Unnecessary references for casting from immutable reference to mutable reference.

[ticki]

A warning should be generated with:

let mut a = 2;
let mut a_mut = &mut a;
let a_ref: &u8 = &a_mut;

which is more approriately written as:

let mut a = 2;
let mut a_mut = &mut a;
let a_ref: &u8 = a_mut;

(when possible)

[mcarton]

I would have guessed this was already covered by needless_borrow, but it seems like not.

[oli-obk]

I think the issue is that this is a conversion from a &&mut T to a &T. I'll check how we can improve the needless_borrow lint.