[beta] trans: pad const structs to aligned size

[TimNN]

This fixes #37222 on beta.

As pointed out by @eddyb in #37281 (comment), the nightly fix cannot be backported as-is due to other changes.

An alternative to this adapted fix would be to backport #36904 as well, in addition to #37281.

Also cc @rust-lang/compiler, since this is not a straight backport.

[rust-highfive]

r? @Aatch

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[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!

[TimNN]

Since there's no CI here I'll note that this passed bootstrap.py --step check --stage 2 on x86_64-unknown-linux-gnu.

[eddyb]

Why not add an alignment argument to build_const_struct?

[TimNN]

I saw how the padding was handled for layout::General { discr: d, ref variants, .. } => and just applied the same logic to the other two call sites of build_const_struct. I suppose just passing the alignment to build_const_struct is the cleaner solution. I'll see about fixing that.

[eddyb]

@nikomatsakis @brson r=me

[alexcrichton]

Merging as #37281 is beta-accepted as well. Thanks so much for being on top of this @TimNN!