Linking a Rust staticlib unexpectedly changes C math functions from libm to bundled ones from compiler-builtins

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Code

Given a staticlib with the source:

#[unsafe(no_mangle)]
pub extern "C" fn demo() -> f32 {
    let x : f32 = 1.1;
    x.ceil()
}

compiled as crate-type = ["staticlib"] and the C program:

#include <math.h>
#include <stdio.h>

float demo();

int main(int argc, char** argv) {
    float x = -1.2;
    printf("%f\n", ceilf(x));
    printf("%f\n", demo());
    return 0;
}

When linking the program like this: $(CC) main.o as-crate/target/debug/libas_crate.a -lm -o out/tgt-from-crate the resulting binary ends up using ceilf from compiler-builtins rather than libm.

I expected to see this happen: The binary should still get ceilf from libm, eg

$ nm -uD out/tgt-from-crate | grep ceilf || true
                 U ceilf@GLIBC_2.2.5

Instead, this happened: Instead, the ceilf symbol was satisfied by the matching symbol in compiler-builtins, and so the C call was surprise-migrated to a different implementation.

Version it worked on

1.86
rustc --version --verbose:

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-unknown-linux-gnu
release: 1.86.0
LLVM version: 19.1.7

Version with regression

1.87
rustc --version --verbose:

rustc 1.87.0 (17067e9ac 2025-05-09)
binary: rustc
commit-hash: 17067e9ac6d7ecb70e50f92c1944e545188d2359
commit-date: 2025-05-09
host: x86_64-unknown-linux-gnu
release: 1.87.0
LLVM version: 20.1.1

Additional information

I suspect this is related to some work tracked in #137578. I also found out Chromium has already stumbled on this: https://issues.chromium.org/issues/419258012#comment5 and for them it caused observable behavior changes. Apologies for not catching this sooner - we hadn't upgraded compiler-builtins for a while and didn't notice until a couple of weeks ago. :(

Reproduce the staticlib and C program from the issue with Rust 1.86 and 1.87, then inspect the generated symbols and link behavior around compiler-builtins and libm. Read the work tracked in #137578 for context; done means the C call to ceilf remains satisfied by libm rather than being replaced by compiler-builtins.