Optimization of deinitialization became worse in nightly

[theemathas]

Code

I tried this code (not from real code, but just experimenting):

use std::mem::MaybeUninit;

#[unsafe(no_mangle)]
fn demo(x: &mut MaybeUninit<u32>) -> MaybeUninit<u32> {
    *x = MaybeUninit::uninit();
    *x
}

In beta, this compiles to (Godbolt):

demo:
        ret

In nightly, this compiles to (Godbolt):

demo:
        mov     eax, dword ptr [rdi]
        ret

Since we've deinitialized the memory that x points to, returning a copy of that memory should result in returning uninit. The compiler should be free to return whatever garbage data it wants, so just ret is a valid compilation, and is probably the best-performing compilation result.

Version it worked on

It most recently worked on Godbolt full compiler version:

rustc 1.92.0-beta.3 (f3f12444a 2025-11-09)
binary: rustc
commit-hash: f3f12444a017add0468f683f3a10656d29438a05
commit-date: 2025-11-09
host: x86_64-unknown-linux-gnu
release: 1.92.0-beta.3
LLVM version: 21.1.3
Internal compiler ID: beta

Version with regression

Godbolt full compiler version:

rustc 1.93.0-nightly (83e49b75e 2025-12-03)
binary: rustc
commit-hash: 83e49b75e7daf827e4390ae0ccbcb0d0e2c96493
commit-date: 2025-12-03
host: x86_64-unknown-linux-gnu
release: 1.93.0-nightly
LLVM version: 21.1.5
Internal compiler ID: nightly

[theemathas]

Bisected with cargo bisect-rustc --start beta --end main --prompt -- asm and manually inspecting the asm.

searched toolchains 75948c8bb3bd37f1e8ee20273a04edea4c1f84f8 through d427ddfe90367eaa6d2ed7bb8a16559f0230f47a


********************************************************************************
Regression in e9acbd99d384280874129fb7fa0da9faeae0d051
********************************************************************************

Attempting to search unrolled perf builds
ERROR: couldn't find perf build comment

Regressed in #147827. CC @saethlin

[theemathas]

Another similar case:

use std::mem::MaybeUninit;

#[unsafe(no_mangle)]
fn demo(x: &mut MaybeUninit<u32>) {
    *x = MaybeUninit::new(1);
    *x = MaybeUninit::uninit();
}

On beta (Godbolt):

demo:
        ret

On nightly (Godbolt):

demo:
        mov     dword ptr [rdi], 1
        ret