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