use the disambiguated idx from last impl trait

compiler/rustc_ty_utils/src/assoc.rs

@@ -177,17 +177,6 @@ impl<'tcx> Visitor<'tcx> for RPITVisitor<'tcx> {
     }
 }
 
-struct DisambiguatorIdxVisitor {
-    depth: u32,
-}
-
-impl<'tcx> Visitor<'tcx> for DisambiguatorIdxVisitor {
-    fn visit_opaque_ty(&mut self, opaque: &'tcx hir::OpaqueTy<'tcx>) -> Self::Result {
-        self.depth += 1;
-        intravisit::walk_opaque_ty(self, opaque)
-    }
-}
-
 /// Given an `fn_def_id` of a trait or a trait implementation:
 ///
 /// if `fn_def_id` is a function defined inside a trait, then it synthesizes
@@ -221,18 +210,25 @@ fn associated_types_for_impl_traits_in_associated_fn(
                 // ```
                 let disambiguator_idx = trait_item_refs
                     .iter()
-                    .take_while(|item| item.id.owner_id.def_id != fn_def_id)
-                    .filter(|item| {
-                        matches!(item.kind, hir::AssocItemKind::Fn { .. })
+                    .rev()
+                    .skip_while(|item| item.id.owner_id.def_id != fn_def_id)
+                    .skip(1)
+                    .find_map(|item| {
+                        if matches!(item.kind, hir::AssocItemKind::Fn { .. })
                             && def_path_id(item.id.owner_id.def_id) == def_path_data
-                    })
-                    .last()
-                    .map(|item| {
-                        let output = tcx.hir_get_fn_output(item.id.owner_id.def_id).unwrap();
-                        let mut visitor = DisambiguatorIdxVisitor { depth: 0 };
-                        visitor.visit_fn_ret_ty(output);
-                        tcx.def_key(item.id.owner_id.def_id).disambiguated_data.disambiguator
-                            + visitor.depth
+                            && let Some(last_impl_trait_id) = tcx
+                                .associated_types_for_impl_traits_in_associated_fn(
+                                    item.id.owner_id.def_id,
+                                )
+                                .last()
+                        {
+                            Some(
+                                tcx.def_key(last_impl_trait_id).disambiguated_data.disambiguator
+                                    + 1,
+                            )
+                        } else {
+                            None
+                        }
                     })
                     .unwrap_or_default();
 

tests/ui/impl-trait/in-trait/rpitit-duplicate-associated-fn-with-nested.rs

@@ -43,4 +43,17 @@ trait Qux {
     //~^^^^ ERROR: the name `foo` is defined multiple times
 }
 
+trait T0<T> {
+    type Target;
+}
+trait T1<T> {}
+
+trait X {
+    fn a() -> impl T0<(), Target = impl T1<()>>;
+    fn a() -> impl T0<(), Target = impl T1<()>>;
+    //~^ ERROR the name `a` is defined multiple times
+    fn a() -> impl T0<(), Target = impl T1<()>>;
+    //~^ ERROR the name `a` is defined multiple times
+}
+
 fn main() {}

tests/ui/impl-trait/in-trait/rpitit-duplicate-associated-fn-with-nested.stderr

@@ -44,6 +44,27 @@ LL | |         >;
    |
    = note: `foo` must be defined only once in the value namespace of this trait
 
-error: aborting due to 4 previous errors
+error[E0428]: the name `a` is defined multiple times
+  --> $DIR/rpitit-duplicate-associated-fn-with-nested.rs:53:5
+   |
+LL |     fn a() -> impl T0<(), Target = impl T1<()>>;
+   |     -------------------------------------------- previous definition of the value `a` here
+LL |     fn a() -> impl T0<(), Target = impl T1<()>>;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `a` redefined here
+   |
+   = note: `a` must be defined only once in the value namespace of this trait
+
+error[E0428]: the name `a` is defined multiple times
+  --> $DIR/rpitit-duplicate-associated-fn-with-nested.rs:55:5
+   |
+LL |     fn a() -> impl T0<(), Target = impl T1<()>>;
+   |     -------------------------------------------- previous definition of the value `a` here
+...
+LL |     fn a() -> impl T0<(), Target = impl T1<()>>;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `a` redefined here
+   |
+   = note: `a` must be defined only once in the value namespace of this trait
+
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0428`.