Rollup merge of #123934 - WaffleLapkin:graph-mini-refactor, r=fmease

`rustc_data_structures::graph` mini refactor

Who doesn't love to breathe dust from the ancient times?

compiler/rustc_borrowck/src/constraints/graph.rs

@@ -216,23 +216,14 @@ impl<'s, 'tcx, D: ConstraintGraphDirection> Iterator for Successors<'s, 'tcx, D>
 
 impl<'s, 'tcx, D: ConstraintGraphDirection> graph::DirectedGraph for RegionGraph<'s, 'tcx, D> {
     type Node = RegionVid;
-}
 
-impl<'s, 'tcx, D: ConstraintGraphDirection> graph::WithNumNodes for RegionGraph<'s, 'tcx, D> {
     fn num_nodes(&self) -> usize {
         self.constraint_graph.first_constraints.len()
     }
 }
 
-impl<'s, 'tcx, D: ConstraintGraphDirection> graph::WithSuccessors for RegionGraph<'s, 'tcx, D> {
-    fn successors(&self, node: Self::Node) -> <Self as graph::GraphSuccessors<'_>>::Iter {
+impl<'s, 'tcx, D: ConstraintGraphDirection> graph::Successors for RegionGraph<'s, 'tcx, D> {
+    fn successors(&self, node: Self::Node) -> impl Iterator<Item = Self::Node> {
         self.outgoing_regions(node)
     }
 }
-
-impl<'s, 'tcx, D: ConstraintGraphDirection> graph::GraphSuccessors<'_>
-    for RegionGraph<'s, 'tcx, D>
-{
-    type Item = RegionVid;
-    type Iter = Successors<'s, 'tcx, D>;
-}

compiler/rustc_borrowck/src/dataflow.rs

@@ -1,5 +1,5 @@
 use rustc_data_structures::fx::FxIndexMap;
-use rustc_data_structures::graph::WithSuccessors;
+use rustc_data_structures::graph;
 use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::{
     self, BasicBlock, Body, CallReturnPlaces, Location, Place, TerminatorEdges,
@@ -262,7 +262,7 @@ impl<'tcx> PoloniusOutOfScopePrecomputer<'_, 'tcx> {
 
         // We first handle the cases where the loan doesn't go out of scope, depending on the issuing
         // region's successors.
-        for successor in self.regioncx.region_graph().depth_first_search(issuing_region) {
+        for successor in graph::depth_first_search(&self.regioncx.region_graph(), issuing_region) {
             // 1. Via applied member constraints
             //
             // The issuing region can flow into the choice regions, and they are either:

compiler/rustc_borrowck/src/region_infer/reverse_sccs.rs

@@ -1,8 +1,8 @@
 use crate::constraints::ConstraintSccIndex;
 use crate::RegionInferenceContext;
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
+use rustc_data_structures::graph;
 use rustc_data_structures::graph::vec_graph::VecGraph;
-use rustc_data_structures::graph::WithSuccessors;
 use rustc_middle::ty::RegionVid;
 use std::ops::Range;
 
@@ -23,8 +23,7 @@ impl ReverseSccGraph {
         scc0: ConstraintSccIndex,
     ) -> impl Iterator<Item = RegionVid> + 'a {
         let mut duplicates = FxIndexSet::default();
-        self.graph
-            .depth_first_search(scc0)
+        graph::depth_first_search(&self.graph, scc0)
             .flat_map(move |scc1| {
                 self.scc_regions
                     .get(&scc1)

compiler/rustc_borrowck/src/type_check/liveness/trace.rs

@@ -1,5 +1,4 @@
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
-use rustc_data_structures::graph::WithSuccessors;
 use rustc_index::bit_set::BitSet;
 use rustc_index::interval::IntervalSet;
 use rustc_infer::infer::canonical::QueryRegionConstraints;
@@ -64,7 +63,10 @@ pub(super) fn trace<'mir, 'tcx>(
         // Traverse each issuing region's constraints, and record the loan as flowing into the
         // outlived region.
         for (loan, issuing_region_data) in borrow_set.iter_enumerated() {
-            for succ in region_graph.depth_first_search(issuing_region_data.region) {
+            for succ in rustc_data_structures::graph::depth_first_search(
+                &region_graph,
+                issuing_region_data.region,
+            ) {
                 // We don't need to mention that a loan flows into its issuing region.
                 if succ == issuing_region_data.region {
                     continue;