Separate clauses and goals from the Predicate type

[oli-obk]

Split up the PredicateKind type into Goal and Clauses similar (or exactly :D) like what chalk does. The TLDR is that ParamEnv only contains a small set of PrecicateKind variants and we want to encode this in the type system by only returning Clauses from it.

This projection has been accepted as an MCP: rust-lang/compiler-team#531

Implementation steps

• [SP] Split out Trait, TypeOutlives, RegionOutlives and Projection into Clause enum: #104846
• [SP] #104911 inferred_outlives_crate returns predicates, but they are always only TypeOutlives or RegionOutlives
  • may kill perf, so let's see if it works to just return ty::Clause where it now returns ty::Predicate and build the predicates at the caller site of inferred_outlives_of
• doing the same thing for explicit_predicates_of is gonna be a bit more involved
• Improve perf of #104846
• start with gather_explicit_predicates_of and convert at the caller sites
• Bounds::predicates should return an iterator over Clauses (Make Bound::predicates use Clause #112734)

Follow-up work after #112938:

• Investigate making TypeFoldable<TyCtxt<'tcx>> for ty::Clause<'tcx> implementation less weird:

  rust/compiler/rustc_middle/src/ty/structural_impls.rs

  Line 672 in 2efe091

  .expect("no sensible folder would do this"))

• Clean up the elaborator since it should only be emitting child clauses, not predicates
• Rename identifiers like pred and predicates to clause if they're actually clauses around the codebase
• Validate that all of the ToPredicate impls are acutally still needed, or prune them if they're not

leave a comment when you start working on one of these steps so that no one else duplicates work.

[spastorino]

@oli-obk thanks for opening this, I'd be happy to review and/or help/mentor if needed.

@obeis feel free to reach out on Zulip.

[GentBinaku]

@rustbot claim

[BoxyUwU]

PredicateKind::ConstEvaluatable and PredicateKind::WellFormed can all appear in the param_env/predicates_of so should be moved to Clause as well (PredicateKind::ConstEquate might too although we might also end up removing that so 🤷‍♀️)

[spastorino]

@GentBinaku are you still actively working on this?