create error code E0754

Author: csmoe

src/librustc_error_codes/error_codes.rs

@@ -435,6 +435,7 @@ E0750: include_str!("./error_codes/E0750.md"),
 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"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

src/librustc_error_codes/error_codes/E0754.md

@@ -0,0 +1,29 @@
+`async fn`/`impl trait` return type cannot contain a projection or `Self` that references lifetimes from a parent scope.
+
+Erroneous code example:
+
+```compile_fail,E0754,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