unused_assignments warning: false negative for dead stores in struct fields

[matthiaskrgr]

code like this causes a proper warning

fn main() {
	let mut a;
	let b = 4;
	let c = 5;

    	a = b;
    	a = c;

	println!("{}", a);
}

=>

warning: value assigned to `a` is never read
 --> src/main.rs:6:5
  |
6 |     a = b;
  |     ^
  |
  = note: #[warn(unused_assignments)] on by default

however if a is a struct field, I get no warning at all.

struct Foo {
    x: i32,
}

fn main() {
	let mut strct = Foo {
	    x: 0, // maybe warn here, too?
	};
	let b = 4;
	let c = 5;

  	strct.x = b; // please warn!
   	strct.x = c;

	println!("{}", strct.x);
}

playground: https://play.rust-lang.org/?gist=c904ff202754b2691e2968977620fd7a&version=nightly

I ran into bugs in my code that could have been prevented by warning about this :(

[Boscop]

I also ran into a bug where it was a situation like this, also no warning that a is never read:

struct Foo {
    a: i32
}

impl Foo {
    fn bar(&mut self) {
        self.a = 42;
    }
}

fn main() {
    let mut foo = Foo { a: 1 };
    foo.bar();
}

https://play.rust-lang.org/?gist=dbe81aa3431e467557fc2bf930827c52&version=stable