| // Test that we syntactically reject *equality predicates*. |
| // |
| // It's a feature that was originally proposed as part of accepted RFC 135 (2014). It's never been |
| // fully implemented and in the meantime design & implementation concerns have been raised. |
| // Between Feb 2017 and Jun 2026 we accidentally accepted such predicates syntactically |
| // (indeed, without any pre-expansion feature gate). |
| // |
| // We *might* add a more restricted version of this feature to the language in the future. |
| // See discussions in tracking issue <https://github.com/rust-lang/rust/issues/20041> for details. |
| |
| #[cfg(false)] |
| fn f<T: Iterator>(mut xs: T) -> Option<u8> |
| where |
| <T as Iterator>::Item == u8 |
| //~^ ERROR general type equality constraints are not supported |
| { |
| xs.next() |
| } |
| |
| fn main() {} |
