Missed optimization with `*x += 0;`

This issue was discovered by @ruriww

I tried this code:

#[unsafe(no_mangle)]
pub fn f(x: &mut u32) {
    *x = *x;
    if *x != 0 {
        *x = 0;
    }
}

#[unsafe(no_mangle)]
pub fn g(x: &mut u32) {
    *x = *x;
    if *x != 0 {
        *x = 0;
    } else {
        *x += 0;
    }
}

I expected both functions to compile to the same assembly. Note that the *x = *x; gives the compiler license to write to *x (without running into opsem questions about whether spurious writes are allowed).

Instead I got the following with -Copt-level=3 (Godbolt):

f:
        mov     dword ptr [rdi], 0
        ret

g:
        cmp     dword ptr [rdi], 0
        je      .LBB1_2
        mov     dword ptr [rdi], 0
.LBB1_2:
        ret

Meta

Godbolt compiler version:

rustc 1.94.0-nightly (fa5eda19b 2025-12-12)
binary: rustc
commit-hash: fa5eda19b95201468f5b1c5c035ec2fc06fccd66
commit-date: 2025-12-12
host: x86_64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.5
Internal compiler ID: nightly

Start with the Rust reproducer in the issue and compare the optimized assembly for functions f and g using the linked Godbolt example. Trace the compiler optimization path responsible for simplifying the two cases, then add or update a regression test if the relevant test location is identified. Done means g receives the same optimization as f without changing the intended semantics.