Idea: keyword gate annotation #![keywords(...)]

[steveklabnik]

My comment from that thread is still relevant:

I wonder if we can take one more small step in this direction:

• When stabilizing a feature in edition N that requires new keywords, the feature flag is usable on stable and is needed to opt in to the new keyword.
• Switching to edition N+1 automatically opts in to all previous feature flags.

This was one of the original models we proposed for editions generally, but was rejected. What's changed since then? It'd be worth re-exploring.

I'm also very wary of doing this now. Yes, the keyword thing is a pain right now. However, this would be making a pretty sweeping change to a system of change we're using for a long time, but the keyword issue will diminish over time. I'm not sure that optimizing for fixing this problem is the right tradeoff.