Add lint for & -> &mut transmutes

[huonw]

Doing a &T to &mut T transmute is now undefined behaviour (it needs to use the Unsafe type), and so there could be a lint for it.

Unfortunately, such a lint is likely to miss many such casts, but it will theoretically catch common cases like transmute::<&Foo, &mut Foo>(self).

[nikomatsakis]

The proposed changes to intrinsics help here, because they wlil effectively forbid transmutes between types unless we can be sure of the size, which implies that one cannot transmute arbitrary types T to U and so on. We could even make these sorts of const casts illegal -- though somehow we have to make the Unsafe type able to do it. :)

[nikomatsakis]

Oh, hmm, the RFC doesn't have the interesting parts. Nm.

[emberian]

(I have this half-implemented in https://github.com/cmr/rust/tree/unsafe-transmutes)

[nikomatsakis]

Ah, the proposed changes to transmute are in #12898, not the RFC.

[steveklabnik]

Triage: nothing has changed, these transmutes still silently work:

use std::mem;

fn main() {
    let x = 5;

    let y = &x;

    let z = unsafe { mem::transmute::<&i32, &mut i32>(y) };
}