Next-generation trait solver

Summary

Continue work towards the stabilization of -Znext-solver=globally, collecting and resolving remaining blockers. Extend its use in lints and rustdoc.

Motivation

The next-generation trait solver is intended to fully replace the existing type system components responsible for proving trait bounds, normalizing associated types, and much more. This should fix many long-standing (soundness) bugs, enable future type system improvements, and improve compile-times. It's tracking issue is #107374(https://github.com/rust-lang/rust/issues/107374).

The status quo

There are multiple type system unsoundnesses blocked on the next-generation trait solver: project board. Desirable features such as coinductive trait semantics and perfect derive, where-bounds on binders, and better handling of higher-ranked bounds and types are also stalled due to shortcomings of the existing implementation.

Fixing these issues in the existing implementation is prohibitively difficult as the required changes are interconnected and require major changes to the underlying structure of the trait solver. The Types Team therefore decided to rewrite the trait solver in-tree, and has been working on it since EOY 2022.

The next six months

• finish and merge in-flight opaque types changes: #139587(https://github.com/rust-lang/rust/pull/139587) and #140497(https://github.com/rust-lang/rust/pull/140497)
  • decide and document inference guidance for unconstrained opaques in the defining scope
• continue work on performance
  • fix last known exponential slowdown when reevaluating goals due to changed provisional results
  • get all benchmarks to be neutral or improvements
• fix remaining minor issues from user reports, crater, and our test suite
• move additional lints and rustdoc to use the new solver by default
• document the new solver and work on the stabilization report
• publicly ask for testing of -Znext-solver=globally

The "shiny future" we are working towards

• we are able to remove the existing trait solver implementation and significantly cleanup the type system in general, e.g. removing most normalize in the caller by handling unnormalized types in the trait system
• all remaining type system unsoundnesses are fixed
• many future type system improvements are unblocked and get implemented
• the type system is more performant, resulting in better compile times

Design axioms

In order of importance, the next-generation trait solver should be:

• sound: the new trait solver is sound and its design enables us to fix all known type system unsoundnesses
• backwards-compatible: the breakage caused by the switch to the new solver should be minimal
• maintainable: the implementation is maintainable, extensible, and approachable to new contributors
• performant: the implementation is efficient, improving compile-times

Ownership and team asks

Owner: lcnr

Add'l implementation work: Michael Goulet

Task	Owner(s) or team(s)	Notes
Discussion and moral support	types
Implementation	lcnr, Michael Goulet, Boxy
Standard reviews	types
FCP decision(s)	types	for necessary refactorings

Support needed from the project

• Types team
  • review design decisions
  • provide technical feedback and suggestion

Outputs and milestones

See next few steps :3 while we don't expect to fully stabilize the new solver this year, we expect to get quite close.

Frequently asked questions

What do I do with this space?

This is a good place to elaborate on your reasoning above -- for example, why did you put the design axioms in the order that you did? It's also a good place to put the answers to any questions that come up during discussion. The expectation is that this FAQ section will grow as the goal is discussed and eventually should contain a complete summary of the points raised along the way.