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| <h1 id="caching-and-subtle-considerations-therewith"><a class="header" href="#caching-and-subtle-considerations-therewith">Caching and subtle considerations therewith</a></h1> |
| <p>In general, we attempt to cache the results of trait selection. This |
| is a somewhat complex process. Part of the reason for this is that we |
| want to be able to cache results even when all the types in the trait |
| reference are not fully known. In that case, it may happen that the |
| trait selection process is also influencing type variables, so we have |
| to be able to not only cache the <em>result</em> of the selection process, |
| but <em>replay</em> its effects on the type variables.</p> |
| <h2 id="an-example"><a class="header" href="#an-example">An example</a></h2> |
| <p>The high-level idea of how the cache works is that we first replace |
| all unbound inference variables with placeholder versions. Therefore, |
| if we had a trait reference <code>usize : Foo<$t></code>, where <code>$t</code> is an unbound |
| inference variable, we might replace it with <code>usize : Foo<$0></code>, where |
| <code>$0</code> is a placeholder type. We would then look this up in the cache.</p> |
| <p>If we found a hit, the hit would tell us the immediate next step to |
| take in the selection process (e.g. apply impl #22, or apply where |
| clause <code>X : Foo<Y></code>).</p> |
| <p>On the other hand, if there is no hit, we need to go through the <a href="./resolution.html#selection">selection |
| process</a> from scratch. Suppose, we come to the conclusion that the only |
| possible impl is this one, with def-id 22:</p> |
| <pre><code class="language-rust ignore">impl Foo<isize> for usize { ... } // Impl #22</code></pre> |
| <p>We would then record in the cache <code>usize : Foo<$0> => ImplCandidate(22)</code>. Next |
| we would <a href="./resolution.html#confirmation">confirm</a> <code>ImplCandidate(22)</code>, which would (as a side-effect) unify |
| <code>$t</code> with <code>isize</code>.</p> |
| <p>Now, at some later time, we might come along and see a <code>usize : Foo<$u></code>. When replaced with a placeholder, this would yield <code>usize : Foo<$0></code>, just as |
| before, and hence the cache lookup would succeed, yielding |
| <code>ImplCandidate(22)</code>. We would confirm <code>ImplCandidate(22)</code> which would |
| (as a side-effect) unify <code>$u</code> with <code>isize</code>.</p> |
| <h2 id="where-clauses-and-the-local-vs-global-cache"><a class="header" href="#where-clauses-and-the-local-vs-global-cache">Where clauses and the local vs global cache</a></h2> |
| <p>One subtle interaction is that the results of trait lookup will vary |
| depending on what where clauses are in scope. Therefore, we actually |
| have <em>two</em> caches, a local and a global cache. The local cache is |
| attached to the <a href="../typing_parameter_envs.html"><code>ParamEnv</code></a>, and the global cache attached to the |
| <a href="../ty.html"><code>tcx</code></a>. We use the local cache whenever the result might depend on the |
| where clauses that are in scope. The determination of which cache to |
| use is done by the method <code>pick_candidate_cache</code> in <code>select.rs</code>. At |
| the moment, we use a very simple, conservative rule: if there are any |
| where-clauses in scope, then we use the local cache. We used to try |
| and draw finer-grained distinctions, but that led to a series of |
| annoying and weird bugs like <a href="https://github.com/rust-lang/rust/issues/22019">#22019</a> and <a href="https://github.com/rust-lang/rust/issues/18290">#18290</a>. This simple rule seems |
| to be pretty clearly safe and also still retains a very high hit rate |
| (~95% when compiling rustc).</p> |
| <p><strong>TODO</strong>: it looks like <code>pick_candidate_cache</code> no longer exists. In |
| general, is this section still accurate at all?</p> |
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