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| <h1 id="chalk-based-trait-solving"><a class="header" href="#chalk-based-trait-solving">Chalk-based trait solving</a></h1> |
| <p><a href="https://github.com/rust-lang/chalk">Chalk</a> is an experimental trait solver for Rust that is |
| (as of <!-- date-check --> May 2022) under development by the <a href="https://github.com/rust-lang/types-team">Types team</a>. |
| Its goal is to enable a lot of trait system features and bug fixes |
| that are hard to implement (e.g. GATs or specialization). If you would like to |
| help in hacking on the new solver, drop by on the rust-lang Zulip in the <a href="https://rust-lang.zulipchat.com/#narrow/stream/144729-t-types"><code>#t-types</code></a> |
| stream and say hello!</p> |
| <p>The new-style trait solver is based on the work done in <a href="https://github.com/rust-lang/chalk">chalk</a>. Chalk |
| recasts Rust's trait system explicitly in terms of logic programming. It does |
| this by "lowering" Rust code into a kind of logic program we can then execute |
| queries against.</p> |
| <p>The key observation here is that the Rust trait system is basically a |
| kind of logic, and it can be mapped onto standard logical inference |
| rules. We can then look for solutions to those inference rules in a |
| very similar fashion to how e.g. a <a href="https://en.wikipedia.org/wiki/Prolog">Prolog</a> solver works. It turns out |
| that we can't <em>quite</em> use Prolog rules (also called Horn clauses) but |
| rather need a somewhat more expressive variant.</p> |
| <p>You can read more about chalk itself in the |
| <a href="https://rust-lang.github.io/chalk/book/">Chalk book</a> section.</p> |
| <h2 id="ongoing-work"><a class="header" href="#ongoing-work">Ongoing work</a></h2> |
| <p>The design of the new-style trait solving happens in two places:</p> |
| <p><strong>chalk</strong>. The <a href="https://github.com/rust-lang/chalk">chalk</a> repository is where we experiment with new ideas |
| and designs for the trait system.</p> |
| <p><strong>rustc</strong>. Once we are happy with the logical rules, we proceed to |
| implementing them in rustc. We map our struct, trait, and impl declarations |
| into logical inference rules in the lowering module in rustc.</p> |
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