Macro bind_mat 

macro_rules! bind_mat {
    ($pass:expr, $materials:expr, $instances:expr, $handle:expr) => { ... };
}

The “material instance → material” chain every draw call repeats: look up $instances.get($handle), then its material via the instance’s own target handle, then set_pipeline + set_bind_group(0, ...) on $pass. Returns from the enclosing function (via or_return!) if either lookup isn’t ready yet — an asset that hasn’t finished uploading is a normal, common case (a couple frames on load), not a bug.

Evaluates to the looked-up &GPUMaterialInstance, so callers that also need .update(name, data) on it (e.g. a per-frame camera uniform) can still bind that:

let instance = bind_mat!(render_pass, materials, instances, instance_handle);