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Don't build `ParamEnv` and do trait solving in `ItemCtxt`s when lowering IATs Fixes rust-lang/rust#108491 Fixes rust-lang/rust#125879 This was due to updating inhabited predicate stuff which I had to do to make constructing ADTs with IATs in fields not ICE Fixes rust-lang/rust#136678 (but no test added, I don't rly care about weird IAT edge cases under GCE) Fixes rust-lang/rust#138131 Avoids doing "fully correct" candidate selection for IATs during hir ty lowering when in item signatures as it almost always leads to a query cycle from trying to build a `ParamEnv`. I replaced it with a use `DeepRejectCtxt` which should be able to handle this kind of conservative "could these types unify" while in a context where we don't want to do type equality. This is a relatively simple scheme and should be forwards compatible with doing something more complex/powerful. I'm not really sure how this interacts with rust-lang/rust#126651, though I'm also not really sure its super important to support projecting IATs from IAT self types given we don't even support `T::Assoc::Other` for trait-associated types so didn't give much thought to how this might fit in with that. r? `@compiler-errors` cc `@fmease`
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.