extend NLL checker to understand 'empty combined with universes

src/librustc_mir/borrow_check/region_infer/mod.rs

@@ -67,6 +67,12 @@ pub struct RegionInferenceContext<'tcx> {
     /// compute the values of each region.
     constraint_sccs: Rc<Sccs<RegionVid, ConstraintSccIndex>>,
 
+    /// SCCs in "dependency order" (or "post order"), meaning that if S1 -> S2,
+    /// then S2 appears first. If you process the SCCs in this order, then you
+    /// are always ensured that when you proces a given SCC, all of its
+    /// successors have been processed.
+    scc_dependency_order: Vec<ConstraintSccIndex>,
+
     /// Reverse of the SCC constraint graph --  i.e., an edge `A -> B` exists if
     /// `B: A`. This is used to compute the universal regions that are required
     /// to outlive a given SCC. Computed lazily.
@@ -277,7 +283,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             scc_values.merge_liveness(scc, region, &liveness_constraints);
         }
 
-        let scc_universes = Self::compute_scc_universes(&constraint_sccs, &definitions);
+        let scc_dependency_order = Self::compute_scc_dependency_order(&constraint_sccs);
+
+        let scc_universes =
+            Self::compute_scc_universes(&constraint_sccs, &scc_dependency_order, &definitions);
 
         let scc_representatives = Self::compute_scc_representatives(&constraint_sccs, &definitions);
 
@@ -290,6 +299,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             constraints,
             constraint_graph,
             constraint_sccs,
+            scc_dependency_order,
             rev_scc_graph: None,
             member_constraints,
             member_constraints_applied: Vec::new(),
@@ -307,6 +317,43 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         result
     }
 
+    /// Returns a vector of all scc-ids in "dependency" or "post order". See the
+    /// `scc_dependency_order` field for more details.
+    fn compute_scc_dependency_order(
+        constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
+    ) -> Vec<ConstraintSccIndex> {
+        let mut visited = &mut BitSet::new_empty(constraints_scc.num_sccs());
+        let mut output = vec![];
+
+        for scc in constraints_scc.all_sccs() {
+            Self::compute_scc_dependency_order_if_new(
+                constraints_scc,
+                scc,
+                &mut visited,
+                &mut output,
+            );
+        }
+
+        output
+    }
+
+    fn compute_scc_dependency_order_if_new(
+        constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
+        index: ConstraintSccIndex,
+        visited: &mut BitSet<ConstraintSccIndex>,
+        output: &mut Vec<ConstraintSccIndex>,
+    ) {
+        if !visited.insert(index) {
+            return;
+        }
+
+        for &succ in constraints_scc.successors(index) {
+            Self::compute_scc_dependency_order_if_new(constraints_scc, succ, visited, output);
+        }
+
+        output.push(index);
+    }
+
     /// Each SCC is the combination of many region variables which
     /// have been equated. Therefore, we can associate a universe with
     /// each SCC which is minimum of all the universes of its
@@ -315,10 +362,11 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// SCC could have as well. This implies that the SCC must have
     /// the minimum, or narrowest, universe.
     fn compute_scc_universes(
-        constraints_scc: &Sccs<RegionVid, ConstraintSccIndex>,
+        constraint_sccs: &Sccs<RegionVid, ConstraintSccIndex>,
+        scc_dependency_order: &[ConstraintSccIndex],
         definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
     ) -> IndexVec<ConstraintSccIndex, ty::UniverseIndex> {
-        let num_sccs = constraints_scc.num_sccs();
+        let num_sccs = constraint_sccs.num_sccs();
         let mut scc_universes = IndexVec::from_elem_n(ty::UniverseIndex::MAX, num_sccs);
 
         debug!("compute_scc_universes()");
@@ -327,7 +375,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         // that contains R is "no bigger" than U. This effectively sets the universe
         // for each SCC to be the minimum of the regions within.
         for (region_vid, region_definition) in definitions.iter_enumerated() {
-            let scc = constraints_scc.scc(region_vid);
+            let scc = constraint_sccs.scc(region_vid);
             let scc_universe = &mut scc_universes[scc];
             let scc_min = std::cmp::min(region_definition.universe, *scc_universe);
             if scc_min != *scc_universe {
@@ -372,8 +420,8 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         // constraint lowers the universe of `R1` to `U0`, which in turn
         // means that the `R1: !1` constraint will (later) cause
         // `R1` to become `'static`.
-        for scc_a in constraints_scc.all_sccs() {
-            for &scc_b in constraints_scc.successors(scc_a) {
+        for &scc_a in scc_dependency_order {
+            for &scc_b in constraint_sccs.successors(scc_a) {
                 let scc_universe_a = scc_universes[scc_a];
                 let scc_universe_b = scc_universes[scc_b];
                 let scc_universe_min = std::cmp::min(scc_universe_a, scc_universe_b);
@@ -616,47 +664,26 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         // SCC. For each SCC, we visit its successors and compute
         // their values, then we union all those values to get our
         // own.
-        let visited = &mut BitSet::new_empty(self.constraint_sccs.num_sccs());
-        for scc_index in self.constraint_sccs.all_sccs() {
-            self.propagate_constraint_sccs_if_new(scc_index, visited);
+        for i in 0..self.scc_dependency_order.len() {
+            self.compute_value_for_scc(self.scc_dependency_order[i]);
         }
 
         // Sort the applied member constraints so we can binary search
         // through them later.
         self.member_constraints_applied.sort_by_key(|applied| applied.member_region_scc);
     }
 
-    /// Computes the value of the SCC `scc_a` if it has not already
-    /// been computed. The `visited` parameter is a bitset
-    #[inline]
-    fn propagate_constraint_sccs_if_new(
-        &mut self,
-        scc_a: ConstraintSccIndex,
-        visited: &mut BitSet<ConstraintSccIndex>,
-    ) {
-        if visited.insert(scc_a) {
-            self.propagate_constraint_sccs_new(scc_a, visited);
-        }
-    }
-
     /// Computes the value of the SCC `scc_a`, which has not yet been
-    /// computed. This works by first computing all successors of the
-    /// SCC (if they haven't been computed already) and then unioning
-    /// together their elements.
-    fn propagate_constraint_sccs_new(
-        &mut self,
-        scc_a: ConstraintSccIndex,
-        visited: &mut BitSet<ConstraintSccIndex>,
-    ) {
+    /// computed, by unioning the values of its successors.
+    /// Assumes that all successors have been computed already
+    /// (which is assured by iterating over SCCs in dependency order).
+    fn compute_value_for_scc(&mut self, scc_a: ConstraintSccIndex) {
         let constraint_sccs = self.constraint_sccs.clone();
 
         // Walk each SCC `B` such that `A: B`...
         for &scc_b in constraint_sccs.successors(scc_a) {
             debug!("propagate_constraint_sccs: scc_a = {:?} scc_b = {:?}", scc_a, scc_b);
 
-            // ...compute the value of `B`...
-            self.propagate_constraint_sccs_if_new(scc_b, visited);
-
             // ...and add elements from `B` into `A`. One complication
             // arises because of universes: If `B` contains something
             // that `A` cannot name, then `A` can only contain `B` if