implement the skeleton of the updated trait solver by lcnr · Pull Request #105661 · rust-lang/rust (original) (raw)

cc @rust-lang/initiative-trait-system-refactor

This is mostly following the architecture discussed in the types team meetup.

After discussing the desired changes for the trait solver, we encountered cyclic dependencies between them. Most notably between changing evaluate to be canonical and returning inference constraints. We cannot canonicalize evaluate without returning inference constraints due to coinductive cycles. However, caching inference constraints also relies on canonicalization. Implementing both of these changes at once in-place is not feasible.

This somewhat closely mirrors the current evaluate implementation with the following notable differences:

• it moves project into the core solver, allowing us to correctly deal with coinductive projections (will be required for implied bounds, perfect derive)
• it changes trait solver overflow to be non-fatal (required to backcompat breakage from changes to the iteration order of nested goals, deferred projection equality, generally very useful)
• it returns inference constraints and canonicalizes inputs and outputs (required for a lot things, most notably merging fulfill and evaluate, and deferred projection equality)
• it is implemented to work with lazy normalization

A lot of things aren't yet implemented, but the remaining FIXMEs should all be fairly self-contained and parallelizable. If the architecture looks correct and is what we want here, I would like to quickly merge this and then split the work.

r? @compiler-errors / @rust-lang/types :3