Making requests with WEnum::Unknown

[andriyDev]

There is currently no way (as far as I can tell) to pass along unknown enum variants through requests. All APIs which take an enum expect a known enum variant.

Motivation

The zwlr_output_head_v1 protocol reports head transforms giving me WEnum<Transform>. I want to write these to disk, then in a future run, send them as part of a request in the zwlr_output_configuration_head_v1 protocol. Unfortunately, since the set_mode protocol takes a Transform instead of a WEnum<Transform>, my conversion is lossy. If the Transform enum is ever extended, my code breaks!

This is not limited to unstable protocols. In theory, you could want to do something similar with the wl_surface protocol (e,g, take the PreferredBufferTransform, and conditionally replace it to some other transform).

[ids1024]

This is relevant for wl_shm::format, where it's probably valid for the compositor to advertise and the client to use a format that isn't part of the enum, but this isn't really documented: https://gitlab.freedesktop.org/wayland/wayland/-/merge_requests/425.

Exactly how enums like this are supposed to be used seems a bit ambiguous in Wayland, generally. Hopefully https://gitlab.freedesktop.org/wayland/wayland/-/issues/497 can address this. I think we'd want to use WEneum for "open" enums, but just the contained type for closed ones? Though I don't know how that would work with something like zwlr_output_head_v1 where the enum is defined in a different protocol.