It looks like you need to use conditional compilation to use Loom, which probably works okay for testing one library, but is pretty intrusive:

    #[cfg(loom)]
    pub(crate) use loom::sync::atomic::AtomicUsize;

    #[cfg(not(loom))]
    pub(crate) use std::sync::atomic::AtomicUsize;

I wonder if there are any languages that do a better job of letting you use your own scheduler?