unused_assignments on MIR warns on earlier compound assignment in a read-modify-write chain

[chenyukang]

I tried this code:

fn main() {
    let mut a = 10.;
    let b = 13.;
    let c = 11.;

    a += b;
    a -= c;
}

I expected only the second assignment (a -= c) to trigger unused_assignments, matching old stable behavior (rustc 1.91.1 (ed61e7d 2025-11-07)).

The second compound assignment reads the previous value of a, so the value produced by a += b appears to be used as part of evaluating a -= c.

Instead, this happened:

warning: value assigned to `a` is never read
 --> /tmp/haha.rs:6:5
  |
6 |     a += b;
  |     ^^^^^^
  |
  = help: maybe it is overwritten before being read?

warning: value assigned to `a` is never read
 --> /tmp/haha.rs:7:5
  |
7 |     a -= c;
  |     ^^^^^^
  |
  = help: maybe it is overwritten before being read?

Find this when working on #154456

The question here is about the lint result itself: when a compound assignment is immediately followed by another compound assignment on the same place, should the earlier assignment still be considered "never read", even though the later operation reads the old value as part of its read-modify-write semantics?

corresponding test case:

rust/tests/ui/liveness/liveness-unused.rs

Lines 172 to 197 in ac40f5e

	fn f9() {
	let mut a = 10;
	//~^ ERROR variable `a` is assigned to, but never used
	let b = 13;
	let c = 13;
	let d = 13;
	let e = 13;
	let f = 13;
	let g = 13;
	let h = 13;
	a += b;
	//~^ ERROR value assigned to `a` is never read
	a -= c;
	//~^ ERROR value assigned to `a` is never read
	a *= d;
	//~^ ERROR value assigned to `a` is never read
	a /= e;
	//~^ ERROR value assigned to `a` is never read
	a |= f;
	//~^ ERROR value assigned to `a` is never read
	a &= g;
	//~^ ERROR value assigned to `a` is never read
	a %= h;
	//~^ ERROR value assigned to `a` is never read
	}

[chenyukang]

link #142390 for root cause.