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

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 (75530e9f7 2025-03-18))

Reproduce the failure from the provided Cargo.toml, build.rs, foo.c, and lib.rs files with CFLAGS=-flto. Start by tracing how rustc handles the C-generated rlib and its native symbols when the FFI is in a library crate. Done means the minimal project builds successfully and has a regression test covering this library-crate case.