Confusing diagnostic for invalid casts of pointers to trait objects

[WaffleLapkin]

Code

fn main() {
    let ptr: *const dyn Fn() = &|| ();
    _ = ptr as *const dyn Fn(u8);
}

Current output

error[E0308]: mismatched types
 --> src/main.rs:3:9
  |
3 |     _ = ptr as *const dyn Fn(u8);
  |         ^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found `(u8,)`
  |
  = note: expected trait object `dyn Fn()`
             found trait object `dyn Fn(u8)`

Desired output

error[E0308]: invalid cast from `*const dyn Fn()` to `*const dyn Fn(u8)`
 --> src/main.rs:3:9
  |
3 |     _ = ptr as *const dyn Fn(u8);
  |         ^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found `(u8,)`
  |
  = note: `Fn()` and `Fn(u8)` might have different vtables, leading to unsoundness
  = note: if you are absolutely sure you are using the resulting value safely, use `transmute`

Rationale and extra context

In #120248 I made checks for casting pointers to trait objects more strict: now they need to have the same generic arguments (this is due to the fact that for the cast to be safe, we need vtables to match).

The check is however made in a bit of a crude way, which results in this "mismatched types" diagnostic.

We should modify it so that the intent of the error is clear.

Rust Version

Reproduces on both stable 1.81.0 and 1.83.0-nightly (2024-09-05 9c01301).