Handle return-position impl Trait in traits properly in register_hidden_type

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -1,6 +1,7 @@
 use crate::errors::OpaqueHiddenTypeDiag;
 use crate::infer::{DefiningAnchor, InferCtxt, InferOk};
 use crate::traits;
+use hir::def::DefKind;
 use hir::def_id::{DefId, LocalDefId};
 use hir::{HirId, OpaqueTyOrigin};
 use rustc_data_structures::sync::Lrc;
@@ -552,7 +553,12 @@ impl<'tcx> InferCtxt<'tcx> {
                 ty_op: |ty| match *ty.kind() {
                     // We can't normalize associated types from `rustc_infer`,
                     // but we can eagerly register inference variables for them.
-                    ty::Projection(projection_ty) if !projection_ty.has_escaping_bound_vars() => {
+                    // FIXME(RPITIT): Don't replace RPITITs with inference vars.
+                    ty::Projection(projection_ty)
+                        if !projection_ty.has_escaping_bound_vars()
+                            && tcx.def_kind(projection_ty.item_def_id)
+                                != DefKind::ImplTraitPlaceholder =>
+                    {
                         self.infer_projection(
                             param_env,
                             projection_ty,
@@ -568,6 +574,12 @@ impl<'tcx> InferCtxt<'tcx> {
                     {
                         hidden_ty
                     }
+                    // FIXME(RPITIT): This can go away when we move to associated types
+                    ty::Projection(proj)
+                        if def_id.to_def_id() == proj.item_def_id && substs == proj.substs =>
+                    {
+                        hidden_ty
+                    }
                     _ => ty,
                 },
                 lt_op: |lt| lt,

src/test/ui/impl-trait/in-trait/default-body-type-err-2.rs

@@ -0,0 +1,13 @@
+// edition:2021
+
+#![allow(incomplete_features)]
+#![feature(async_fn_in_trait)]
+
+pub trait Foo {
+    async fn woopsie_async(&self) -> String {
+        42
+        //~^ ERROR mismatched types
+    }
+}
+
+fn main() {}

src/test/ui/impl-trait/in-trait/default-body-type-err-2.stderr

@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/default-body-type-err-2.rs:8:9
+   |
+LL |         42
+   |         ^^- help: try using a conversion method: `.to_string()`
+   |         |
+   |         expected struct `String`, found integer
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

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

@@ -0,0 +1,13 @@
+#![allow(incomplete_features)]
+#![feature(return_position_impl_trait_in_trait)]
+
+use std::ops::Deref;
+
+pub trait Foo {
+    fn lol(&self) -> impl Deref<Target = String> {
+        //~^ type mismatch resolving `<&i32 as Deref>::Target == String`
+        &1i32
+    }
+}
+
+fn main() {}

src/test/ui/impl-trait/in-trait/default-body-type-err.stderr

@@ -0,0 +1,12 @@
+error[E0271]: type mismatch resolving `<&i32 as Deref>::Target == String`
+  --> $DIR/default-body-type-err.rs:7:22
+   |
+LL |     fn lol(&self) -> impl Deref<Target = String> {
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `i32`, found struct `String`
+LL |
+LL |         &1i32
+   |         ----- return type was inferred to be `&i32` here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0271`.

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

@@ -1,4 +1,4 @@
-// known-bug: #102688
+// check-pass
 // edition:2021
 
 #![feature(async_fn_in_trait, return_position_impl_trait_in_trait)]