`const`-ify `BytesMut::len`, `BytesMut::is_empty`, and `BytesMut::capacity`,

[Cyborus04]

[taiki-e]

Is there a way to create BytesMut in const? If not, this does not seem to be available in practice.

[taiki-e]

Ah, it was previously proposed, but I closed it for compatibility with future implementation changes. https://github.com/tokio-rs/bytes/pull/472#pullrequestreview-632946191

[Cyborus04]

I guess there isn't, you're right. I think new could be made const, but I'm not sure how useful that would be.

[Cyborus04]

Ah, yep, haha