Catch transmuting Option<&T> to Option<&mut T>

[fhartwig]

Rustc already has a lint (mutable_transmutes) that catches transmutes from &T to &mut T, which can cause undefined behaviour. However, it doesn't catch minor variations of this problem, like Option<&T> -> Option<&mut T>, which are presumably just as dangerous. I've seen this particular example in real code.
It would be nice to have a lint that catches this case (and ideally, other instances of the same problem, although this could get quite complex in the fully general case).

[Manishearth]

This basically involves using ty.walk() in the transmute lint somehow.

[samueltardieu]

@rustbot label +good-first-issue

[mendelsshop]

@rustbot claim