Promise array::from_fn is generated in order of increasing indices

[scottmcm]

Fixes #139061

I agree this needs to be documented because of the FnMut, either with a guarantee or to explicitly disclaim one.

I'm pretty sure this will be non-controversial (like the other "well sure you could do it in a different order, but why?" things were), but I couldn't find any previous libs-api decision on it so it's seemingly a new promise that will need FCP.

Basically, yes, it would be plausible to fill in the reverse order, but there's no obvious way we could ever know that that might even be a good idea, so forward seems like an easy thing to promise. We could always add a from_fn_rev or something later if there's ever a strong enough need, but it seems unlikely.

Let's just do the obvious thing so it matches what [gen(0), gen(1), …, gen(N-1)] does.

[rustbot]

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[Amanieu]

There seems to be little downside to guaranteeing this.

@rfcbot merge

[rfcbot]

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

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• @m-ou-se

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[rfcbot]

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