Rollup merge of #104239 - b-naber:sccs-info, r=jackh726

Better debug logs for borrowck constraint graph

It's really cumbersome to work with `RegionVar`s when trying to debug borrowck code or when trying to understand how the borrowchecker works. This PR collects some region information (behind `cfg(debug_assertions)`) for created `RegionVar`s (NLL region vars, this PR doesn't touch canonicalization) and prints the nodes and edges of the strongly connected constraints graph using representatives that use that region information (either lifetime names, locations in MIR or spans).

compiler/rustc_data_structures/src/graph/scc/mod.rs

@@ -27,7 +27,7 @@ pub struct Sccs<N: Idx, S: Idx> {
     scc_data: SccData<S>,
 }
 
-struct SccData<S: Idx> {
+pub struct SccData<S: Idx> {
     /// For each SCC, the range of `all_successors` where its
     /// successors can be found.
     ranges: IndexVec<S, Range<usize>>,
@@ -43,6 +43,14 @@ impl<N: Idx, S: Idx + Ord> Sccs<N, S> {
         SccsConstruction::construct(graph)
     }
 
+    pub fn scc_indices(&self) -> &IndexVec<N, S> {
+        &self.scc_indices
+    }
+
+    pub fn scc_data(&self) -> &SccData<S> {
+        &self.scc_data
+    }
+
     /// Returns the number of SCCs in the graph.
     pub fn num_sccs(&self) -> usize {
         self.scc_data.len()
@@ -115,6 +123,14 @@ impl<S: Idx> SccData<S> {
         self.ranges.len()
     }
 
+    pub fn ranges(&self) -> &IndexVec<S, Range<usize>> {
+        &self.ranges
+    }
+
+    pub fn all_successors(&self) -> &Vec<S> {
+        &self.all_successors
+    }
+
     /// Returns the successors of the given SCC.
     fn successors(&self, scc: S) -> &[S] {
         // Annoyingly, `range` does not implement `Copy`, so we have