Add lint for u8 as *mut cast

[mattico]

This is an easy error to make: #42901 #42827

A cast from u8 to a pointer is probably always an error. In the rare case that you're creating a pointer into the first 256 bytes of address space, u8 as usize as *mut is more clear, or you can be troubled to #[allow(u8_to_ptr)].

[retep998]

This lint should apply to any cast from an integer type that is not usize/isize to a raw pointer, especially because Rust has awful footgun behavior for such casts.

-1i8 as isize gives 0xFFFFFFFFFFFFFFFF
-1i8 as usize gives 0xFFFFFFFFFFFFFFFF
-1i8 as *const () as usize rather surprisingly gives 0xFF

If we compare this to C++ for example:

(intptr_t)int8_t{-1} gives 0xFFFFFFFFFFFFFFFF
(uintptr_t)int8_t{-1} gives 0xFFFFFFFFFFFFFFFF
(uintptr_t)(void*)int8_t{-1} gives the naturally expected 0xFFFFFFFFFFFFFFFF

So basically Rust has a giant footgun in that casting from non pointer sized integers to pointers differs from C/C++ and casts from non pointer sized integers to pointer sized integers. This lint is essential to catch this footgun and warn people about this hazard!

[retep998]

Related footgun: #43291

[Enselic]

Triage: Lint remains unimplemented. Seems useful to have and easy to implement. Maybe first in clippy? Not sure. Playground:

fn main() {
    let _ = 42u8 as *mut u8;
}