Add Default support for arrays of all sizes

[saeubank]

Proposal

Problem statement

Arrays over the size of 32 do not support Default.

Motivation, use-cases

let arr = <[u8; 64]>::default();

Solution sketches

[T::default(); N]
This does not work because default only gets called once.
If for some reason an implementation of default is like a generator, this will not behave as expected since the whole array will be the same value rather than being the generated value.

Links and related work

rust-lang/rust#106145

What happens now?

This issue is part of the libs-api team API change proposal process. Once this issue is filed the libs-api team will review open proposals in its weekly meeting. You should receive feedback within a week or two.

[cuviper]

We can't do this until we have a solution for [T; 0] without T: Default, or a convincing argument to make that breaking change.

[saeubank]

Yeah it looks like it will be a breaking change until negative Traits are implemented.

impl<T> Default for [T; 0] where 
 T: !Default {
    fn default() -> Self {
        []
    }
}

I think that this is worth making a breaking change for.
Is it worth using @craterbot to check if people actually use default for [T; 0] where T does not implement Default?

[cuviper]

I didn't have the issue at hand before, but that's rust-lang/rust#61415 for a lot more history.

[dtolnay]

I'm confident the library API team is on board with arbitrarily large [T; N] implementing Default. The missing piece is just figuring out a way to implement it — as mentioned above, this is tracked in rust-lang/rust#61415.