foo.c

int pants() {
    char *foo = crud();
    printf("Crud: %s\n", foo);
    rust_free(foo);
}

foo.rs

#[no_mangle]
pub extern fn crud() -> *const i8 {
    CString::new("Hello!").unwrap().into_raw()
}
#[no_mangle]
pub extern fn rust_free(mem: *mut i8) {
    unsafe { CString::from_raw(mem); }
}