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| <main> |
| <h1 id="the-solver"><a class="header" href="#the-solver">The solver</a></h1> |
| <p>Also consider reading the documentation for <a href="https://rust-lang.github.io/chalk/book/recursive.html">the recursive solver in chalk</a> |
| as it is very similar to this implementation and also talks about limitations of this |
| approach.</p> |
| <h2 id="a-rough-walkthrough"><a class="header" href="#a-rough-walkthrough">A rough walkthrough</a></h2> |
| <p>The entry-point of the solver is <code>InferCtxtEvalExt::evaluate_root_goal</code>. This |
| function sets up the root <code>EvalCtxt</code> and then calls <code>EvalCtxt::evaluate_goal</code>, |
| to actually enter the trait solver.</p> |
| <p><code>EvalCtxt::evaluate_goal</code> handles <a href="./canonicalization.html">canonicalization</a>, caching, |
| overflow, and solver cycles. Once that is done, it creates a nested <code>EvalCtxt</code> with a |
| separate local <code>InferCtxt</code> and calls <code>EvalCtxt::compute_goal</code>, which is responsible for the |
| ‘actual solver behavior’. We match on the <code>PredicateKind</code>, delegating to a separate function |
| for each one.</p> |
| <p>For trait goals, such a <code>Vec<T>: Clone</code>, <code>EvalCtxt::compute_trait_goal</code> has |
| to collect all the possible ways this goal can be proven via |
| <code>EvalCtxt::assemble_and_evaluate_candidates</code>. Each candidate is handled in |
| a separate “probe”, to not leak inference constraints to the other candidates. |
| We then try to merge the assembled candidates via <code>EvalCtxt::merge_candidates</code>.</p> |
| <h2 id="important-concepts-and-design-patterns"><a class="header" href="#important-concepts-and-design-patterns">Important concepts and design patterns</a></h2> |
| <h3 id="evalctxtadd_goal"><a class="header" href="#evalctxtadd_goal"><code>EvalCtxt::add_goal</code></a></h3> |
| <p>To prove nested goals, we don’t directly call <code>EvalCtxt::compute_goal</code>, but instead |
| add the goal to the <code>EvalCtxt</code> with <code>EvalCtxt::all_goal</code>. We then prove all nested |
| goals together in either <code>EvalCtxt::try_evaluate_added_goals</code> or |
| <code>EvalCtxt::evaluate_added_goals_and_make_canonical_response</code>. This allows us to handle |
| inference constraints from later goals.</p> |
| <p>E.g. if we have both <code>?x: Debug</code> and <code>(): ConstrainToU8<?x></code> as nested goals, |
| then proving <code>?x: Debug</code> is initially ambiguous, but after proving <code>(): ConstrainToU8<?x></code> |
| we constrain <code>?x</code> to <code>u8</code> and proving <code>u8: Debug</code> succeeds.</p> |
| <h3 id="matching-on-tykind"><a class="header" href="#matching-on-tykind">Matching on <code>TyKind</code></a></h3> |
| <p>We lazily normalize types in the solver, so we always have to assume that any types |
| and constants are potentially unnormalized. This means that matching on <code>TyKind</code> can easily |
| be incorrect.</p> |
| <p>We handle normalization in two different ways. When proving <code>Trait</code> goals when normalizing |
| associated types, we separately assemble candidates depending on whether they structurally |
| match the self type. Candidates which match on the self type are handled in |
| <code>EvalCtxt::assemble_candidates_via_self_ty</code> which recurses via |
| <code>EvalCtxt::assemble_candidates_after_normalizing_self_ty</code>, which normalizes the self type |
| by one level. In all other cases we have to match on a <code>TyKind</code> we first use |
| <code>EvalCtxt::try_normalize_ty</code> to normalize the type as much as possible.</p> |
| <h3 id="higher-ranked-goals"><a class="header" href="#higher-ranked-goals">Higher ranked goals</a></h3> |
| <p>In case the goal is higher-ranked, e.g. <code>for<'a> F: FnOnce(&'a ())</code>, <code>EvalCtxt::compute_goal</code> |
| eagerly instantiates <code>'a</code> with a placeholder and then recursively proves |
| <code>F: FnOnce(&'!a ())</code> as a nested goal.</p> |
| <h3 id="dealing-with-choice"><a class="header" href="#dealing-with-choice">Dealing with choice</a></h3> |
| <p>Some goals can be proven in multiple ways. In these cases we try each option in |
| a separate “probe” and then attempt to merge the resulting responses by using |
| <code>EvalCtxt::try_merge_responses</code>. If merging the responses fails, we use |
| <code>EvalCtxt::flounder</code> instead, returning ambiguity. For some goals, we try to |
| incompletely prefer some choices over others in case <code>EvalCtxt::try_merge_responses</code> |
| fails.</p> |
| <h2 id="learning-more"><a class="header" href="#learning-more">Learning more</a></h2> |
| <p>The solver should be fairly self-contained. I hope that the above information provides a |
| good foundation when looking at the code itself. Please reach out on Zulip if you get stuck |
| while doing so or there are some quirks and design decisions which were unclear and deserve |
| better comments or should be mentioned here.</p> |
| |
| </main> |
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