winit: "internal error: entered unreachable code" when using Wayland backend

Attempting to create a Window on the Wayland backend on my system results in the following:

thread 'main' panicked at 'internal error: entered unreachable code', /home/slabity/.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.23.0/src/platform_impl/linux/wayland/window/mod.rs:218:77
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
[wayland-client error] A handler for wl_keyboard panicked.

Which seems to be coming from this:

// To make our window usable for drawing right away we must `ack` a `configure`
// from the server, the acking part here is done by SCTK window frame, so we just
// need to sync with server so it'll be done automatically for us.
event_loop_handle.with_source(&wayland_source, |event_queue| {
    let event_queue = event_queue.queue();
    let _ = event_queue.sync_roundtrip(&mut *winit_state, |_, _, _| unreachable!());
});

Can anyone explain what this little piece of code is doing and what would cause the it to be reachable?