traverse region graph instead of SCCs to compute polonius loan scopes

By using SCC for better performance, we also have to take into account
SCCs whose representative is an existential region but also contains a
placeholder.

By only checking the representative, we may miss that the loan escapes
the function. This can be fixed by picking a better representative, or
removing placeholders from the main path.

This is the simplest fix: forgo efficiency and traverse the region graph
instead of the SCCs.

1 files changed, 3 insertions, 3 deletions

diff --git a/compiler/rustc_borrowck/src/dataflow.rs b/compiler/rustc_borrowck/src/dataflow.rs
index 16814950b0d..8676d2ba7c4 100644
--- a/compiler/rustc_borrowck/src/dataflow.rs
+++ b/compiler/rustc_borrowck/src/dataflow.rs
@@ -273,11 +273,10 @@ impl<'tcx> PoloniusOutOfScopePrecomputer<'_, 'tcx> {
     ) {
         let sccs = self.regioncx.constraint_sccs();
         let universal_regions = self.regioncx.universal_regions();
-        let issuing_region_scc = sccs.scc(issuing_region);
 
         // We first handle the cases where the loan doesn't go out of scope, depending on the issuing
         // region's successors.
-        for scc in sccs.depth_first_search(issuing_region_scc) {
+        for successor in self.regioncx.region_graph().depth_first_search(issuing_region) {
             // 1. Via applied member constraints
             //
             // The issuing region can flow into the choice regions, and they are either:
@@ -290,6 +289,7 @@ impl<'tcx> PoloniusOutOfScopePrecomputer<'_, 'tcx> {
             // For additional insurance via fuzzing and crater, we verify that the constraint's min
             // choice indeed escapes the function. In the future, we could e.g. turn this check into
             // a debug assert and early return as an optimization.
+            let scc = sccs.scc(successor);
             for constraint in self.regioncx.applied_member_constraints(scc) {
                 if universal_regions.is_universal_region(constraint.min_choice) {
                     return;
@@ -300,7 +300,7 @@ impl<'tcx> PoloniusOutOfScopePrecomputer<'_, 'tcx> {
             //
             // If the issuing region outlives such a region, its loan escapes the function and
             // cannot go out of scope. We can early return.
-            if self.regioncx.scc_is_live_at_all_points(scc) {
+            if self.regioncx.is_region_live_at_all_points(successor) {
                 return;
             }
         }