Don't ICE when trying to copy unsized value in const prop

[compiler-errors]

When we have a trivially false where-clause predicate like Self: Sized where Self = dyn Trait, we sometimes don't throw an error during typeck for an illegal operation such as copying an unsized type.

This, unfortunately, cannot be made into an error (at least not without some migration -- see #95611 for example), but we should at least not ICE, since this function will never actually be reachable from main, for example.

r? @RalfJung since I think you added these assertions? but feel free to reassign.

Fixes #102553

[rustbot]

Some changes occurred to the CTFE / Miri engine

cc @rust-lang/miri

[oli-obk]

I think #101168 will just take care of this, but that's a bit far off to being merged. Maybe we can avoid this function from being called at all from const prop by checking something in the const propagator instead of modifying the mir interpreter?

[RalfJung]

The better fix would be to change ConstProp to not hit this part of the interpreter. I am not a fan of compromising the interpreter to account for all the ways in which ConstProp is misusing its APIs...

But given that we already have SizeOfUnsizedType I guess this is okay. Please add FIXME comments explaining that this is only needed because of ConstProp misusing interpreter APIs.

[rustbot]

Some changes occurred in src/tools/cargo

cc @ehuss

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[compiler-errors]

Oops, updated submodules by accident. 🤦

[compiler-errors]

@oli-obk: This isn't fixed by #101168, or at least the draft version that's in that PR right now. We could alternatively check for any trivial predicates in a function's param-env that look like T: Sized where the layout of T is known to be unsized, and skip const-prop if so. Not sure if all that extra work is worth it, compared to just bailing during const eval like this PR is doing.

[oli-obk]

@bors r+ rollup

[bors]

📌 Commit c7d1ec0 has been approved by oli-obk

It is now in the queue for this repository.