Auto merge of #111775 - compiler-errors:triple-check, r=Nilstrieb

Add extra debug assertions for equality for Adt/Variant/FieldDef

Would've made it easier to both catch and test rust-lang/rust#111494. Maybe not worth it, since it does mean that the compiler is doing extra work when debug-assertions are enabled, but also that's what debug assertions are for :^)

This is a revival of #111523 because I think I pushed an empty branch and bors got a bit too excited it closed the PR.

Author: bors