[WIP] compute region values using SCCs not iterative flow

This is marked as [WIP] because it contains a number of minor
refactorings that ought to be broken out.

Author: nikomatsakis

Diff for: src/librustc_mir/borrow_check/nll/constraint_generation.rs

@@ -210,7 +210,7 @@ impl<'cx, 'cg, 'gcx, 'tcx> ConstraintGeneration<'cx, 'cg, 'gcx, 'tcx> {
         for (region, location, cause) in liveness_set {
             debug!("generate: {:#?} is live at {:#?}", region, location);
             let region_vid = regioncx.to_region_vid(region);
-            regioncx.add_live_point(region_vid, *location, &cause);
+            regioncx.add_live_element(region_vid, *location, &cause);
         }
 
         if let Some(all_facts) = all_facts {
@@ -242,7 +242,7 @@ impl<'cx, 'cg, 'gcx, 'tcx> ConstraintGeneration<'cx, 'cg, 'gcx, 'tcx> {
             .tcx
             .for_each_free_region(&live_ty, |live_region| {
                 let vid = live_region.to_region_vid();
-                self.regioncx.add_live_point(vid, location, &cause);
+                self.regioncx.add_live_element(vid, location, &cause);
             });
     }
 }

Diff for: src/librustc_mir/borrow_check/nll/constraints/full_graph.rs

@@ -0,0 +1,96 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use borrow_check::nll::constraints::{ConstraintIndex, ConstraintSet};
+use rustc::ty::RegionVid;
+use rustc_data_structures::graph;
+use rustc_data_structures::indexed_vec::IndexVec;
+
+/// An initial graph which contains a node for every constraint index.
+/// This is basically just represented as a set of linked lists; for
+/// each region R, there is a linked list of constraints that lead to
+/// other regions. (The graph is always defined relative to some set.)
+crate struct FullConstraintGraph<'s> {
+    set: &'s ConstraintSet,
+    first_constraints: IndexVec<RegionVid, Option<ConstraintIndex>>,
+    next_constraints: IndexVec<ConstraintIndex, Option<ConstraintIndex>>,
+}
+
+impl<'s> FullConstraintGraph<'s> {
+    /// Constraint a graph where each region constraint `R1: R2` is
+    /// treated as an edge `R2 -> R1`. This is useful for cheaply
+    /// finding dirty constraints.
+    crate fn new(set: &'s ConstraintSet, num_region_vars: usize) -> Self {
+        let mut first_constraints = IndexVec::from_elem_n(None, num_region_vars);
+        let mut next_constraints = IndexVec::from_elem(None, &set.constraints);
+
+        for (idx, constraint) in set.constraints.iter_enumerated().rev() {
+            let mut head = &mut first_constraints[constraint.sup];
+            let mut next = &mut next_constraints[idx];
+            debug_assert!(next.is_none());
+            *next = *head;
+            *head = Some(idx);
+        }
+
+        Self { set, first_constraints, next_constraints }
+    }
+
+    crate fn sub_regions(
+        &self,
+        region_sup: RegionVid,
+    ) -> Successors<'_> {
+        let first = self.first_constraints[region_sup];
+        Successors { graph: self, pointer: first }
+    }
+}
+
+crate struct Successors<'s> {
+    graph: &'s FullConstraintGraph<'s>,
+    pointer: Option<ConstraintIndex>,
+}
+
+impl<'s> Iterator for Successors<'s> {
+    type Item = RegionVid;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some(p) = self.pointer {
+            let r = self.graph.set[p].sub;
+            self.pointer = self.graph.next_constraints[p];
+            Some(r)
+        } else {
+            None
+        }
+    }
+}
+
+impl<'s> graph::DirectedGraph for FullConstraintGraph<'s> {
+    type Node = RegionVid;
+}
+
+impl<'s> graph::WithNumNodes for FullConstraintGraph<'s> {
+    fn num_nodes(&self) -> usize {
+        self.first_constraints.len()
+    }
+}
+
+impl<'s> graph::WithSuccessors for FullConstraintGraph<'s> {
+    fn successors<'graph>(
+        &'graph self,
+        node: Self::Node,
+    ) -> <Self as graph::GraphSuccessors<'graph>>::Iter {
+        self.sub_regions(node)
+    }
+}
+
+impl<'s, 'graph> graph::GraphSuccessors<'graph> for FullConstraintGraph<'s> {
+    type Item = RegionVid;
+    type Iter = Successors<'graph>;
+}
+

Diff for: src/librustc_mir/borrow_check/nll/constraints/mod.rs

@@ -9,19 +9,22 @@
 // except according to those terms.
 
 use rustc::ty::RegionVid;
+use rustc_data_structures::graph::scc::Sccs;
 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
 use borrow_check::nll::type_check::Locations;
 
 use std::fmt;
 use std::ops::Deref;
 
+mod full_graph;
+
 #[derive(Clone, Default)]
 crate struct ConstraintSet {
     constraints: IndexVec<ConstraintIndex, OutlivesConstraint>,
 }
 
 impl ConstraintSet {
-    pub fn push(&mut self, constraint: OutlivesConstraint) {
+    crate fn push(&mut self, constraint: OutlivesConstraint) {
         debug!(
             "ConstraintSet::push({:?}: {:?} @ {:?}",
             constraint.sup, constraint.sub, constraint.locations
@@ -32,12 +35,19 @@ impl ConstraintSet {
         }
         self.constraints.push(constraint);
     }
+
+    crate fn compute_sccs(&self, num_region_vars: usize) -> Sccs<RegionVid, ConstraintSccIndex> {
+        let graph = &full_graph::FullConstraintGraph::new(self, num_region_vars);
+        Sccs::new(graph)
+    }
 }
 
 impl Deref for ConstraintSet {
     type Target = IndexVec<ConstraintIndex, OutlivesConstraint>;
 
-    fn deref(&self) -> &Self::Target { &self.constraints }
+    fn deref(&self) -> &Self::Target {
+        &self.constraints
+    }
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
@@ -68,45 +78,4 @@ impl fmt::Debug for OutlivesConstraint {
 
 newtype_index!(ConstraintIndex { DEBUG_FORMAT = "ConstraintIndex({})" });
 
-crate struct ConstraintGraph {
-    first_constraints: IndexVec<RegionVid, Option<ConstraintIndex>>,
-    next_constraints: IndexVec<ConstraintIndex, Option<ConstraintIndex>>,
-}
-
-impl ConstraintGraph {
-    /// Constraint a graph where each region constraint `R1: R2` is
-    /// treated as an edge `R2 -> R1`. This is useful for cheaply
-    /// finding dirty constraints.
-    crate fn new(set: &ConstraintSet, num_region_vars: usize) -> Self {
-        let mut first_constraints = IndexVec::from_elem_n(None, num_region_vars);
-        let mut next_constraints = IndexVec::from_elem(None, &set.constraints);
-
-        for (idx, constraint) in set.constraints.iter_enumerated().rev() {
-            let mut head = &mut first_constraints[constraint.sub];
-            let mut next = &mut next_constraints[idx];
-            debug_assert!(next.is_none());
-            *next = *head;
-            *head = Some(idx);
-        }
-
-        ConstraintGraph { first_constraints, next_constraints }
-    }
-
-    /// Invokes `op` with the index of any constraints of the form
-    /// `region_sup: region_sub`.  These are the constraints that must
-    /// be reprocessed when the value of `R1` changes. If you think of
-    /// each constraint `R1: R2` as an edge `R2 -> R1`, then this
-    /// gives the set of successors to R2.
-    crate fn for_each_dependent(
-        &self,
-        region_sub: RegionVid,
-        mut op: impl FnMut(ConstraintIndex),
-    ) {
-        let mut p = self.first_constraints[region_sub];
-        while let Some(dep_idx) = p {
-            op(dep_idx);
-            p = self.next_constraints[dep_idx];
-        }
-    }
-}
-
+newtype_index!(ConstraintSccIndex { DEBUG_FORMAT = "ConstraintSccIndex({})" });

Diff for: src/librustc_mir/borrow_check/nll/explain_borrow/mod.rs

@@ -185,7 +185,7 @@ impl<'gcx, 'tcx> UseFinder<'gcx, 'tcx> {
 
         stack.push(self.start_point);
         while let Some(p) = stack.pop() {
-            if !self.regioncx.region_contains_point(self.borrow.region, p) {
+            if !self.regioncx.region_contains(self.borrow.region, p) {
                 continue;
             }
 

Diff for: src/librustc_mir/borrow_check/nll/region_infer/error_reporting.rs

@@ -321,7 +321,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
 
         while changed {
             changed = false;
-            for constraint in &*self.constraints {
+            for constraint in self.constraints.iter() {
                 if let Some(n) = result_set[constraint.sup] {
                     let m = n + 1;
                     if result_set[constraint.sub]