a.rs

extern crate b;

extern "C" {
    fn foo() -> i32;
}

#[no_mangle]
pub extern "C" fn main() -> i32 {
    unsafe { foo() + b::bar() }
}

b.rs

extern "C" {
    fn foo() -> i32;
}

pub fn bar() -> i32 {
    unsafe { foo() }
}

helper.c

extern inline int foo() {
	return 1;
}

run.sh

#!/usr/bin/env bash
clang -S helper.c -emit-llvm -o helper.ll -O2
sed -i 's/^define dso_local/define linkonce_odr dso_local/g' helper.ll
llvm-as helper.ll -o helper.bc
rustc +stage1 --crate-type=lib --emit=llvm-bc --emit=metadata b.rs -C opt-level=2 -L .
rustc +stage1 --crate-type=lib --emit=llvm-bc --emit=metadata a.rs -C opt-level=2 -L .
llvm-link b.bc helper.bc -o b.m.bc
clang -c b.m.bc -o b.o -O2
llvm-link a.bc helper.bc -o a.m.bc
clang -c a.m.bc -o a.o -O2
clang a.o b.o -o final.o