d3df8ff851
Properly deal with weak alias types as self types of impls Fixes #114216. Fixes #116100. Not super happy about the two ad hoc “normalization” implementations for weak alias types: 1. In `inherent_impls`: The “peeling”, normalization to [“WHNF”][whnf]: Semantically that's exactly what we want (neither proper normalization nor shallow normalization would be correct here). Basically a weak alias type is “nominal” (well...^^) if the WHNF is nominal. [#97974](https://github.com/rust-lang/rust/pull/97974) followed the same approach. 2. In `constrained_generic_params`: Generic parameters are constrained by a weak alias type if the corresp. “normalized” type constrains them (where we only normalize *weak* alias types not arbitrary ones). Weak alias types are injective if the corresp. “normalized” type is injective. Both have ad hoc overflow detection mechanisms. **Coherence** is handled in #117164. r? `@oli-obk` or types [whnf]: https://en.wikipedia.org/wiki/Lambda_calculus_definition#Weak_head_normal_form |
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For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.