`~const` is allowed on structs (and other bad positions) within `const fn`.

[compiler-errors]

We now silently accept ~const Trait in illegal positions when they're nested in legal positions, because AST validation is not smart enough. I think it would be nice if we made validation more robust; perhaps moving it to ast lowering is the best thing to do.

Originally posted by @compiler-errors in #131985 (comment)