Function optional_device_features 

pub fn optional_device_features() -> Features

The wgpu::Features a device request opportunistically asks for when the adapter exposes them.

PIPELINE_CACHE alone: it is what create_pipeline_cache needs to seed the renderer’s shader-pipeline compilation from a persisted blob. An optional feature is only ever added when the adapter already offers it, so this can never turn a working adapter into a failed device request.