filter redundant requirements through unit propagation

Author: LorrensP-2158466

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -589,7 +589,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     fn check_type_tests(
         &self,
         infcx: &InferCtxt<'tcx>,
-        mut propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
+        propagated_outlives_requirements: Option<&mut Vec<ClosureOutlivesRequirement<'tcx>>>,
         errors_buffer: &mut RegionErrors<'tcx>,
     ) {
         let tcx = infcx.tcx;
@@ -599,6 +599,9 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         // the user. Avoid that.
         let mut deduplicate_errors = FxIndexSet::default();
 
+        let mut conjunctive_propagated_outlives_requirements =
+            propagated_outlives_requirements.is_some().then_some(vec![]);
+
         for type_test in &self.type_tests {
             debug!("check_type_test: {:?}", type_test);
 
@@ -612,8 +615,13 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                 continue;
             }
 
-            if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements
-                && self.try_promote_type_test(infcx, type_test, propagated_outlives_requirements)
+            if let Some(conjunctive_propagated_outlives_requirements) =
+                &mut conjunctive_propagated_outlives_requirements
+                && self.try_promote_type_test(
+                    infcx,
+                    type_test,
+                    conjunctive_propagated_outlives_requirements,
+                )
             {
                 continue;
             }
@@ -637,6 +645,69 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                 errors_buffer.push(RegionErrorKind::TypeTestError { type_test: type_test.clone() });
             }
         }
+
+        if let Some(mut conjunctive_requirements) = conjunctive_propagated_outlives_requirements
+            && !conjunctive_requirements.is_empty()
+        {
+            // We can simplify this list of list of requirements.
+            //
+            // Say we did some number of type tests and it results in following requirements:
+            //
+            // R1: (T: 'a OR T: 'b)
+            // R2: (T: 'a)
+            //
+            // * See `try_promote_type_test` below on why we obtain OR requirements implicitly.
+            //
+            // Full requirement is then: R1 AND R2. *BUT*, we can remove R1 entirely, because we already
+            // require `T: 'a`, which implies `T:'a OR T: 'b`, making R1 redundant.
+            //
+            // The requirements can be seen as a boolean conjunctive normal form expression:
+            // Treat a requirement `T: 'region` as a boolean value, then this problem is (almost)
+            // equivalant to "Unit Propagation". However, this problem we are trying to solve is much
+            // simpler: Unit Propagation considers any form of subexpression, even containing negation
+            // of values, making it a multi-pass algorithm. The only subexpressions we encounter are of
+            // the form (R1 OR ... OR RN), thus if even on R is required on their own (a unit), this
+            // whole subexpression can be removed.
+            //
+            // So we can filter redundant OR requirements with the following algorithm:
+            // Collect every Unit requirement. Then for every other requirement, if one of the units is
+            // contained within them we can remove the entire requirement from the list.
+
+            fn requirement_key<'a>(subject: ClosureOutlivesRequirement<'a>) -> (Ty<'a>, RegionVid) {
+                let ClosureOutlivesSubject::Ty(ClosureOutlivesSubjectTy { inner: ty }) =
+                    subject.subject
+                else {
+                    unreachable!("ClosureOutliveSubject of a type test is always a Ty");
+                };
+                (ty, subject.outlived_free_region)
+            }
+
+            let units: Vec<_> = conjunctive_requirements
+                .iter()
+                .filter_map(|r| {
+                    let [r] = r.as_slice() else { return None };
+                    Some(requirement_key(*r))
+                })
+                .collect();
+
+            // Remove the `or_requirement`s that contain any of the unit requirements.
+            conjunctive_requirements.retain(|or_requirement| {
+                or_requirement.len() == 1
+                    || !or_requirement.iter().any(|r| {
+                        let key = requirement_key(*r);
+                        units.contains(&key)
+                    })
+            });
+
+            assert!(
+                !conjunctive_requirements.is_empty(),
+                "It should not be possible to remove every requirement."
+            );
+            // Propagate all requirements as is.
+            propagated_outlives_requirements
+                .expect("conjunctive_requirements is `Some`, so this should be as well")
+                .extend(conjunctive_requirements.into_iter().flatten());
+        }
     }
 
     /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot
@@ -668,7 +739,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         &self,
         infcx: &InferCtxt<'tcx>,
         type_test: &TypeTest<'tcx>,
-        propagated_outlives_requirements: &mut Vec<ClosureOutlivesRequirement<'tcx>>,
+        propagated_outlives_requirements: &mut Vec<Vec<ClosureOutlivesRequirement<'tcx>>>,
     ) -> bool {
         let tcx = infcx.tcx;
         let TypeTest { generic_kind, lower_bound, span: blame_span, verify_bound: _ } = *type_test;
@@ -694,58 +765,55 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         if let Some(p) = self.scc_values.placeholders_contained_in(r_scc).next() {
             debug!("encountered placeholder in higher universe: {:?}, requiring 'static", p);
             let static_r = self.universal_regions().fr_static;
-            propagated_outlives_requirements.push(ClosureOutlivesRequirement {
+            propagated_outlives_requirements.push(vec![ClosureOutlivesRequirement {
                 subject,
                 outlived_free_region: static_r,
                 blame_span,
                 category: ConstraintCategory::Boring,
-            });
+            }]);
 
             // we can return here -- the code below might push add'l constraints
             // but they would all be weaker than this one.
             return true;
         }
 
+        let universal_regions: Vec<_> =
+            self.scc_values.universal_regions_outlived_by(r_scc).collect();
+        debug!(?universal_regions);
+
+        // Filter to only the "minimal" universal regions:
+        // Drop any region `a` that strictly outlives another region `b`.
+        let minimal_universal_regions: Vec<_> = universal_regions
+            .iter()
+            .copied()
+            .filter(|&a| {
+                !universal_regions.iter().copied().any(|b| {
+                    !self.universal_region_relations.outlives(a, b)
+                        && self.universal_region_relations.outlives(b, a)
+                })
+            })
+            .collect();
+
+        assert!(
+            !minimal_universal_regions.is_empty(),
+            "There should always be at least 1 minimal region"
+        );
+
         // For each region outlived by lower_bound find a non-local,
         // universal region (it may be the same region) and add it to
         // `ClosureOutlivesRequirement`.
         let mut found_outlived_universal_region = false;
-        for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
+        for ur in minimal_universal_regions {
             found_outlived_universal_region = true;
             debug!("universal_region_outlived_by ur={:?}", ur);
-            let mut non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur);
+            let non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur);
             debug!(?non_local_ub);
 
-            // We don't need to propagate every `T: 'ub` for soundness:
-            // Say we have the following outlives constraints given (`'b: 'a` == `a -> b`):
-            //
-            // a
-            //  \
-            //   -> c
-            //  /
-            // b
-            //
-            // And we are doing the type test `T: 'a`.
-            //
-            // The `lower_bound_universal_regions` of `'a` are `['a, 'c]`. The `upper_bounds` of `'a`
-            // is `['a]`, so we propagate `T: 'a`.
-            // The `upper_bounds` of `'c` are `['a, 'b]`, propagating `T: 'a` is correct;
-            // `T: 'b` is redundant because it provides no value to proving `T: 'a`.
-            //
-            // So we filter the set of upper_bounds to regions already outliving `lower_bound`,
-            // but if this subset is empty, we fall back to the original one.
-            let subset: Vec<_> = non_local_ub
-                .iter()
-                .copied()
-                .filter(|ub| self.eval_outlives(*ub, lower_bound))
-                .collect();
-            non_local_ub = if subset.is_empty() { non_local_ub } else { subset };
-            debug!(?non_local_ub, "upper_bounds after filtering");
-
             // This is slightly too conservative. To show T: '1, given `'2: '1`
             // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
             // avoid potential non-determinism we approximate this by requiring
             // T: '1 and T: '2.
+            let mut or_requirements = Vec::with_capacity(non_local_ub.len());
             for upper_bound in non_local_ub {
                 debug_assert!(self.universal_regions().is_universal_region(upper_bound));
                 debug_assert!(!self.universal_regions().is_local_free_region(upper_bound));
@@ -757,8 +825,9 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                     category: ConstraintCategory::Boring,
                 };
                 debug!(?requirement, "adding closure requirement");
-                propagated_outlives_requirements.push(requirement);
+                or_requirements.push(requirement);
             }
+            propagated_outlives_requirements.push(or_requirements);
         }
         // If we succeed to promote the subject, i.e. it only contains non-local regions,
         // and fail to prove the type test inside of the closure, the `lower_bound` has to

tests/ui/borrowck/unconstrained-closure-lifetime-generic.rs

@@ -13,8 +13,6 @@ impl Foo {
         //~| ERROR the parameter type `impl for<'a> Fn(&'a usize) -> Box<I>` may not live long enough
         //~| ERROR the parameter type `impl for<'a> Fn(&'a usize) -> Box<I>` may not live long enough
         //~| ERROR the parameter type `I` may not live long enough
-        //~| ERROR the parameter type `I` may not live long enough
-        //~| ERROR the parameter type `I` may not live long enough
         //~| ERROR `f` does not live long enough
     }
 }

tests/ui/borrowck/unconstrained-closure-lifetime-generic.stderr

@@ -69,34 +69,6 @@ help: consider adding an explicit lifetime bound
 LL |     pub fn ack<I: 'static>(&mut self, f: impl for<'a> Fn(&'a usize) -> Box<I>) {
    |                 +++++++++
 
-error[E0310]: the parameter type `I` may not live long enough
-  --> $DIR/unconstrained-closure-lifetime-generic.rs:10:35
-   |
-LL |         self.bar = Box::new(|baz| Box::new(f(baz)));
-   |                                   ^^^^^^^^^^^^^^^^
-   |                                   |
-   |                                   the parameter type `I` must be valid for the static lifetime...
-   |                                   ...so that the type `I` will meet its required lifetime bounds
-   |
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
-help: consider adding an explicit lifetime bound
-   |
-LL |     pub fn ack<I: 'static>(&mut self, f: impl for<'a> Fn(&'a usize) -> Box<I>) {
-   |                 +++++++++
-
-error[E0311]: the parameter type `I` may not live long enough
-  --> $DIR/unconstrained-closure-lifetime-generic.rs:10:35
-   |
-LL |     pub fn ack<I>(&mut self, f: impl for<'a> Fn(&'a usize) -> Box<I>) {
-   |                   --------- the parameter type `I` must be valid for the anonymous lifetime defined here...
-LL |         self.bar = Box::new(|baz| Box::new(f(baz)));
-   |                                   ^^^^^^^^^^^^^^^^ ...so that the type `I` will meet its required lifetime bounds
-   |
-help: consider adding an explicit lifetime bound
-   |
-LL |     pub fn ack<'a, I: 'a>(&'a mut self, f: impl for<'a> Fn(&'a usize) -> Box<I>) {
-   |                +++  ++++   ++
-
 error[E0597]: `f` does not live long enough
   --> $DIR/unconstrained-closure-lifetime-generic.rs:10:44
    |
@@ -113,7 +85,7 @@ LL |     }
    |
    = note: due to object lifetime defaults, `Box<dyn for<'a> Fn(&'a usize) -> Box<(dyn Any + 'a)>>` actually means `Box<(dyn for<'a> Fn(&'a usize) -> Box<(dyn Any + 'a)> + 'static)>`
 
-error: aborting due to 8 previous errors
+error: aborting due to 6 previous errors
 
-Some errors have detailed explanations: E0310, E0311, E0597.
+Some errors have detailed explanations: E0310, E0597.
 For more information about an error, try `rustc --explain E0310`.

tests/ui/nll/closure-requirements/type-test-issue-154267.rs

@@ -10,6 +10,7 @@ impl<'a, 'b, 'c, T> Arg<'a, 'b, 'c, T> {
     fn constrain(self)
     where
         T: 'a,
+        T: 'c,
     {
     }
 }