interpreter: improve comments and error message in mir_assign_valid_types

[RalfJung]

I looked at this while debugging #155477, but this makes no progress on that issue.

[rustbot]

Some changes occurred to the CTFE machinery

cc @oli-obk, @lcnr

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

[rustbot]

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[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
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Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[RalfJung]

Strangely, codegen does not seem to see the same types as the interpreter / Miri. I can't reproduce the ICE there.

[RalfJung]

Ah, transmute is implemented differently in codegen so we're not asking the mir_assign_valid_types question as part of the transmute. So there's not really a place outside the interpreter that could hit #155477.

It could still be worth checking mir_assign_valid_types during codegen, as a sanity check? I can also remove the 2nd commit again if you prefer. (But note that this does resolve an existing FIXME saying we need to find a better place for these utility functions.)

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[RalfJung]

Given that relate_types is apparently buggy I think maybe we shouldn't make it more widely available then. ;) So I removed the 2nd commit.