Insufficient diagnostic info when casting usize to fat pointer

[Noratrieb]

Given the following code: playground

fn main() {
    maybe_sized_t::<()>();
}

fn maybe_sized_t<T: ?Sized>() {
    let _ptr = 0 as *mut T; 
}

The current output is:

error[E0606]: casting `usize` as `*mut T` is invalid
 --> src/main.rs:6:16
  |
6 |     let _ptr = 0 as *mut T; 
  |                ^^^^^^^^^^^

For more information about this error, try `rustc --explain E0606`.
error: could not compile `playground` due to previous error

error[E0606]: casting `usize` as `*mut T` is invalid
 --> src/main.rs:6:16
  |
6 |     let _ptr = 0 as *mut T; 
  |                ^^^^^^^^^^^

For more information about this error, try `rustc --explain E0606`.
error: could not compile `playground` due to previous error

note: `*mut T` is a fat pointer, and can therefore not be built from a `usize`

Casting a usize to a regular pointer is possible, so it's a bit confusing at first that this doesn't work for fat pointers, especially if it's not obvious that *mut T is a fat pointer here.

[compiler-errors]

Perhaps it should say "*mut T may be a fat pointer", since it's ?Sized (and so it could either be 1xusize or 2xusize depending on the choice of T).

Another possibly useful thing to point out for the programmer: tracking down the definition of the pointee type, and if it's a generic parameter, pointing to the ?Sized bound.

[BGR360]

@rustbot label +D-terse +D-confusing +A-coercions

[Noratrieb]

Nice, thank you for your work @compiler-errors!