c067287049
`simplify_type` improvements and cursed docs the existing `TreatParams` enum pretty much mixes everything up. Not sure why this looked right to me in #94057 This also includes two changes which impact perf: - `ty::Projection` with inference vars shouldn't be treated as a rigid type, even if fully normalized - `ty::Placeholder` only unifies with itself, so actually return `Some` for them r? `@nikomatsakis`
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.