16de1fddee
implied_bounds: deal with inference vars fixes #101951 while computing implied bounds for `<<T as ConstructionFirm>::Builder as BuilderFn<'_>>::Output` normalization replaces a projection with an inference var (adding a `Projection` obligation). Until we prove that obligation, this inference var remains unknown, which caused us to miss an implied bound necessary to prove that the unnormalized projection from the trait method signature is wf. r? types
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.