warning: the feature `unsized_locals` is incomplete and may not be safe to use and/or cause compiler crashes
  --> $DIR/issue-50940-with-feature.rs:1:12
   |
LL | #![feature(unsized_locals, unsized_fn_params)]
   |            ^^^^^^^^^^^^^^
   |
   = note: see issue #48055 <https://github.com/rust-lang/rust/issues/48055> for more information
   = note: `#[warn(incomplete_features)]` on by default

error[E0277]: the size for values of type `str` cannot be known at compilation time
  --> $DIR/issue-50940-with-feature.rs:6:5
   |
LL |     A as fn(str) -> A<str>;
   |     ^ doesn't have a size known at compile-time
   |
   = help: within `A<str>`, the trait `Sized` is not implemented for `str`
note: required because it appears within the type `A<str>`
  --> $DIR/issue-50940-with-feature.rs:5:12
   |
LL |     struct A<X: ?Sized>(X);
   |            ^
   = note: the return type of a function must have a statically known size

error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0277`.