fn_to_numeric_cast incorrect suggestion

[phansch]

Given the following code:

#![warn(clippy::fn_to_numeric_cast)]

fn fn_with_fn_args(f: fn(i32) -> i32) -> i32 {
    f as i32
}

Clippy gives the following suggestion:

error: casting function pointer `f` to `i32`, which truncates the value
  --> $DIR/fn_to_numeric_cast.rs:53:5
   |
LL |     f as i32
   |     ^^^^^^^^ help: try: `f as usize`

Applying this, results in a compilation error:

error[E0308]: mismatched types
  --> tests/ui/fn_to_numeric_cast.fixed:53:5
   |
52 | fn fn_with_fn_args(f: fn(i32) -> i32) -> i32 {
   |                                          --- expected `i32` because of return type
53 |     f as usize
   |     ^^^^^^^^^^ expected i32, found usize

I'm not really sure if Clippy can suggest to change the return type of the function as well. Or maybe Clippy should mark this suggestion as MaybeIncorrect if the cast is the return value of the function?

(Split off from #3630)

edit: Also make sure this is fixed for fn_to_numeric_cast_with_truncation

I think MaybeIncorrect is the way to go. Even if you fixed the signature to return usize, you'd now have to fix all the places where the function is used which isn't possible.