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| <h1 id="opaque-types-in-the-new-solver"><a class="header" href="#opaque-types-in-the-new-solver">Opaque types in the new solver</a></h1> |
| <p>The way <a href="../opaque-types-type-alias-impl-trait.html">opaque types</a> are handled in the new solver differs from the old implementation. |
| This should be a self-contained explanation of the behavior in the new solver.</p> |
| <h2 id="opaques-are-alias-types"><a class="header" href="#opaques-are-alias-types">opaques are alias types</a></h2> |
| <p>Opaque types are treated the same as other aliases, most notabily associated types, |
| whenever possible. There should be as few divergences in behavior as possible.</p> |
| <p>This is desirable, as they are very similar to other alias types, in that they can be |
| normalized to their hidden type and also have the same requirements for completeness. |
| Treating them this way also reduces the complexity of the type system by sharing code. |
| Having to deal with opaque types separately results in more complex rules and new kinds |
| of interactions. As we need to treat them like other aliases in the implicit-negative |
| mode, having significant differences between modes also adds complexity.</p> |
| <p><em>open question: is there an alternative approach here, maybe by treating them more like rigid |
| types with more limited places to instantiate them? they would still have to be ordinary |
| aliases during coherence</em></p> |
| <h3 id="normalizes-to-for-opaques"><a class="header" href="#normalizes-to-for-opaques"><code>normalizes-to</code> for opaques</a></h3> |
| <p><a href="https://github.com/rust-lang/rust/blob/384d26fc7e3bdd7687cc17b2662b091f6017ec2a/compiler/rustc_trait_selection/src/solve/normalizes_to/opaque_types.rs#L13">source</a></p> |
| <p><code>normalizes-to</code> is used to define the one-step normalization behavior for aliases in the new |
| solver: <code><<T as IdInner>::Assoc as IdOuter>::Assoc</code> first normalizes to <code><T as IdInner>::Assoc</code> |
| which then normalizes to <code>T</code>. It takes both the <code>AliasTy</code> which is getting normalized and the |
| expected <code>Term</code>. To use <code>normalizes-to</code> for actual normalization, the expected term can simply |
| be an unconstrained inference variable.</p> |
| <p>For opaque types in the defining scope and in the implicit-negative coherence mode, this is |
| always done in two steps. Outside of the defining scope <code>normalizes-to</code> for opaques always |
| returns <code>Err(NoSolution)</code>.</p> |
| <p>We start by trying to assign the expected type as a hidden type.</p> |
| <p>In the implicit-negative coherence mode, this currently always results in ambiguity without |
| interacting with the opaque types storage. We could instead add allow 'defining' all opaque types, |
| discarding their inferred types at the end, changing the behavior of an opaque type is used |
| multiple times during coherence: <a href="https://github.com/rust-lang/rust/blob/HEAD/tests/ui/type-alias-impl-trait/coherence/coherence_different_hidden_ty.rs">example</a></p> |
| <p>Inside of the defining scope we start by checking whether the type and const arguments of the |
| opaque are all placeholders: <a href="https://github.com/rust-lang/rust/blob/384d26fc7e3bdd7687cc17b2662b091f6017ec2a/compiler/rustc_trait_selection/src/solve/normalizes_to/opaque_types.rs#L33">source</a>. If this check is ambiguous, |
| return ambiguity, if it fails, return <code>Err(NoSolution)</code>. This check ignores regions which are |
| only checked at the end of borrowck. If it succeeds, continue.</p> |
| <p>We then check whether we're able to <em>semantically</em> unify the generic arguments of the opaque |
| with the arguments of any opaque type already in the opaque types storage. If so, we unify the |
| previously stored type with the expected type of this <code>normalizes-to</code> call: <a href="https://github.com/rust-lang/rust/blob/384d26fc7e3bdd7687cc17b2662b091f6017ec2a/compiler/rustc_trait_selection/src/solve/normalizes_to/opaque_types.rs#L51-L59">source</a><sup class="footnote-reference" id="fr-1-1"><a href="#footnote-1">1</a></sup>.</p> |
| <p>If not, we insert the expected type in the opaque types storage: <a href="https://github.com/rust-lang/rust/blob/384d26fc7e3bdd7687cc17b2662b091f6017ec2a/compiler/rustc_trait_selection/src/solve/normalizes_to/opaque_types.rs#L68">source</a><sup class="footnote-reference" id="fr-2-1"><a href="#footnote-2">2</a></sup>. |
| Finally, we check whether the item bounds of the opaque hold for the expected type: |
| <a href="https://github.com/rust-lang/rust/blob/384d26fc7e3bdd7687cc17b2662b091f6017ec2a/compiler/rustc_trait_selection/src/solve/normalizes_to/opaque_types.rs#L69-L74">source</a>.</p> |
| <h3 id="using-alias-bounds-of-normalizable-aliases"><a class="header" href="#using-alias-bounds-of-normalizable-aliases">using alias-bounds of normalizable aliases</a></h3> |
| <p>https://github.com/rust-lang/trait-system-refactor-initiative/issues/77</p> |
| <p>Using an <code>AliasBound</code> candidate for normalizable aliases is generally not possible as an |
| associated type can have stronger bounds then the resulting type when normalizing via a |
| <code>ParamEnv</code> candidate.</p> |
| <p>These candidates would change our exact normalization strategy to be user-facing. It is otherwise |
| pretty much unobservable whether we eagerly normalize. Where we normalize is something we likely |
| want to change that after removing support for the old solver, so that would be undesirable.</p> |
| <h2 id="opaque-types-can-be-defined-anywhere"><a class="header" href="#opaque-types-can-be-defined-anywhere">opaque types can be defined anywhere</a></h2> |
| <p>Opaque types in their defining-scope can be defined anywhere, whether when simply relating types |
| or in the trait solver. This removes order dependence and incompleteness. Without this the result |
| of a goal can differ due to subtle reasons, e.g. whether we try to evaluate a goal using the |
| opaque before the first defining use of the opaque.</p> |
| <h2 id="higher-ranked-opaque-types-in-their-defining-scope"><a class="header" href="#higher-ranked-opaque-types-in-their-defining-scope">higher ranked opaque types in their defining scope</a></h2> |
| <p>These are not supported and trying to define them right now should always error.</p> |
| <p>FIXME: Because looking up opaque types in the opaque type storage can now unify regions, |
| we have to eagerly check that the opaque types does not reference placeholders. We otherwise |
| end up leaking placeholders.</p> |
| <h2 id="member-constraints"><a class="header" href="#member-constraints">member constraints</a></h2> |
| <p>The handling of member constraints does not change in the new solver. See the |
| <a href="../borrow_check/region_inference/member_constraints.html">relevant existing chapter</a> for that.</p> |
| <h2 id="calling-methods-on-opaque-types"><a class="header" href="#calling-methods-on-opaque-types">calling methods on opaque types</a></h2> |
| <p>FIXME: We need to continue to support calling methods on still unconstrained |
| opaque types in their defining scope. It's unclear how to best do this.</p> |
| <pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>use std::future::Future; |
| use futures::FutureExt; |
| |
| fn go(i: usize) -> impl Future<Output = ()> + Send + 'static { |
| async move { |
| if i != 0 { |
| // This returns `impl Future<Output = ()>` in its defining scope, |
| // we don't know the concrete type of that opaque at this point. |
| // Currently treats the opaque as a known type and succeeds, but |
| // from the perspective of "easiest to soundly implement", it would |
| // be good for this to be ambiguous. |
| go(i - 1).boxed().await; |
| } |
| } |
| } |
| <span class="boring">}</span></code></pre></pre> |
| <hr> |
| <ol class="footnote-definition"><li id="footnote-1"> |
| <p>FIXME: this should ideally only result in a unique candidate given that we require the args to be placeholders and regions are always inference vars <a href="#fr-1-1">↩</a></p> |
| </li> |
| <li id="footnote-2"> |
| <p>FIXME: why do we check whether the expected type is rigid for this. <a href="#fr-2-1">↩</a></p> |
| </li> |
| </ol> |
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