document guarantee about evaluation of associated consts and const blocks

[RalfJung]

Fixes rust-lang/rust#112090

I think this has been our intention for a while (ever since rust-lang/rust#67191 landed), but was never properly decided, so it likely needs t-lang FCP.

[RalfJung]

Blocked on rust-lang/rust#124971

[scottmcm]

I think that we'll plausibly end up with a stronger guarantee than what's discussed here, but the basic one of "if it's 'executed' it'll definitely be evaluated" as proposed here seems like an entirely reasonable and hopefully uncontroversial minimal first step.

@rfcbot fcp merge

[joshtriplett]

@rfcbot merge

[rfcbot]

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @tmandry

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[tmandry]

@rfcbot reviewed

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

psst @joshtriplett, I wasn't able to add the final-comment-period label, please do so.

[nikomatsakis]

@rfcbot reviewed

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

psst @joshtriplett, I wasn't able to add the finished-final-comment-period label, please do so.

[ehuss]

Thanks!