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| <h1 id="effects-and-const-condition-checking"><a class="header" href="#effects-and-const-condition-checking">Effects and const condition checking</a></h1> |
| <h2 id="the-hosteffect-predicate"><a class="header" href="#the-hosteffect-predicate">The <code>HostEffect</code> predicate</a></h2> |
| <p><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/predicate/struct.HostEffectPredicate.html"><code>HostEffectPredicate</code></a>s are a kind of predicate from <code>~const Tr</code> or <code>const Tr</code> |
| bounds. It has a trait reference, and a <code>constness</code> which could be <code>Maybe</code> or |
| <code>Const</code> depending on the bound. Because <code>~const Tr</code>, or rather <code>Maybe</code> bounds |
| apply differently based on whichever contexts they are in, they have different |
| behavior than normal bounds. Where normal trait bounds on a function such as |
| <code>T: Tr</code> are collected within the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.predicates_of"><code>predicates_of</code></a> query to be proven when a |
| function is called and to be assumed within the function, bounds such as |
| <code>T: ~const Tr</code> will behave as a normal trait bound and add <code>T: Tr</code> to the result |
| from <code>predicates_of</code>, but also adds a <code>HostEffectPredicate</code> to the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.const_conditions"><code>const_conditions</code></a> query.</p> |
| <p>On the other hand, <code>T: const Tr</code> bounds do not change meaning across contexts, |
| therefore they will result in <code>HostEffect(T: Tr, const)</code> being added to |
| <code>predicates_of</code>, and not <code>const_conditions</code>.</p> |
| <h2 id="the-const_conditions-query"><a class="header" href="#the-const_conditions-query">The <code>const_conditions</code> query</a></h2> |
| <p><code>predicates_of</code> represents a set of predicates that need to be proven to use an |
| item. For example, to use <code>foo</code> in the example below:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>fn foo<T>() where T: Default {} |
| <span class="boring">}</span></code></pre> |
| <p>We must be able to prove that <code>T</code> implements <code>Default</code>. In a similar vein, |
| <code>const_conditions</code> represents a set of predicates that need to be proven to use |
| an item <em>in const contexts</em>. If we adjust the example above to use <code>const</code> trait |
| bounds:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>const fn foo<T>() where T: ~const Default {} |
| <span class="boring">}</span></code></pre> |
| <p>Then <code>foo</code> would get a <code>HostEffect(T: Default, maybe)</code> in the <code>const_conditions</code> |
| query, suggesting that in order to call <code>foo</code> from const contexts, one must |
| prove that <code>T</code> has a const implementation of <code>Default</code>.</p> |
| <h2 id="enforcement-of-const_conditions"><a class="header" href="#enforcement-of-const_conditions">Enforcement of <code>const_conditions</code></a></h2> |
| <p><code>const_conditions</code> are currently checked in various places.</p> |
| <p>Every call in HIR from a const context (which includes <code>const fn</code> and <code>const</code> |
| items) will check that <code>const_conditions</code> of the function we are calling hold. |
| This is done in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/fn_ctxt/struct.FnCtxt.html#method.enforce_context_effects"><code>FnCtxt::enforce_context_effects</code></a>. Note that we don’t check |
| if the function is only referred to but not called, as the following code needs |
| to compile:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>const fn hi<T: ~const Default>() -> T { |
| T::default() |
| } |
| const X: fn() -> u32 = hi::<u32>; |
| <span class="boring">}</span></code></pre> |
| <p>For a trait <code>impl</code> to be well-formed, we must be able to prove the |
| <code>const_conditions</code> of the trait from the <code>impl</code>’s environment. This is checked |
| in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_analysis/check/wfcheck/fn.check_impl.html"><code>wfcheck::check_impl</code></a>.</p> |
| <p>Here’s an example:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>const trait Bar {} |
| const trait Foo: ~const Bar {} |
| // `const_conditions` contains `HostEffect(Self: Bar, maybe)` |
| |
| impl const Bar for () {} |
| impl const Foo for () {} |
| // ^ here we check `const_conditions` for the impl to be well-formed |
| <span class="boring">}</span></code></pre> |
| <p>Methods of trait impls must not have stricter bounds than the method of the |
| trait that they are implementing. To check that the methods are compatible, a |
| hybrid environment is constructed with the predicates of the <code>impl</code> plus the |
| predicates of the trait method, and we attempt to prove the predicates of the |
| impl method. We do the same for <code>const_conditions</code>:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>const trait Foo { |
| fn hi<T: ~const Default>(); |
| } |
| |
| impl<T: ~const Clone> Foo for Vec<T> { |
| fn hi<T: ~const PartialEq>(); |
| // ^ we can't prove `T: ~const PartialEq` given `T: ~const Clone` and |
| // `T: ~const Default`, therefore we know that the method on the impl |
| // is stricter than the method on the trait. |
| } |
| <span class="boring">}</span></code></pre> |
| <p>These checks are done in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_analysis/check/compare_impl_item/fn.compare_method_predicate_entailment.html"><code>compare_method_predicate_entailment</code></a>. A similar |
| function that does the same check for associated types is called |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_analysis/check/compare_impl_item/fn.compare_type_predicate_entailment.html"><code>compare_type_predicate_entailment</code></a>. Both of these need to consider |
| <code>const_conditions</code> when in const contexts.</p> |
| <p>In MIR, as part of const checking, <code>const_conditions</code> of items that are called |
| are revalidated again in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/check_consts/check/struct.Checker.html#method.revalidate_conditional_constness"><code>Checker::revalidate_conditional_constness</code></a>.</p> |
| <h2 id="explicit_implied_const_bounds-on-associated-types-and-traits"><a class="header" href="#explicit_implied_const_bounds-on-associated-types-and-traits"><code>explicit_implied_const_bounds</code> on associated types and traits</a></h2> |
| <p>Bounds on associated types, opaque types, and supertraits such as</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>trait Foo: ~const PartialEq { |
| type X: ~const PartialEq; |
| } |
| |
| fn foo() -> impl ~const PartialEq { |
| // ^ unimplemented syntax |
| } |
| <span class="boring">}</span></code></pre> |
| <p>Have their bounds represented differently. Unlike <code>const_conditions</code> which need |
| to be proved for callers, and can be assumed inside the definition (e.g. trait |
| bounds on functions), these bounds need to be proved at definition (at the impl, |
| or when returning the opaque) but can be assumed for callers. The non-const |
| equivalent of these bounds are called <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.explicit_item_bounds"><code>explicit_item_bounds</code></a>.</p> |
| <p>These bounds are checked in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_analysis/check/compare_impl_item/fn.check_type_bounds.html"><code>compare_impl_item::check_type_bounds</code></a> for HIR |
| typeck, <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_trait_selection/traits/effects/fn.evaluate_host_effect_from_item_bounds.html"><code>evaluate_host_effect_from_item_bounds</code></a> in the old solver and |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_next_trait_solver/solve/assembly/trait.GoalKind.html#tymethod.consider_additional_alias_assumptions"><code>consider_additional_alias_assumptions</code></a> in the new solver.</p> |
| <h2 id="proving-hosteffectpredicates"><a class="header" href="#proving-hosteffectpredicates">Proving <code>HostEffectPredicate</code>s</a></h2> |
| <p><code>HostEffectPredicate</code>s are implemented both in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_trait_selection/traits/effects.rs.html">old solver</a> and the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_next_trait_solver/solve/effect_goals.rs.html">new |
| trait solver</a>. In general, we can prove a <code>HostEffect</code> predicate when either of |
| these conditions are met:</p> |
| <ul> |
| <li>The predicate can be assumed from caller bounds;</li> |
| <li>The type has a <code>const</code> <code>impl</code> for the trait, <em>and</em> that const conditions on |
| the impl holds, <em>and</em> that the <code>explicit_implied_const_bounds</code> on the trait |
| holds; or</li> |
| <li>The type has a built-in implementation for the trait in const contexts. For |
| example, <code>Fn</code> may be implemented by function items if their const conditions |
| are satisfied, or <code>Destruct</code> is implemented in const contexts if the type can |
| be dropped at compile time.</li> |
| </ul> |
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