Calculate ProjectionTy::trait_def_id for return-position impl Trait in trait correctly

compiler/rustc_middle/src/ty/sty.rs

@@ -1133,9 +1133,13 @@ pub struct ProjectionTy<'tcx> {
 
 impl<'tcx> ProjectionTy<'tcx> {
     pub fn trait_def_id(&self, tcx: TyCtxt<'tcx>) -> DefId {
-        let parent = tcx.parent(self.item_def_id);
-        assert_eq!(tcx.def_kind(parent), DefKind::Trait);
-        parent
+        match tcx.def_kind(self.item_def_id) {
+            DefKind::AssocTy | DefKind::AssocConst => tcx.parent(self.item_def_id),
+            DefKind::ImplTraitPlaceholder => {
+                tcx.parent(tcx.impl_trait_in_trait_parent(self.item_def_id))
+            }
+            kind => bug!("unexpected DefKind in ProjectionTy: {kind:?}"),
+        }
     }
 
     /// Extracts the underlying trait reference and own substs from this projection.

src/test/ui/impl-trait/in-trait/issue-102140.rs

@@ -0,0 +1,30 @@
+#![feature(return_position_impl_trait_in_trait)]
+#![allow(incomplete_features)]
+
+trait Marker {}
+impl Marker for u32 {}
+
+trait MyTrait {
+    fn foo(&self) -> impl Marker
+    where
+        Self: Sized;
+}
+
+struct Outer;
+
+impl MyTrait for Outer {
+    fn foo(&self) -> impl Marker {
+        42
+    }
+}
+
+impl dyn MyTrait {
+    fn other(&self) -> impl Marker {
+        MyTrait::foo(&self)
+        //~^ ERROR the trait bound `&dyn MyTrait: MyTrait` is not satisfied
+        //~| ERROR the trait bound `&dyn MyTrait: MyTrait` is not satisfied
+        //~| ERROR the trait bound `&dyn MyTrait: MyTrait` is not satisfied
+    }
+}
+
+fn main() {}

src/test/ui/impl-trait/in-trait/issue-102140.stderr

@@ -0,0 +1,29 @@
+error[E0277]: the trait bound `&dyn MyTrait: MyTrait` is not satisfied
+  --> $DIR/issue-102140.rs:23:22
+   |
+LL |         MyTrait::foo(&self)
+   |         ------------ -^^^^
+   |         |            |
+   |         |            the trait `MyTrait` is not implemented for `&dyn MyTrait`
+   |         |            help: consider removing the leading `&`-reference
+   |         required by a bound introduced by this call
+
+error[E0277]: the trait bound `&dyn MyTrait: MyTrait` is not satisfied
+  --> $DIR/issue-102140.rs:23:9
+   |
+LL |         MyTrait::foo(&self)
+   |         ^^^^^^^^^^^^^^^^^^^ the trait `MyTrait` is not implemented for `&dyn MyTrait`
+   |
+   = help: the trait `MyTrait` is implemented for `Outer`
+
+error[E0277]: the trait bound `&dyn MyTrait: MyTrait` is not satisfied
+  --> $DIR/issue-102140.rs:23:9
+   |
+LL |         MyTrait::foo(&self)
+   |         ^^^^^^^^^^^^ the trait `MyTrait` is not implemented for `&dyn MyTrait`
+   |
+   = help: the trait `MyTrait` is implemented for `Outer`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.