Uninformative error message when casting usize to pointer to generic `?Sized` type

[rodrimati1992]

Trying to compile this code

pub fn bogus_ptr<U: ?Sized>() -> *const U {
    0usize as *const U
}

produces this error message

error[E0606]: casting `usize` as `*const U` is invalid
  --> <source>:2:5
  |
2 |     0usize as *const U
  |     ^^^^^^^^^^^^^^^^^^

It would be better if the error message stated that usize cannot be cast to a pointer to a ?Sized type because it's potentially unsized.

Meta

Tried the code in these Rust versions and they all produced the same error message:

• 1.37.0 stable

• 1.45.0 stable

• 1.46.0-beta.1 (2020-07-15 e51b714)

• 1.47.0-nightly (2020-07-22 bbebe73)

[estebank]

Current output:

error[E0606]: cannot cast `usize` to a pointer that may be wide
 --> src/lib.rs:4:15
  |
4 |     0usize as *const U
  |               ^^^^^^^^ creating a `*const U` requires both an address and type-specific metadata

[estebank]

Current output:

error[E0606]: cannot cast `usize` to a pointer that may be wide
 --> src/lib.rs:4:15
  |
4 |     0usize as *const U
  |     ------    ^^^^^^^^ creating a `*const U` requires both an address and type-specific metadata
  |     |
  |     consider casting this expression to `*const ()`, then using `core::ptr::from_raw_parts`