error[E0277]: `&mut ()` is not a tuple
  --> $DIR/issue-57404.rs:6:41
   |
LL |     handlers.unwrap().as_mut().call_mut(&mut ());
   |                                -------- ^^^^^^^ the trait `Tuple` is not implemented for `&mut ()`
   |                                |
   |                                required by a bound introduced by this call
   |
note: required by a bound in `call_mut`
  --> $SRC_DIR/core/src/ops/function.rs:LL:COL
help: consider removing the leading `&`-reference
   |
LL -     handlers.unwrap().as_mut().call_mut(&mut ());
LL +     handlers.unwrap().as_mut().call_mut(());
   |

error: aborting due to previous error

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