staticlib with lto=fat now includes compiler_builtins as llvm bitcode after rustup update

[behdad]

In HarfBuzz we use cargo to build a staticlib, then link it to harfbuzz using meson. We use lto="fat" and that's essential to the performance of the generated rust code binary.

This used to work with gcc as the final linker with rustc 1.91.0-nightly (898aff7 2025-08-14), but after updating to rustc 1.91.0-nightly (51ff895 2025-09-02), now the generated libharfbuzz_rust.a includes: a harfbuzz_rust .o file that is an ELF binary, but also a compiler_builtins .o that is in LLVM IR bitcode. This makes the gcc linker fail linking with unknown file format error. Is this intentional?

Meta

rustc --version --verbose:

rustc 1.91.0-nightly (51ff89506 2025-09-02)
binary: rustc
commit-hash: 51ff895062ba60a7cba53f57af928c3fb7b0f2f4
commit-date: 2025-09-02
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0

[Noratrieb]

likely regressed in #145368, there's already been another issue about that PR #146109 @rcvalle @dianqk

[lqd]

If so, this should be fixed soon by the revert #146133

[apiraino]

Assigning priority (discussion on Zulip).

@rustbot label -I-prioritize +P-critical

[behdad]

Note that even trying to do the final link with lld fails, with:

ld.lld: error: src/rust/libharfbuzz_rust.a(compiler_builtins-e82ecba48a5a77b2.compiler_builtins.c83520de7ece3beb-cgu.00.rcgu.o): Unknown attribute kind (102) (Producer: 'LLVM21.1.0-rust-1.91.0-nightly' Reader: 'LLVM 19.1.7')