RegionInferenceContext: remove Rc from rev_scc_graph field

Author: klensy

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -76,7 +76,7 @@ pub struct RegionInferenceContext<'tcx> {
     /// 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.
-    rev_scc_graph: Option<Rc<ReverseSccGraph>>,
+    rev_scc_graph: Option<ReverseSccGraph>,
 
     /// The "R0 member of [R1..Rn]" constraints, indexed by SCC.
     member_constraints: Rc<MemberConstraintSet<'tcx, ConstraintSccIndex>>,
@@ -813,9 +813,9 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         // free region that must outlive the member region `R0` (`UB:
         // R0`). Therefore, we need only keep an option `O` if `UB: O`
         // for all UB.
-        let rev_scc_graph = self.reverse_scc_graph();
+        self.compute_reverse_scc_graph();
         let universal_region_relations = &self.universal_region_relations;
-        for ub in rev_scc_graph.upper_bounds(scc) {
+        for ub in self.rev_scc_graph.as_ref().unwrap().upper_bounds(scc) {
             debug!(?ub);
             choice_regions.retain(|&o_r| universal_region_relations.outlives(ub, o_r));
         }

compiler/rustc_borrowck/src/region_infer/reverse_sccs.rs

@@ -8,7 +8,6 @@ use rustc_data_structures::graph::vec_graph::VecGraph;
 use rustc_data_structures::graph::WithSuccessors;
 use rustc_middle::ty::RegionVid;
 use std::ops::Range;
-use std::rc::Rc;
 
 pub(crate) struct ReverseSccGraph {
     graph: VecGraph<ConstraintSccIndex>,
@@ -40,10 +39,10 @@ impl ReverseSccGraph {
 }
 
 impl RegionInferenceContext<'_> {
-    /// Compute and return the reverse SCC-based constraint graph (lazily).
-    pub(super) fn reverse_scc_graph(&mut self) -> Rc<ReverseSccGraph> {
-        if let Some(g) = &self.rev_scc_graph {
-            return g.clone();
+    /// Compute the reverse SCC-based constraint graph (lazily).
+    pub(super) fn compute_reverse_scc_graph(&mut self) {
+        if matches!(self.rev_scc_graph, Some(_)) {
+            return;
         }
 
         let graph = self.constraint_sccs.reverse();
@@ -63,8 +62,6 @@ impl RegionInferenceContext<'_> {
             start += group_size;
         }
 
-        let rev_graph = Rc::new(ReverseSccGraph { graph, scc_regions, universal_regions });
-        self.rev_scc_graph = Some(rev_graph.clone());
-        rev_graph
+        self.rev_scc_graph = Some(ReverseSccGraph { graph, scc_regions, universal_regions });
     }
 }