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| <h1 id="canonicalization"><a class="header" href="#canonicalization">Canonicalization</a></h1> |
| <p>Canonicalization is the process of <em>isolating</em> a value from its context and is necessary |
| for global caching of goals which include inference variables.</p> |
| <p>The idea is that given the goals <code>u32: Trait<?x></code> and <code>u32: Trait<?y></code>, where <code>?x</code> and <code>?y</code> |
| are two different currently unconstrained inference variables, we should get the same result |
| for both goals. We can therefore prove <em>the canonical query</em> <code>exists<T> u32: Trait<T></code> once |
| and reuse the result.</p> |
| <p>Let's first go over the way canonical queries work and then dive into the specifics of |
| how canonicalization works.</p> |
| <h2 id="a-walkthrough-of-canonical-queries"><a class="header" href="#a-walkthrough-of-canonical-queries">A walkthrough of canonical queries</a></h2> |
| <p>To make this a bit easier, let's use the trait goal <code>u32: Trait<?x></code> as an example with the |
| assumption that the only relevant impl is <code>impl<T> Trait<Vec<T>> for u32</code>.</p> |
| <h3 id="canonicalizing-the-input"><a class="header" href="#canonicalizing-the-input">Canonicalizing the input</a></h3> |
| <p>We start by <em>canonicalizing</em> the goal, replacing inference variables with existential and |
| placeholders with universal bound variables. This would result in the <em>canonical goal</em> |
| <code>exists<T> u32: Trait<T></code>.</p> |
| <p>We remember the original values of all bound variables in the original context. Here this would |
| map <code>T</code> back to <code>?x</code>. These original values are used later on when dealing with the query |
| response.</p> |
| <p>We now call the canonical query with the canonical goal.</p> |
| <h3 id="instantiating-the-canonical-goal-inside-of-the-query"><a class="header" href="#instantiating-the-canonical-goal-inside-of-the-query">Instantiating the canonical goal inside of the query</a></h3> |
| <p>To actually try to prove the canonical goal we start by instantiating the bound variables with |
| inference variables and placeholders again.</p> |
| <p>This happens inside of the query in a completely separate <code>InferCtxt</code>. Inside of the query we |
| now have a goal <code>u32: Trait<?0></code>. We also remember which value we've used to instantiate the bound |
| variables in the canonical goal, which maps <code>T</code> to <code>?0</code>.</p> |
| <p>We now compute the goal <code>u32: Trait<?0></code> and figure out that this holds, but we've constrained |
| <code>?0</code> to <code>Vec<?1></code>. We finally convert this result to something useful to the caller.</p> |
| <h3 id="canonicalizing-the-query-response"><a class="header" href="#canonicalizing-the-query-response">Canonicalizing the query response</a></h3> |
| <p>We have to return to the caller both whether the goal holds, and the inference constraints |
| from inside of the query.</p> |
| <p>To return the inference results to the caller we canonicalize the mapping from bound variables |
| to the instantiated values in the query. This means that the query response is <code>Certainty::Yes</code> |
| and a mapping from <code>T</code> to <code>exists<U> Vec<U></code>.</p> |
| <h3 id="instantiating-the-query-response"><a class="header" href="#instantiating-the-query-response">Instantiating the query response</a></h3> |
| <p>The caller now has to apply the constraints returned by the query. For this they first |
| instantiate the bound variables of the canonical response with inference variables and |
| placeholders again, so the mapping in the response is now from <code>T</code> to <code>Vec<?z></code>.</p> |
| <p>It now equates the original value of <code>T</code> (<code>?x</code>) with the value for <code>T</code> in the |
| response (<code>Vec<?z></code>), which correctly constrains <code>?x</code> to <code>Vec<?z></code>.</p> |
| <h2 id="externalconstraints"><a class="header" href="#externalconstraints"><code>ExternalConstraints</code></a></h2> |
| <p>Computing a trait goal may not only constrain inference variables, it can also add region |
| obligations, e.g. given a goal <code>(): AOutlivesB<'a, 'b></code> we would like to return the fact that |
| <code>'a: 'b</code> has to hold.</p> |
| <p>This is done by not only returning the mapping from bound variables to the instantiated values |
| from the query but also extracting additional <code>ExternalConstraints</code> from the <code>InferCtxt</code> context |
| while building the response.</p> |
| <h2 id="how-exactly-does-canonicalization-work"><a class="header" href="#how-exactly-does-canonicalization-work">How exactly does canonicalization work</a></h2> |
| <p>TODO: link to code once the PR lands and elaborate</p> |
| <ul> |
| <li>types and consts: infer to existentially bound var, placeholder to universally bound var, |
| considering universes</li> |
| <li>generic parameters in the input get treated as placeholders in the root universe</li> |
| <li>all regions in the input get all mapped to existentially bound vars and we "uniquify" them. |
| <code>&'a (): Trait<'a></code> gets canonicalized to <code>exists<'0, '1> &'0 (): Trait<'1></code>. We do not care |
| about their universes and simply put all regions into the highest universe of the input.</li> |
| <li>in the output everything in a universe of the caller gets put into the root universe and only |
| gets its correct universe when we unify the var values with the orig values of the caller</li> |
| <li>we do not uniquify regions in the response and don't canonicalize <code>'static</code></li> |
| </ul> |
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