$ rustup default beta
$ cargo new --lib foo
$ cd foo
$ echo "mod qux;" > src/lib.rs
$ echo "mod bar;" > src/qux.rs
$ mkdir src/qux
$ touch src/qux/bar.rs
$ cargo build
   Compiling foo v0.1.0 (/tmp/foo)                                                                                                                                       
    Finished dev [unoptimized + debuginfo] target(s) in 0.33s
$ cbindgen
WARN: Parsing crate `foo`: can't find mod bar`.
#include <cstdint>
#include <cstdlib>