Rollup merge of #115624 - compiler-errors:rtn-path, r=WaffleLapkin

Print the path of a return-position impl trait in trait when `return_type_notation` is enabled

When we're printing a return-position impl trait in trait, we usually just print it like an opaque. This is *usually* fine, but can be confusing when using `return_type_notation`. Print the path of the method from where the RPITIT originates when this feature gate is enabled.

5 files changed, 75 insertions, 2 deletions

diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index ac0c88468fa..7191ed7a0f6 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -1123,6 +1123,17 @@ pub trait PrettyPrinter<'tcx>:
             }
         }
 
+        if self.tcx().features().return_type_notation
+            && let Some(ty::ImplTraitInTraitData::Trait { fn_def_id, .. }) = self.tcx().opt_rpitit_info(def_id)
+            && let ty::Alias(_, alias_ty) = self.tcx().fn_sig(fn_def_id).skip_binder().output().skip_binder().kind()
+            && alias_ty.def_id == def_id
+        {
+            let num_args = self.tcx().generics_of(fn_def_id).count();
+            write!(self, " {{ ")?;
+            self = self.print_def_path(fn_def_id, &args[..num_args])?;
+            write!(self, "() }}")?;
+        }
+
         Ok(self)
     }
 
diff --git a/tests/ui/associated-type-bounds/return-type-notation/basic.without.stderr b/tests/ui/associated-type-bounds/return-type-notation/basic.without.stderr
index c2da4f57696..edce1045e24 100644
--- a/tests/ui/associated-type-bounds/return-type-notation/basic.without.stderr
+++ b/tests/ui/associated-type-bounds/return-type-notation/basic.without.stderr
@@ -13,12 +13,12 @@ error: future cannot be sent between threads safely
 LL |     is_send(foo::<T>());
    |             ^^^^^^^^^^ future returned by `foo` is not `Send`
    |
-   = help: within `impl Future<Output = Result<(), ()>>`, the trait `Send` is not implemented for `impl Future<Output = Result<(), ()>>`
+   = help: within `impl Future<Output = Result<(), ()>>`, the trait `Send` is not implemented for `impl Future<Output = Result<(), ()>> { <T as Foo>::method() }`
 note: future is not `Send` as it awaits another future which is not `Send`
   --> $DIR/basic.rs:13:5
    |
 LL |     T::method().await?;
-   |     ^^^^^^^^^^^ await occurs here on type `impl Future<Output = Result<(), ()>>`, which is not `Send`
+   |     ^^^^^^^^^^^ await occurs here on type `impl Future<Output = Result<(), ()>> { <T as Foo>::method() }`, which is not `Send`
 note: required by a bound in `is_send`
   --> $DIR/basic.rs:17:20
    |
diff --git a/tests/ui/async-await/return-type-notation/normalizing-self-auto-trait-issue-109924.current.stderr b/tests/ui/async-await/return-type-notation/normalizing-self-auto-trait-issue-109924.current.stderr
new file mode 100644
index 00000000000..3b63ec45804
--- /dev/null
+++ b/tests/ui/async-await/return-type-notation/normalizing-self-auto-trait-issue-109924.current.stderr
@@ -0,0 +1,27 @@
+warning: the feature `return_type_notation` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/normalizing-self-auto-trait-issue-109924.rs:8:12
+   |
+LL | #![feature(return_type_notation)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0277]: `impl Future<Output = ()> { <_ as Foo>::bar() }` cannot be sent between threads safely
+  --> $DIR/normalizing-self-auto-trait-issue-109924.rs:23:11
+   |
+LL |     build(Bar);
+   |     ----- ^^^ `impl Future<Output = ()> { <_ as Foo>::bar() }` cannot be sent between threads safely
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `for<'a> Send` is not implemented for `impl Future<Output = ()> { <_ as Foo>::bar() }`
+note: required by a bound in `build`
+  --> $DIR/normalizing-self-auto-trait-issue-109924.rs:20:39
+   |
+LL | fn build<T>(_: T) where T: Foo<bar(): Send> {}
+   |                                       ^^^^ required by this bound in `build`
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/async-await/return-type-notation/normalizing-self-auto-trait-issue-109924.next.stderr b/tests/ui/async-await/return-type-notation/normalizing-self-auto-trait-issue-109924.next.stderr
new file mode 100644
index 00000000000..6fab7178767
--- /dev/null
+++ b/tests/ui/async-await/return-type-notation/normalizing-self-auto-trait-issue-109924.next.stderr
@@ -0,0 +1,11 @@
+warning: the feature `return_type_notation` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/normalizing-self-auto-trait-issue-109924.rs:8:12
+   |
+LL | #![feature(return_type_notation)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+
diff --git a/tests/ui/async-await/return-type-notation/normalizing-self-auto-trait-issue-109924.rs b/tests/ui/async-await/return-type-notation/normalizing-self-auto-trait-issue-109924.rs
new file mode 100644
index 00000000000..b2cd9707db9
--- /dev/null
+++ b/tests/ui/async-await/return-type-notation/normalizing-self-auto-trait-issue-109924.rs
@@ -0,0 +1,24 @@
+// revisions: current next
+//[current] known-bug: #109924
+//[next] check-pass
+//[next] compile-flags: -Ztrait-solver=next
+// edition:2021
+
+#![feature(async_fn_in_trait)]
+#![feature(return_type_notation)]
+//[next]~^ WARN the feature `return_type_notation` is incomplete
+
+trait Foo {
+    async fn bar(&self);
+}
+
+struct Bar;
+impl Foo for Bar {
+    async fn bar(&self) {}
+}
+
+fn build<T>(_: T) where T: Foo<bar(): Send> {}
+
+fn main() {
+    build(Bar);
+}