remove LengthAtMost32 on AsRef/Borrow impl for array

[burrbull]

Since I did not receive a response to #71373 I tried to do PR.

All tests are passed on my PC. Is there any reason not to do this?

Thanks.

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[crlf0710]

When these bounds are removed, the resulting impl surface will not be expressible in current stable Rust. So we have to keep these bounds until const generics is stablized...

[withoutboats]

When these bounds are removed, the resulting impl surface will not be expressible in current stable Rust. So we have to keep these bounds until const generics is stablized...

We don't. By removing these bounds, we would be committing that std will always be able to implement traits for all array types. Since that's always been a completely consensus goal for everyone involved in Rust, and we have no reason to believe we won't eventually stabilize const generics at least powerful enough to implement traits for array types, I think we should remove this bound and provide this functionality to people sooner, rather than later.

The decision to introduce this bound was always unnecessary conservatism.

@rfcbot fcp merge

[rfcbot]

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

• @cramertj
• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @withoutboats

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!

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[withoutboats]

@burrbull if we're going to do this, we should remove LengthAtMost32 entirely and change the implementation for all traits for arrays, not only asref/borrow.

[joshtriplett]

I agree: we're just committing that the standard library will implement such traits, which does not inherently commit us to any particular design for stable const generics outside the standard library; that just means we must provide a language facility the standard library can use for this purpose.

[tesuji]

Does this mean that now we could implement Copy, Hash, etc for all arrays?

[lcnr]

One thing which concerns me is Default for arrays, which afaik we can't implement using const generics rn
as the current Default impl does not have a T: Default bound for N == 0.

So I fear that we will end up with Default being the only trait which is not implemented for all arrays.

[burrbull]

Closed in favour of 74060.