Fix test for default body with impl

compiler/rustc_middle/src/ty/sty.rs

@@ -1160,6 +1160,7 @@ impl<'tcx> ProjectionTy<'tcx> {
         &self,
         tcx: TyCtxt<'tcx>,
     ) -> (ty::TraitRef<'tcx>, &'tcx [ty::GenericArg<'tcx>]) {
+        assert_eq!(tcx.def_kind(def_id), DefKind::Trait);
         let def_id = tcx.parent(self.item_def_id);
         let trait_generics = tcx.generics_of(def_id);
         (

compiler/rustc_privacy/src/lib.rs

@@ -122,8 +122,20 @@ where
         &mut self,
         projection: ty::ProjectionTy<'tcx>,
     ) -> ControlFlow<V::BreakTy> {
-        let (trait_ref, assoc_substs) =
-            projection.trait_ref_and_own_substs(self.def_id_visitor.tcx());
+        let tcx = self.def_id_visitor.tcx();
+        let (trait_ref, assoc_substs) = if tcx.def_kind(projection.item_def_id)
+            != DefKind::ImplTraitPlaceholder
+        {
+            projection.trait_ref_and_own_substs(tcx)
+        } else {
+            // HACK(RPITIT): Remove this when RPITITs are lowered to regular assoc tys
+            let def_id = tcx.impl_trait_in_trait_parent(projection.item_def_id);
+            let trait_generics = tcx.generics_of(def_id);
+            (
+                ty::TraitRef { def_id, substs: projection.substs.truncate_to(tcx, trait_generics) },
+                &projection.substs[trait_generics.count()..],
+            )
+        };
         self.visit_trait(trait_ref)?;
         if self.def_id_visitor.shallow() {
             ControlFlow::CONTINUE

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1325,10 +1325,11 @@ fn assemble_candidate_for_impl_trait_in_trait<'cx, 'tcx>(
 ) {
     let tcx = selcx.tcx();
     if tcx.def_kind(obligation.predicate.item_def_id) == DefKind::ImplTraitPlaceholder {
-        // If we are trying to project an RPITIT with the _identity_ substs,
+        // If we are trying to project an RPITIT with trait's default `Self` parameter,
         // then we must be within a default trait body.
-        if obligation.predicate.substs
+        if obligation.predicate.self_ty()
             == ty::InternalSubsts::identity_for_item(tcx, obligation.predicate.item_def_id)
+                .type_at(0)
         {
             candidate_set.push_candidate(ProjectionCandidate::ImplTraitInTrait(
                 ImplTraitInTraitCandidate::Trait,

src/test/ui/impl-trait/in-trait/default-body.rs

@@ -1,15 +1,21 @@
 // check-pass
 // edition:2021
 
-#![feature(return_position_impl_trait_in_trait)]
+#![feature(async_fn_in_trait, return_position_impl_trait_in_trait)]
 #![allow(incomplete_features)]
 
 use std::fmt::Debug;
 
 trait Foo {
-    async fn baz() -> impl Debug {
-        Self::baz().await
+    async fn baz(&self) -> &str {
+        ""
     }
 }
 
-fn main() {}
+struct Bar;
+
+impl Foo for Bar {}
+
+fn main() {
+    let _ = Bar.baz();
+}