glib: Deprecate main context channel

[sdroege]

It was already removed in git master and giving an advance warning seems useful so people already port now if possible.

[sdroege]

CI failures unrelated. clippy was updated once again 🙄

[upsuper]

There are like 5 different crates that people could use. Not sure we want to prescribe one here.

I have to say I'm quite frustrated by this deprecation, as it is completely unclear what to do, and it took me quite a while to figure out.

It would probably be good to have some examples to show how the functionality can be achieved without this API, or at least mention relevant crates and API to use.

For anyone else finding finding this PR, I ended up using futures_channel's unbounded mpsc channel with MainContext::default().spawn_local to replace the use of MainContext::channel, which seems to work fine.

[sdroege]

@upsuper Does #1242 help? If not, can you suggest an improvement for the deprecation message?

[upsuper]

Yes, that's way more helpful than the deprecation message here. It would probably be good to publish that change to crates.io sooner than later, so that others wouldn't have to struggle with figuring out it again.

[sdroege]

I'll try getting a bugfix release out one of these days, yes. I think there was another useful bugfix too.

If you have any suggestions what else to improve with that message, now would be a good time :)