Miscompile involving function inlining

[robdockins]

Consider the following code:

fn main() {
    println!("{:?}", run_my_check(42, 1));
}

#[inline(never)]
fn run_my_check(v0: u64, v1: u64) -> Result<(), ()> {
    if do_check(v0, v1) {
        Ok(())
    } else {
        Err(())
    }
}

#[inline(always)]
fn do_check(v0: u64, v1: u64) -> bool {
    let mut ok = v0 == 42;
    ok &= v1 == 0;
    ok
}

I expected to see this happen: In all cases, when I compile and run this program, I expect it to print Err(())

Instead, this happened: With rust version 1.92, and when compiled with optimizations, it instead prints Ok(()).

% rustc --version --verbose 
rustc 1.92.0 (ded5c06cf 2025-12-08)
binary: rustc
commit-hash: ded5c06cf21d2b93bffd5d884aa6e96934ee4234
commit-date: 2025-12-08
host: aarch64-apple-darwin
release: 1.92.0
LLVM version: 21.1.3

% rustc main.rs && ./main   
Err(())

% rustc -O main.rs && ./main
Ok(())

This behavior is not observed with 1.91.1

% rustc --version --verbose 
rustc 1.91.1 (ed61e7d7e 2025-11-07)
binary: rustc
commit-hash: ed61e7d7e242494fb7057f2657300d9e77bb4fcb
commit-date: 2025-11-07
host: aarch64-apple-darwin
release: 1.91.1
LLVM version: 21.1.2

% rustc main.rs && ./main   
Err(())

% rustc -O main.rs && ./main
Err(())

The behavior seems to be related to inlining somehow. Marking do_check as inline(never) resolves the issue. So does rewriting do_check to use immutable lets.

[SpriteOvO]

@rustbot label +I-miscompile +E-needs-bisection +S-has-mcve +regression-from-stable-to-stable +T-compiler

[saethlin]

Bisection points to #142915.