bar.c

void foo(void(*)());

void bar() {
}

int main() {
  foo(bar);
  return 0;
}

foo.rs

extern mod rustuv;

use std::rt;
use std::ptr;

#[no_mangle]
pub extern "C" fn foo(c_callback: extern "C" fn()) {
  do rt::start(0, ptr::null()) {
    c_callback();
  };
}

Makefile

all: libfoo.dylib bar.c
	clang bar.c -o bar -L. -lfoo -Wl,-rpath,`pwd`

libfoo.dylib: foo.rs
	rustc --lib foo.rs -Z gen-crate-map
	ln -nsf libfoo-*.dylib libfoo.dylib