Rollup merge of #72260 - csmoe:issue-69276, r=estebank

Spell out `Self` in async function return

Closes #69276
r? @tmandry

src/librustc_error_codes/error_codes.rs

@@ -437,6 +437,7 @@ E0751: include_str!("./error_codes/E0751.md"),
 E0752: include_str!("./error_codes/E0752.md"),
 E0753: include_str!("./error_codes/E0753.md"),
 E0754: include_str!("./error_codes/E0754.md"),
+E0760: include_str!("./error_codes/E0760.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

src/librustc_error_codes/error_codes/E0760.md

@@ -0,0 +1,32 @@
+`async fn`/`impl trait` return type cannot contain a projection
+or `Self` that references lifetimes from a parent scope.
+
+Erroneous code example:
+
+```compile_fail,E0760,edition2018
+struct S<'a>(&'a i32);
+
+impl<'a> S<'a> {
+    async fn new(i: &'a i32) -> Self {
+        S(&22)
+    }
+}
+```
+
+To fix this error we need to spell out `Self` to `S<'a>`:
+
+```edition2018
+struct S<'a>(&'a i32);
+
+impl<'a> S<'a> {
+    async fn new(i: &'a i32) -> S<'a> {
+        S(&22)
+    }
+}
+```
+
+This will be allowed at some point in the future,
+but the implementation is not yet complete.
+See the [issue-61949] for this limitation.
+
+[issue-61949]: https://github.com/rust-lang/rust/issues/61949