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| <h2 id="significant-changes-and-quirks"><a class="header" href="#significant-changes-and-quirks">Significant changes and quirks</a></h2> |
| <p>While some of the items below are already mentioned separately, this page tracks the |
| main changes from the old trait system implementation. This also mentions some ways |
| in which the solver significantly diverges from an idealized implementation. This |
| document simplifies and ignores edge cases. It is recommended to add an implicit |
| "mostly" to each statement.</p> |
| <h3 id="canonicalization"><a class="header" href="#canonicalization">Canonicalization</a></h3> |
| <p>The new solver uses <a href="./canonicalization.html">canonicalization</a> when evaluating nested goals. In case there |
| are possibly multiple candidates, each candidate is eagerly canonicalized. We then |
| attempt to merge their canonical responses. This differs from the old implementation |
| which does not use canonicalization inside of the trait system.</p> |
| <p>This has a some major impacts on the design of both solvers. Without using |
| canonicalization to stash the constraints of candidates, candidate selection has |
| to discard the constraints of each candidate, only applying the constraints by |
| reevaluating the candidate after it has been selected: <a href="https://github.com/rust-lang/rust/blob/47dd709bedda8127e8daec33327e0a9d0cdae845/compiler/rustc_trait_selection/src/traits/select/mod.rs#L1232-L1237">source</a>. |
| Without canonicalization it is also not possible to cache the inference constraints |
| from evaluating a goal. This causes the old implementation to have two systems: |
| <em>evaluate</em> and <em>fulfill</em>. <em>Evaluation</em> is cached, does not apply inference constraints |
| and is used when selecting candidates. <em>Fulfillment</em> applies inference and region |
| constraints is not cached and applies inference constraints.</p> |
| <p>By using canonicalization, the new implementation is able to merge <em>evaluation</em> and |
| <em>fulfillment</em>, avoiding complexity and subtle differences in behavior. It greatly |
| simplifies caching and prevents accidentally relying on untracked information. |
| It allows us to avoid reevaluating candidates after selection and enables us to merge |
| the responses of multiple candidates. However, canonicalizing goals during evaluation |
| forces the new implementation to use a fixpoint algorithm when encountering cycles |
| during trait solving: <a href="https://github.com/rust-lang/rust/blob/df8ac8f1d74cffb96a93ae702d16e224f5b9ee8c/compiler/rustc_trait_selection/src/solve/search_graph.rs#L382-L387">source</a>.</p> |
| <h3 id="deferred-alias-equality"><a class="header" href="#deferred-alias-equality">Deferred alias equality</a></h3> |
| <p>The new implementation emits <code>AliasRelate</code> goals when relating aliases while the |
| old implementation structurally relates the aliases instead. This enables the |
| new solver to stall equality until it is able to normalize the related aliases.</p> |
| <p>The behavior of the old solver is incomplete and relies on eager normalization |
| which replaces ambiguous aliases with inference variables. As this is |
| not possible for aliases containing bound variables, the old implementation does |
| not handle aliases inside of binders correctly, e.g. <a href="https://github.com/rust-lang/rust/issues/102048">#102048</a>. See the chapter on |
| <a href="../normalization.html">normalization</a> for more details.</p> |
| <h3 id="eagerly-evaluating-nested-goals"><a class="header" href="#eagerly-evaluating-nested-goals">Eagerly evaluating nested goals</a></h3> |
| <p>The new implementation eagerly handles nested goals instead of returning |
| them to the caller. The old implementation does both. In evaluation nested |
| goals <a href="https://github.com/rust-lang/rust/blob/master/compiler/rustc_trait_selection/src/traits/select/mod.rs#L1271-L1277">are eagerly handled</a>, while fulfillment simply |
| <a href="https://github.com/rust-lang/rust/blob/df8ac8f1d74cffb96a93ae702d16e224f5b9ee8c/compiler/rustc_trait_selection/src/traits/fulfill.rs#L708-L712">returns them for later processing</a>.</p> |
| <p>As the new implementation has to be able to eagerly handle nested goals for |
| candidate selection, always doing so reduces complexity. It may also enable |
| us to merge more candidates in the future.</p> |
| <h3 id="nested-goals-are-evaluated-until-reaching-a-fixpoint"><a class="header" href="#nested-goals-are-evaluated-until-reaching-a-fixpoint">Nested goals are evaluated until reaching a fixpoint</a></h3> |
| <p>The new implementation always evaluates goals in a loop until reaching a fixpoint. |
| The old implementation only does so in <em>fulfillment</em>, but not in <em>evaluation</em>. |
| Always doing so strengthens inference and is reduces the order dependence of |
| the trait solver. See <a href="https://github.com/rust-lang/trait-system-refactor-initiative/issues/102">trait-system-refactor-initiative#102</a>.</p> |
| <h3 id="proof-trees-and-providing-diagnostics-information"><a class="header" href="#proof-trees-and-providing-diagnostics-information">Proof trees and providing diagnostics information</a></h3> |
| <p>The new implementation does not track diagnostics information directly, |
| instead providing <a href="./proof-trees.html">proof trees</a> which are used to lazily compute the |
| relevant information. This is not yet fully fleshed out and somewhat hacky. |
| The goal is to avoid tracking this information in the happy path to improve |
| performance and to avoid accidentally relying on diagnostics data for behavior.</p> |
| <h2 id="major-quirks-of-the-new-implementation"><a class="header" href="#major-quirks-of-the-new-implementation">Major quirks of the new implementation</a></h2> |
| <h3 id="hiding-impls-if-there-are-any-env-candidates"><a class="header" href="#hiding-impls-if-there-are-any-env-candidates">Hiding impls if there are any env candidates</a></h3> |
| <p>If there is at least one <code>ParamEnv</code> or <code>AliasBound</code> candidate to prove |
| some <code>Trait</code> goal, we discard all impl candidates for both <code>Trait</code> and |
| <code>Projection</code> goals: <a href="https://github.com/rust-lang/rust/blob/03994e498df79aa1f97f7bbcfd52d57c8e865049/compiler/rustc_trait_selection/src/solve/assembly/mod.rs#L785-L789">source</a>. This prevents users from |
| using an impl which is entirely covered by a <code>where</code>-bound, matching the |
| behavior of the old implementation and avoiding some weird errors, |
| e.g. <a href="https://github.com/rust-lang/trait-system-refactor-initiative/issues/76">trait-system-refactor-initiative#76</a>.</p> |
| <h3 id="normalizesto-goals-are-a-function"><a class="header" href="#normalizesto-goals-are-a-function"><code>NormalizesTo</code> goals are a function</a></h3> |
| <p>See the <a href="../normalization.html">normalization</a> chapter. We replace the expected term with an unconstrained |
| inference variable before computing <code>NormalizesTo</code> goals to prevent it from affecting |
| normalization. This means that <code>NormalizesTo</code> goals are handled somewhat differently |
| from all other goal kinds and need some additional solver support. Most notably, |
| their ambiguous nested goals are returned to the caller which then evaluates them. |
| See <a href="https://github.com/rust-lang/rust/pull/122687">#122687</a> for more details.</p> |
| |
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