Avoid relating erased regions in valtree field types

[amirHdev]

Fixes #159688

[rustbot]

changes to the core type system

cc @lcnr

[rustbot]

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[lcnr]

cc @rust-lang/project-const-generics this feels wrong. We should not encounter erased regions inside of the type system. cc https://rust-lang.zulipchat.com/#narrow/channel/618216-t-types.2Fcall-for-participation/topic/adt_const_params.3A.20unnormalized.20type.20of.20valtree/with/613355575 which feels related

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[lcnr]

The type used for const values should be the non-erased type of the expected result type I think. Feel free to start a discussion about this on zulip 🤔 not fully sure

[BoxyUwU]

r? me

[amirHdev]

cc @rust-lang/project-const-generics this feels wrong. We should not encounter erased regions inside of the type system. cc https://rust-lang.zulipchat.com/#narrow/channel/618216-t-types.2Fcall-for-participation/topic/adt_const_params.3A.20unnormalized.20type.20of.20valtree/with/613355575 which feels related

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I tested the #159688 regression on top of #160526 without the wf.rs change from this patch It still ICEs with:
cannot relate region: LUB('static, '{erased})
#160526 doesn't appear to cover this erased-region case

[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.