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Dana JansensandRichard Smith
6326bbdbe1
A nested designator like `.(X.X1).(Y.Y1)` results in nested ImplWitnessAccess instructions, which can produce cycles in the toolchain easily when replacing `.Self`. First, when constructing a facet type like `V:! Z where .Z1 impls (Y where .Y1 = U)` we substitute replace `.Self` in the nested facet type, and in this case we replace `.Self` with `.Z1` which contains a `.Self` of its own. This was coming from us being lazy about replacing `.Self` in an `impl as` declaration, such as `impl C as Z where .Z1 = .Self`. The self type is known there, so we can more eagerly replace `.Self` as we do in a `require impls` declaration. Then the replacement for `.Self` never comes with a `.Self` that needs to also be replaced. Any resulting `.Self` would always be the top-level one. Second, when evaluating ImplWitnessAccess, we were replacing .Self in the LHS of rewrite constraints, but the `.Self` may itself have a type that contains rewrite constraints. If one of those rewrite constraints has nested ImplWitnessAccess instructions, we evaluate the new ImplWitnessAccess, which again finds rewrite constraints to replace `.Self` in, and we repeat forever. For this one we just stop replacing .Self in the LHS of rewrite constraints. Since they are always against .Self, we can always look in the access facet's type for a value. While fixing ImplWitness access, also correct the lookup to search through the types of nested ImplWitnessAccess instructions to find a rewrite value, since it may find it at any level up to the eventual `.Self`. --------- Co-authored-by: Richard Smith <richard@metafoo.co.uk>