error[E0121]: the placeholder `_` is not allowed within types on item signatures for functions --> $DIR/bad-infer-in-trait-impl.rs:6:15 | LL | fn bar(s: _) {} | ^ not allowed in type signatures | help: use type parameters instead | LL | fn bar(s: T) {} | +++ ~ error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0121`.