Missed optimization with Option

[theemathas]

I tried this code:

#[unsafe(no_mangle)]
pub fn f(x: &mut Option<u32>) {
    let mut y = *x;
    y = y.map(|a| a + 1);
    *x = y;
}

#[unsafe(no_mangle)]
pub fn g(x: &mut Option<u32>) {
    let mut y = *x;
    if let Some(a) = &mut y {
        *a += 1;
    }
    *x = y;
}

I expected both code to generate the same assembly, but instead I got the following with -Copt-level=3 (Godbolt):

f:
        inc     dword ptr [rdi + 4]
        ret

g:
        mov     eax, dword ptr [rdi]
        add     dword ptr [rdi + 4], eax
        ret

Since both functions are unconditionally doing a typed copy to *x, the compiler should be free to mutate the uninitialized bytes in a None value.

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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