segment is the same as the trait defining the item,
             // use the `<Self as ..>` syntax in the error.
-            debug!("qpath_to_ty: self.item_def_id()={:?}",  self.item_def_id());
+            let is_part_of_self_trait_constraints = def_id == Some(trait_def_id);
+            let is_part_of_fn_in_self_trait = parent_def_id == Some(trait_def_id);
 
-            let type_name = if self.item_def_id() == Some(trait_def_id) {
+            let type_name = if is_part_of_self_trait_constraints || is_part_of_fn_in_self_trait {
                 "Self"
             } else {
                 "Type"

src/test/ui/associated-types/associated-types-in-ambiguous-context.rs

@@ -10,6 +10,9 @@ trait Grab {
     type Value;
     fn grab(&self) -> Grab::Value;
     //~^ ERROR ambiguous associated type
+
+    fn get(&self) -> Get::Value;
+    //~^ ERROR ambiguous associated type
 }
 
 trait Bar {}

src/test/ui/associated-types/associated-types-in-ambiguous-context.stderr

@@ -5,13 +5,13 @@ LL | fn get<T:Get,U:Get>(x: T, y: U) -> Get::Value {}
    |                                    ^^^^^^^^^^ help: use fully-qualified syntax: `<Type as Get>::Value`
 
 error[E0223]: ambiguous associated type
-  --> $DIR/associated-types-in-ambiguous-context.rs:17:17
+  --> $DIR/associated-types-in-ambiguous-context.rs:20:17
    |
 LL | trait Foo where Foo::Assoc: Bar {
    |                 ^^^^^^^^^^ help: use fully-qualified syntax: `<Self as Foo>::Assoc`
 
 error[E0223]: ambiguous associated type
-  --> $DIR/associated-types-in-ambiguous-context.rs:22:10
+  --> $DIR/associated-types-in-ambiguous-context.rs:25:10
    |
 LL | type X = std::ops::Deref::Target;
    |          ^^^^^^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<Type as std::ops::Deref>::Target`
@@ -20,8 +20,14 @@ error[E0223]: ambiguous associated type
   --> $DIR/associated-types-in-ambiguous-context.rs:11:23
    |
 LL |     fn grab(&self) -> Grab::Value;
-   |                       ^^^^^^^^^^^ help: use fully-qualified syntax: `<Type as Grab>::Value`
+   |                       ^^^^^^^^^^^ help: use fully-qualified syntax: `<Self as Grab>::Value`
 
-error: aborting due to 4 previous errors
+error[E0223]: ambiguous associated type
+  --> $DIR/associated-types-in-ambiguous-context.rs:14:22
+   |
+LL |     fn get(&self) -> Get::Value;
+   |                      ^^^^^^^^^^ help: use fully-qualified syntax: `<Type as Get>::Value`
+
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0223`.