Rollup merge of #103355 - compiler-errors:rpitit-default-check, r=oli-obk

Handle return-position `impl Trait` in traits properly in `register_hidden_type`

The bounds that we get by calling `bound_explicit_item_bounds` from an RPITIT have projections, not opaques, but when we're *registering* an opaque, we want to treat it like an opaque.

Coincidentally fixes #102688 as well, which makes sense, since that was failing because we were inferring an opaque type to be equal to itself (opaque cycle error => "cannot resolve opaque type").

Fixes #103352

r? ```@oli-obk```

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)]

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

@@ -1,12 +0,0 @@
-error[E0720]: cannot resolve opaque type
-  --> $DIR/default-body-with-rpit.rs:10:28
-   |
-LL |     async fn baz(&self) -> impl Debug {
-   |                            ^^^^^^^^^^ cannot resolve opaque type
-   |
-   = note: these returned values have a concrete "never" type
-   = help: this error will resolve once the item's body returns a concrete type
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0720`.