Provide array impls for arrays up to length 44

[alex]

Currently various traits are provided for arrays up to length 32: https://github.com/rust-lang/rust/blob/master/src/libcore/array.rs#L259-L264

I'm requesting this be extended up to length 44.

Why? Because bindgen relies on AsMut<[T]> for fixed-size arrays, and the linux kernel has some structs with 44-element arrays. Providing impls up to length 44 would permit bindgening the full kernel headers without needing to whitelist/blacklist elements. I realize this is vaguely whack-a-mole, but it's a valuable use-case in my, 100% biased, opinion.

If this idea is acceptable, I'm happy to prepare a patch.

[varkor]

Once const generics are implemented, we'll be able to implement traits on arrays for any length. It's probably better to wait till then, rather than special-casing for a particular codebase now.

[alex]

Yes, that's clearly the right long term solution. The nice thing about the short term solution is that I could have a patch in in an hour and be able to use it in tomorrow's nightly :-)

[alex]

I've gone ahead and submitted #51318. If the core team has interest in this, the PR is available for consideration.

[mark-i-m]

@alex How are you bindgen-ing linux kernel headers? TMK linux relies on a handful of gcc optimizations, attributes, and non-standard extensions, so you can't use clang to compile it. Is that outdated knowledge?

[mark-i-m]

@varkor Is there an idea of how long it will be before basic const generics are ready? (btw, thanks for all your work on that!)

[varkor]

@mark-i-m: before the focus switched to the refactorings (which are now almost done), I felt we were relatively close to an initial implementation. It's hard to give a good estimate, because quite a lot has changed since then, but personally I'd really like to aim for something working by July or so (as soon as #48149 is merged, I think we'll be in a position to make swift headway).

[alex]

@mark-i-m you can look at https://github.com/alex/linux-kernel-module-rust to see how it works. It seems to be working at any rate 🤷‍♂️

[mark-i-m]

@alex It looks like it is close, but not quite there yet. https://lwn.net/Articles/734071/... It will probably work until you need touch a weirder data structure like struct page...

Anyway I don't want to derail the thread...

Given that const generics on nightly might be only a month away, my vote would be to wait...

[varkor]

There's a proper tracking issue for this here now: #61415.