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It is optional and is needed only when a layer has physical device extensions that may be unknown to the loader. This simplifies the layer a bit, but more importantly, it works around a bug in the loader when there is another layer in the layer chain that wraps VkInstance. Reviewed-by: Dave Airlie <airlied@redhat.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16307>