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| <h1 id="propagating-closure-constraints"><a class="header" href="#propagating-closure-constraints">Propagating closure constraints</a></h1> |
| <p>When we are checking the type tests and universal regions, we may come |
| across a constraint that we can't prove yet if we are in a closure |
| body! However, the necessary constraints may actually hold (we just |
| don't know it yet). Thus, if we are inside a closure, we just collect |
| all the constraints we can't prove yet and return them. Later, when we |
| are borrow check the MIR node that created the closure, we can also |
| check that these constraints hold. At that time, if we can't prove |
| they hold, we report an error.</p> |
| <h2 id="how-this-is-implemented"><a class="header" href="#how-this-is-implemented">How this is implemented</a></h2> |
| <p>While borrow-checking a closure inside of <code>RegionInferenceContext::solve</code> we separately try to propagate type-outlives and region-outlives constraints to the parent if we're unable to prove them locally.</p> |
| <h3 id="region-outlive-constraints"><a class="header" href="#region-outlive-constraints">Region-outlive constraints</a></h3> |
| <p>If <code>RegionInferenceContext::check_universal_regions</code> fails to prove some outlives constraint <code>'longer_fr: 'shorter_fr</code>, we try to propagate it in <code>fn try_propagate_universal_region_error</code>. Both these universal regions are either local to the closure or an external region.</p> |
| <p>In case <code>'longer_fr</code> is a local universal region, we search for the largest external region <code>'fr_minus</code> which is outlived by <code>'longer_fr</code>, i.e. <code>'longer_fr: 'fr_minus</code>. In case there are multiple such regions, we pick the <code>mutual_immediate_postdominator</code>: the fixpoint of repeatedly computing the GLB of all GLBs, see <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/transitive_relation/struct.TransitiveRelation.html#method.postdom_upper_bound">TransitiveRelation::postdom_upper_bound</a> for more details.</p> |
| <p>If <code>'fr_minus</code> exists we require it to outlive all non-local upper bounds of <code>'shorter_fr</code>. There will always be at least one non-local upper bound <code>'static</code>.</p> |
| <h3 id="type-outlive-constraints"><a class="header" href="#type-outlive-constraints">Type-outlive constraints</a></h3> |
| <p>Type-outlives constraints are proven in <code>check_type_tests</code>. This happens after computing the outlives graph, which is now immutable.</p> |
| <p>For all type tests we fail to prove via <code>fn eval_verify_bound</code> inside of the closure we call <code>try_promote_type_test</code>. A <code>TypeTest</code> represents a type-outlives bound <code>generic_kind: lower_bound</code> together with a <code>verify_bound</code>. If the <code>VerifyBound</code> holds for the <code>lower_bound</code>, the constraint is satisfied. <code>try_promote_type_test</code> does not care about the <code> verify_bound</code>.</p> |
| <p>It starts by calling <code>fn try_promote_type_test_subject</code>. This function takes the <code>GenericKind</code> and tries to transform it to a <code>ClosureOutlivesSubject</code> which is no longer references anything local to the closure. This is done by replacing all free regions in that type with either <code>'static</code> or region parameters which are equal to that free region. This operation fails if the <code>generic_kind</code> contains a region which cannot be replaced.</p> |
| <p>We then promote the <code>lower_bound</code> into the context of the caller. If the lower bound is equal to a placeholder, we replace it with <code>'static</code></p> |
| <p>We then look at all universal regions <code>uv</code> which are required to be outlived by <code>lower_bound</code>, i.e. for which borrow checking added region constraints. For each of these we then emit a <code>ClosureOutlivesRequirement</code> for all non-local universal regions which are known to outlive <code>uv</code>.</p> |
| <p>As we've already built the region graph of the closure at this point and separately check that it is consistent, we are also able to assume the outlive constraints <code>uv: lower_bound</code> here.</p> |
| <p>So if we have a type-outlives bounds we can't prove, e.g. <code>T: 'local_infer</code>, we use the region graph to go to universal variables <code>'a</code> with <code>'a: local_infer</code>. In case <code>'a</code> are local, we then use the assumed outlived constraints to go to non-local ones.</p> |
| <p>We then store the list of promoted type tests in the <code>BorrowCheckResults</code>. |
| We then apply them in while borrow-checking its parent in <code>TypeChecker::prove_closure_bounds</code>.</p> |
| <p>TODO: explain how exactly that works :3</p> |
| |
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