Bump bitflags to v2

[daxpedda]

Changelog: https://github.com/bitflags/bitflags/blob/2.0.0/CHANGELOG.md#200.

This is a breaking change. The API that is implemented for Transformations by the macro is now different.

[fintelia]

Thanks for this PR. Since it is a breaking change, it'll have to wait for the next major release.

[sjackman]

Since it is a breaking change, it'll have to wait for the next major release.

Any plans for a version 0.18.0 that includes this change? Thank you for your work on this useful crate!

[fintelia]

Uh, not any immediate plans. There's a number of API changes I'd like to make at the same time, but it'll probably be a while before I have a chance to fully design what those should be

[nyurik]

@fintelia thx for your work on this. Is there a reason why you want to bundle all breaking changes together? Version numbers are "cheap" - it's ok to have several breaking versions as they become ready for release -- in my opinion, rapid iteration is better than stalled dependencies... Or am I missing something? Thx!

[fintelia]

No, we do tend to be overly hesitant to do breaking changes.

Part of it is not getting a ton of user feedback beyond things like download counts which show some users being very slow to update (we still get a significant number of downloads for the 0.16.x series?!). There's also complications around whether to backport fixes or evaluating whether API design changes are actually improvements. But another big aspect is limited maintainer bandwidth that makes it easy to focus on issues/changes that can be handled immediately and let PRs with breaking changes drift to the end of the TODO list

[anforowicz]

Any updates? :-)

(One other breaking change that I'd like to see merged is #421 - maybe it's time to pull the trigger on both of them? I haven't looked closely at other PRs that may be waiting for their turn because they introduce breaking changes - e.g. I don't have a strong opinion whether #488 is ready or not.)