unused_assignments does not care about unused assignments to reference derefs

[oli-obk]

pub fn foo(b: bool, x: &mut i32) {
    if b {
        *x = 6;
    }
    *x = 7;
}

does not emit the unused_assignments lint. I understand that this is not possible for DerefMut in general, because every call to deref_mut (via desugaring *) can result in a different reference, but for pure references everything should be good.

A case of not linting this causing a bug was found in rustc:

rust/src/librustc_mir/transform/inline.rs

Lines 614 to 620 in 2f1ef9e

	if idx < self.args.len() {
	match self.args[idx] {
	Operand::Consume(Lvalue::Local(l)) => *local = l,
	ref op => bug!("Arg operand `{:?}` is {:?}, not local", idx, op)
	}
	}
	*local = self.local_map[Local::new(idx - self.args.len())];

[briansmith]

Simplified:

pub fn foo(x: &mut i32) {
    *x = 6;
    *x = 7;
}