Make empty arrays Copy

[tbu-]

This PR is based on #30130.

The relevant commit from this PR is tbu-@1769c98.

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

Can this just be folded into #30130? It seems pretty harmless.

Also why does this need a compiler change? Can this not just be a trait impl?

[tbu-]

I'll put this into #30130.

[alexcrichton]

To copy over my concerns from #30130, I was a little hesitant when I realized that other marker traits like Send and Sync could in theory also be implemented for [T; 0] with no restrictions on T, but they currently aren't either (a tad inconsistent).

I'm gonna tag this with T-libs so it comes up during triage and we can discuss.

[tbu-]

The difference to Send and Sync is that you can still force a struct containing a member that does not implement the trait to be Send or Sync anyway. You can't do the same for Copy.

[bors]

☔ The latest upstream changes (presumably #30247) made this pull request unmergeable. Please resolve the merge conflicts.

[aturon]

We discussed this in the libs team meeting this week, and would like to see a stronger rationale for making this change before we land it.

[alexcrichton]

Closing due to @aturon's previous comment.