Add a validity testcase for uninhabited variants

[meithecatte]

Found this testcase in a random unmerged branch of mine. Looks like I wrote it as part of my work on rust-lang/rust#138961, and then forgot to submit it when plans wrt the scope of that PR have changed.

I have double checked by grepping for the error message and it looks like we still don't have any similar tests in miri.

[rustbot]

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[RalfJung]

I have double checked by grepping for the error message and it looks like we still don't have any similar tests in miri.

Yeah, we probably mostly test this via const-eval tests... but can't hurt to also have a Miri test for this.

[RalfJung]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[meithecatte]

So, having looked into this more closely, there's already a quite comprehensive rustc test for this at tests/ui/consts/const-eval/ub-enum.rs, not sure if duplicating it makes sense...

[RalfJung]

Having at least a sort of smoke test also in Miri does make sense I think.

[meithecatte]

Okay, I've switched it over to a transmute and added a comment to note where the more comprehensive tests can be found.

@rustbot ready

[RalfJung]

Thanks!