traits.md

Trait solving (new-style)

🚧 This chapter describes "new-style" trait solving. This is still in the process of being implemented; this chapter serves as a kind of in-progress design document. If you would prefer to read about how the current trait solver works, check out this other chapter. (By the way, if you would like to help in hacking on the new solver, you will find instructions for getting involved in the Traits Working Group tracking issue.) 🚧

Trait solving is based around a few key ideas:

• Lowering to logic, which expresses Rust traits in terms of standard logical terms.
  • The goals and clauses chapter describes the precise form of rules we use, and lowering rules gives the complete set of lowering rules in a more reference-like form.
• Canonical queries, which allow us to solve trait problems (like "is Foo implemented for the type Bar?") once, and then apply that same result independently in many different inference contexts.
• Lazy normalization, which is the technique we use to accommodate associated types when figuring out whether types are equal.
• Region constraints, which are accumulated during trait solving but mostly ignored. This means that trait solving effectively ignores the precise regions involved, always -- but we still remember the constraints on them so that those constraints can be checked by thet type checker.

Note: this is not a complete list of topics. See the sidebar for more.