ices/57404.rs: fixed with errors

[github-actions]

Issue: rust-lang/rust#57404

#![feature(unboxed_closures)]
#![feature(fn_traits)]

fn main() {
    let handlers: Option<Box<dyn for<'a> FnMut<&'a mut (), Output=()>>> = None;
    handlers.unwrap().as_mut().call_mut(&mut ());
}

=== stdout ===
=== stderr ===
error[E0277]: `&mut ()` is not a tuple
 --> /home/runner/work/glacier/glacier/ices/57404.rs:6:41
  |
6 |     handlers.unwrap().as_mut().call_mut(&mut ());
  |                                -------- -^^^^^^
  |                                |        |
  |                                |        the trait `Tuple` is not implemented for `&mut ()`
  |                                |        help: consider removing the leading `&`-reference
  |                                required by a bound introduced by this call
  |
note: required by a bound in `call_mut`

error: aborting due to previous error

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