suggest const_in_array_repeat_expression flag

[davidtwco]

This PR adds a suggestion to add the #![feature(const_in_array_repeat_expression)] attribute to the crate when a promotable expression is used in a repeat expression and the feature gate is not enabled.

Unfortunately, this ended up being a little bit more complex than I anticipated, which may not have been worth it given that this would all be removed when the feature is stabilized. However, with #65732 and #65737 being open, and the feature gate having not been being suggested to potential users, the feature might not be stabilized in a while, so maybe this is worth landing.

cc @Centril (addresses this comment)
r? @ecstatic-morse (opened issues related to RFC 2203 recently)

[ecstatic-morse]

There's a newly merged, top-down way to compute promotability in transform::promote_consts that would make this a few lines in borrow_check. You need to run collect_temps_and_candidates and pass the vector of TempStates to a Validator with explicit set to false. Then call validate_operand on the operand to the array repeat expression which will tell you if it is implicitly promotable. You might want to wait until #65839 gets merged (it's currently in the bors queue), which will make initializing a Validator a lot simpler.

We should be able to remove all the infrastructure in qualify_consts soon, so I think we should try to avoid adding new features that depend on it. That said, we could merge this as is, and I could port it to use eddyb's new stuff later.

[davidtwco]

@ecstatic-morse I've marked this as S-blocked to wait until #65839 is merged, but if you want to merge this as-is and then port it to the new stuff later, feel free to.

[ecstatic-morse]

@davidtwco #65839 has been merged. Let me know on Zulip if you have any questions. We may be able to similarly improve error messages when rvalue static promotion fails and 'static is required, but I don't know enough about lifetime inference to do this.

Sorry about the timing :/

[davidtwco]

@ecstatic-morse I've rebased atop that PR and implemented the suggested approach.

[ecstatic-morse]

Thanks @davidtwco!

r=me after we discuss the help message.

Note that this will be my first review approval as a new member of the rust-lang org. This change is low impact and in code that I am very familiar with, so I think this is fine.

[ecstatic-morse]

Looks good to me as well.

@bors r+

[bors]

📌 Commit 92b1512 has been approved by ecstatic-morse

[likebike]

There is a critical spelling mistake in the suggested attribute. Right now, the compiler suggests this:

= help: add #![feature(const_in_array_repeat_expression)] to the crate attributes to enable

...but the real attribute name is const_in_array_repeat_expressions (with an 's' at the end).

This mistake confused me for many hours. Please correct it ASAP.

[estebank]

@likebike, could you create a new ticket for this linking to your comment here? I'm currently on mobile so I can't create it myself. It is easier to keep track of things that need to be done when there's a ticket for them.