[beta] Backport union stabilization PRs

[petrochenkov]

Supersedes #42321

• Stabilize unions with Copy fields and no destructor #42068
• Make assignments to Copy union fields safe #42083

cc @aturon
r? @brson

[rust-highfive]

Warning

• Pull requests are usually filed against the master branch for this repo, but this one is against beta. Please double check that you specified the right target!

[est31]

I think this should be put into the release notes, no?

[brson]

I'm going to update the relnotes and backport as part of this pr.

[brson]

This isn't backporting the reference docs, and it probably should.

[brson]

There's no beta branch of the reference to backport the reference docs to.

I'm feeling a bit uncomfortable about this.

[nikomatsakis]

I'll copy my comment from #42068:

"I don't strongly object, but I'm surprised we would backport this. I feel like we try to keep backports to regressions or important bug fixes, and only rarely backport stabilizations -- the exception would be if some feature is hotly demanded. But otoh this seems pretty darn low risk, so no real objection."

[brson]

I'm going to look into backporting the reference directly from reference/master and see if it makes sense on beta.

[brson]

Here's the reference diff: rust-lang/reference@6b0de90...master

[brson]

Let's not do it. Feels risky to me.

[lilianmoraru]

So, this did not land?
The release notes say that it landed: https://github.com/rust-lang/rust/pull/42503/files#diff-5cafcf46bfb947f8e58fa5c16d39018cR13