error[E0277]: the size for values of type `str` cannot be known at compilation time --> $DIR/issue-50940.rs:3:5 | LL | A as fn(str) -> A; | ^ doesn't have a size known at compile-time | = help: the trait `Sized` is not implemented for `str` = note: all function arguments must have a statically known size = help: unsized fn params are gated as an unstable feature error: aborting due to previous error For more information about this error, try `rustc --explain E0277`.