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Failure to link rust dependency with LTO-enabled C FFI code built with GCC #138681

@glandium

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@glandium
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STR:

  • Create the following files:
  • Cargo.toml
[package]
name = "foo"
version = "0.1.0"
edition = "2024"

[dependencies]
foolib = {path = "foolib" }
  • src/main.rs
fn main() {
    println!("{}", foolib::foo());
}
  • foolib/Cargo.toml
[package]
name = "foolib"
version = "0.1.0"
edition = "2024"

[build-dependencies]
cc = "1"
  • foolib/build.rs
fn main() {
    cc::Build::new().file("foo.c").compile("foo");
}
  • foolib/foo.c
int foo() { return 42; }
  • foolib/src/lib.rs
mod foo {
    unsafe extern "C" {
        pub fn foo() -> std::os::raw::c_int;
    }
}

pub fn foo() -> usize {
    unsafe { foo::foo() as usize }
}
  • Build with CFLAGS=-flto cargo build

I expected to see this happen: success.

Instead, this happened:

  = note: /usr/bin/ld: /tmp/foo/target/debug/deps/libfoolib-591acb1d5285dc4c.rlib(foolib-591acb1d5285dc4c.07n83wzvxeypc13m10c6n8g8q.rcgu.o): in function `foolib::foo':
          /tmp/foo/foolib/src/lib.rs:8: undefined reference to `foo'
          collect2: error: ld returned 1 exit status
          
  = note: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified

Notes:

  • This does not happen if the build script and FFI is in the bin crate. Only when in the lib crate.
  • This can be worked around by running ranlib /tmp/foo/target/debug/deps/libfoolib-591acb1d5285dc4c.rlib after the first failure, and trying again. IOW, what's missing is running a GCC-LTO-aware ranlib for the rlib's symbol table to be filled properly.

Meta

rustc --version --verbose:

rustc 1.85.1 (4eb161250 2025-03-15)
binary: rustc
commit-hash: 4eb161250e340c8f48f66e2b929ef4a5bed7c181
commit-date: 2025-03-15
host: x86_64-unknown-linux-gnu
release: 1.85.1
LLVM version: 19.1.7

(this happens with today's nightly too, rustc 1.87.0-nightly (75530e9 2025-03-18))

Activity

added
needs-triageThis issue may need triage. Remove it if it has been sufficiently triaged.
on Mar 19, 2025
added
A-linkageArea: linking into static, shared libraries and binaries
A-FFIArea: Foreign function interface (FFI)
T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.
A-LTOArea: Link-time optimization (LTO)
and removed
needs-triageThis issue may need triage. Remove it if it has been sufficiently triaged.
on Mar 22, 2025
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    A-FFIArea: Foreign function interface (FFI)A-LTOArea: Link-time optimization (LTO)A-linkageArea: linking into static, shared libraries and binariesC-bugCategory: This is a bug.T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.

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          Failure to link rust dependency with LTO-enabled C FFI code built with GCC · Issue #138681 · rust-lang/rust