Auto merge of #125069 - amandasystems:scc-refactor, r=nikomatsakis

Extend SCC construction to enable extra functionality

Do YOU feel like your SCC construction doesn't do enough? Then I have a patch for you! SCCs can now do *everything*! Well, almost.

This patch has been extracted from #123720. It specifically enhances
`Sccs` to allow tracking arbitrary commutative properties (think min/max mappings on nodes vs arbitrary closures) of strongly connected components, including
- reachable values (max/min)
- SCC-internal values (max/min)

This helps with among other things universe computation. We can now identify
SCC universes as a reasonably straightforward "find max/min" operation during SCC construction. This is also included in this patch.

It's also more or less zero-cost; don't use the new features, don't pay for them.

This commit also vastly extends the documentation of the SCCs module, which I had a very hard time following. It may or may not have gotten easier to read for someone else.

I believe this logic can also be used in leak check, but haven't checked. Ha. ha. Ha.

compiler/rustc_borrowck/src/constraints/mod.rs

@@ -1,13 +1,11 @@
-use rustc_data_structures::graph::scc::Sccs;
+use crate::type_check::Locations;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::mir::ConstraintCategory;
 use rustc_middle::ty::{RegionVid, TyCtxt, VarianceDiagInfo};
 use rustc_span::Span;
 use std::fmt;
 use std::ops::Index;
 
-use crate::type_check::Locations;
-
 pub(crate) mod graph;
 
 /// A set of NLL region constraints. These include "outlives"
@@ -45,18 +43,6 @@ impl<'tcx> OutlivesConstraintSet<'tcx> {
         graph::ConstraintGraph::new(graph::Reverse, self, num_region_vars)
     }
 
-    /// Computes cycles (SCCs) in the graph of regions. In particular,
-    /// find all regions R1, R2 such that R1: R2 and R2: R1 and group
-    /// them into an SCC, and find the relationships between SCCs.
-    pub(crate) fn compute_sccs(
-        &self,
-        constraint_graph: &graph::NormalConstraintGraph,
-        static_region: RegionVid,
-    ) -> Sccs<RegionVid, ConstraintSccIndex> {
-        let region_graph = &constraint_graph.region_graph(self, static_region);
-        Sccs::new(region_graph)
-    }
-
     pub(crate) fn outlives(
         &self,
     ) -> &IndexSlice<OutlivesConstraintIndex, OutlivesConstraint<'tcx>> {