error[E0631]: type mismatch in closure arguments
  --> $DIR/unboxed-closures-vtable-mismatch.rs:16:24
   |
LL |     let f = to_fn_mut(|x: usize, y: isize| -> isize { (x as isize) + y });
   |                       ----------------------------- found signature defined here
LL |
LL |     let z = call_it(3, f);
   |             -------    ^ expected due to this
   |             |
   |             required by a bound introduced by this call
   |
   = note: expected closure signature `fn(isize, _) -> _`
              found closure signature `fn(usize, _) -> _`
note: required by a bound in `call_it`
  --> $DIR/unboxed-closures-vtable-mismatch.rs:7:15
   |
LL | fn call_it<F: FnMut(isize, isize) -> isize>(y: isize, mut f: F) -> isize {
   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `call_it`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0631`.