introduce `ConstraintGraph`, stop mutating constraints in place

Encapsulate the dependencies more cleanly.

6 files changed, 76 insertions, 94 deletions

diff --git a/src/librustc_mir/borrow_check/nll/constraint_set.rs b/src/librustc_mir/borrow_check/nll/constraint_set.rs
index ed30e335437..eab1de07311 100644
--- a/src/librustc_mir/borrow_check/nll/constraint_set.rs
+++ b/src/librustc_mir/borrow_check/nll/constraint_set.rs
@@ -32,39 +32,6 @@ impl ConstraintSet {
         }
         self.constraints.push(constraint);
     }
-
-    /// Once all constraints have been added, `link()` is used to thread together the constraints
-    /// based on which would be affected when a particular region changes. See the next field of
-    /// `OutlivesContraint` for more details.
-    /// link returns a map that is needed later by `each_affected_by_dirty`.
-    pub fn link(&mut self, len: usize) -> IndexVec<RegionVid, Option<ConstraintIndex>> {
-        let mut map = IndexVec::from_elem_n(None, len);
-
-        for (idx, constraint) in self.constraints.iter_enumerated_mut().rev() {
-            let mut head = &mut map[constraint.sub];
-            debug_assert!(constraint.next.is_none());
-            constraint.next = *head;
-            *head = Some(idx);
-        }
-
-        map
-    }
-
-    /// When a region R1 changes, we need to reprocess all constraints R2: R1 to take into account
-    /// any new elements that R1 now has. This method will quickly enumerate all such constraints
-    /// (that is, constraints where R1 is in the "subregion" position).
-    /// To use it, invoke with `map[R1]` where map is the map returned by `link`;
-    /// the callback op will be invoked for each affected constraint.
-    pub fn each_affected_by_dirty(
-        &self,
-        mut opt_dep_idx: Option<ConstraintIndex>,
-        mut op: impl FnMut(ConstraintIndex),
-    ) {
-        while let Some(dep_idx) = opt_dep_idx {
-            op(dep_idx);
-            opt_dep_idx = self.constraints[dep_idx].next;
-        }
-    }
 }
 
 impl Deref for ConstraintSet {
@@ -85,16 +52,6 @@ pub struct OutlivesConstraint {
     /// Region that must be outlived.
     pub sub: RegionVid,
 
-    /// Later on, we thread the constraints onto a linked list
-    /// grouped by their `sub` field. So if you had:
-    ///
-    /// Index | Constraint | Next Field
-    /// ----- | ---------- | ----------
-    /// 0     | `'a: 'b`   | Some(2)
-    /// 1     | `'b: 'c`   | None
-    /// 2     | `'c: 'b`   | None
-    pub next: Option<ConstraintIndex>,
-
     /// Where did this constraint arise?
     pub locations: Locations,
 }
@@ -110,3 +67,46 @@ 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];
+        }
+    }
+}
+
diff --git a/src/librustc_mir/borrow_check/nll/region_infer/dump_mir.rs b/src/librustc_mir/borrow_check/nll/region_infer/dump_mir.rs
index 88d9f46e340..3c73203706d 100644
--- a/src/librustc_mir/borrow_check/nll/region_infer/dump_mir.rs
+++ b/src/librustc_mir/borrow_check/nll/region_infer/dump_mir.rs
@@ -83,7 +83,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                 sup,
                 sub,
                 locations,
-                next: _,
             } = constraint;
             with_msg(&format!(
                 "{:?}: {:?} due to {:?}",
diff --git a/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs b/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs
index 786b6a77d2b..5405ad91296 100644
--- a/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs
@@ -89,8 +89,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         stack: &mut Vec<ConstraintIndex>,
         results: &mut Vec<Vec<ConstraintIndex>>,
     ) {
-        let dependency_map = self.dependency_map.as_ref().unwrap();
-
         // Check if we already visited this region.
         if !visited.insert(current_region) {
             return;
@@ -105,20 +103,19 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             return;
         }
 
-        self.constraints
-            .each_affected_by_dirty(dependency_map[current_region], |constraint| {
-                assert_eq!(self.constraints[constraint].sub, current_region);
-                stack.push(constraint);
-                self.find_constraint_paths_between_regions_helper(
-                    from_region,
-                    self.constraints[constraint].sup,
-                    target_test,
-                    visited,
-                    stack,
-                    results,
-                );
-                stack.pop();
-            });
+        self.constraint_graph.for_each_dependent(current_region, |constraint| {
+            assert_eq!(self.constraints[constraint].sub, current_region);
+            stack.push(constraint);
+            self.find_constraint_paths_between_regions_helper(
+                from_region,
+                self.constraints[constraint].sup,
+                target_test,
+                visited,
+                stack,
+                results,
+            );
+            stack.pop();
+        });
     }
 
     /// This function will return true if a constraint is interesting and false if a constraint
diff --git a/src/librustc_mir/borrow_check/nll/region_infer/mod.rs b/src/librustc_mir/borrow_check/nll/region_infer/mod.rs
index 8cae1fd2a4f..f0ada4a85a1 100644
--- a/src/librustc_mir/borrow_check/nll/region_infer/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/region_infer/mod.rs
@@ -9,7 +9,9 @@
 // except according to those terms.
 
 use super::universal_regions::UniversalRegions;
-use borrow_check::nll::constraint_set::{ConstraintIndex, ConstraintSet, OutlivesConstraint};
+use borrow_check::nll::constraint_set::{
+    ConstraintIndex, ConstraintGraph, ConstraintSet, OutlivesConstraint
+};
 use borrow_check::nll::type_check::Locations;
 use rustc::hir::def_id::DefId;
 use rustc::infer::canonical::QueryRegionConstraint;
@@ -57,16 +59,12 @@ pub struct RegionInferenceContext<'tcx> {
     /// until `solve` is invoked.
     inferred_values: Option<RegionValues>,
 
-    /// For each variable, stores the index of the first constraint
-    /// where that variable appears on the RHS. This is the start of a
-    /// 'linked list' threaded by the `next` field in `Constraint`.
-    ///
-    /// This map is build when values are inferred.
-    dependency_map: Option<IndexVec<RegionVid, Option<ConstraintIndex>>>,
-
     /// The constraints we have accumulated and used during solving.
     constraints: ConstraintSet,
 
+    /// The constraint-set, but organized by regions.
+    constraint_graph: ConstraintGraph,
+
     /// Type constraints that we check after solving.
     type_tests: Vec<TypeTest<'tcx>>,
 
@@ -203,27 +201,26 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         outlives_constraints: ConstraintSet,
         type_tests: Vec<TypeTest<'tcx>>,
     ) -> Self {
-        // The `next` field should not yet have been initialized:
-        debug_assert!(outlives_constraints.iter().all(|c| c.next.is_none()));
-
         let num_region_variables = var_infos.len();
         let num_universal_regions = universal_regions.len();
 
         let elements = &Rc::new(RegionValueElements::new(mir, num_universal_regions));
 
         // Create a RegionDefinition for each inference variable.
-        let definitions = var_infos
+        let definitions: IndexVec<_, _> = var_infos
             .into_iter()
             .map(|info| RegionDefinition::new(info.origin))
             .collect();
 
+        let constraint_graph = ConstraintGraph::new(&outlives_constraints, definitions.len());
+
         let mut result = Self {
             definitions,
             elements: elements.clone(),
             liveness_constraints: RegionValues::new(elements, num_region_variables),
             inferred_values: None,
-            dependency_map: None,
             constraints: outlives_constraints,
+            constraint_graph,
             type_tests,
             universal_regions,
         };
@@ -392,7 +389,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// satisfied. Note that some values may grow **too** large to be
     /// feasible, but we check this later.
     fn propagate_constraints(&mut self, mir: &Mir<'tcx>) {
-        self.dependency_map = Some(self.build_dependency_map());
+        assert!(self.inferred_values.is_none());
         let inferred_values = self.compute_region_values(mir);
         self.inferred_values = Some(inferred_values);
     }
@@ -409,8 +406,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         // constraints we have accumulated.
         let mut inferred_values = self.liveness_constraints.clone();
 
-        let dependency_map = self.dependency_map.as_ref().unwrap();
-
         // Constraints that may need to be repropagated (initially all):
         let mut dirty_list: Vec<_> = self.constraints.indices().collect();
 
@@ -428,14 +423,15 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                 debug!("propagate_constraints:   sub={:?}", constraint.sub);
                 debug!("propagate_constraints:   sup={:?}", constraint.sup);
 
-                self.constraints.each_affected_by_dirty(
-                    dependency_map[constraint.sup],
-                    |dep_idx| {
-                        if clean_bit_vec.remove(dep_idx.index()) {
-                            dirty_list.push(dep_idx);
-                        }
-                    },
-                );
+                // The region of `constraint.sup` changed, so find all
+                // constraints of the form `R: constriant.sup` and
+                // enqueue them as dirty.  We will have to reprocess
+                // them.
+                self.constraint_graph.for_each_dependent(constraint.sup, |dep_idx| {
+                    if clean_bit_vec.remove(dep_idx.index()) {
+                        dirty_list.push(dep_idx);
+                    }
+                });
             }
 
             debug!("\n");
@@ -444,14 +440,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         inferred_values
     }
 
-    /// Builds up a map from each region variable X to a vector with the
-    /// indices of constraints that need to be re-evaluated when X changes.
-    /// These are constraints like Y: X @ P -- so if X changed, we may
-    /// need to grow Y.
-    fn build_dependency_map(&mut self) -> IndexVec<RegionVid, Option<ConstraintIndex>> {
-        self.constraints.link(self.definitions.len())
-    }
-
     /// Once regions have been propagated, this method is used to see
     /// whether the "type tests" produced by typeck were satisfied;
     /// type tests encode type-outlives relationships like `T:
diff --git a/src/librustc_mir/borrow_check/nll/type_check/constraint_conversion.rs b/src/librustc_mir/borrow_check/nll/type_check/constraint_conversion.rs
index 27bd5042777..a5a159fbb1c 100644
--- a/src/librustc_mir/borrow_check/nll/type_check/constraint_conversion.rs
+++ b/src/librustc_mir/borrow_check/nll/type_check/constraint_conversion.rs
@@ -186,7 +186,6 @@ impl<'a, 'gcx, 'tcx> ConstraintConversion<'a, 'gcx, 'tcx> {
             locations: self.locations,
             sub,
             sup,
-            next: None,
         });
     }
 
diff --git a/src/librustc_mir/borrow_check/nll/type_check/mod.rs b/src/librustc_mir/borrow_check/nll/type_check/mod.rs
index 0203349ffab..97a74e0d336 100644
--- a/src/librustc_mir/borrow_check/nll/type_check/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/type_check/mod.rs
@@ -1537,7 +1537,6 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                                     sup: ref_region.to_region_vid(),
                                     sub: borrow_region.to_region_vid(),
                                     locations: location.boring(),
-                                    next: None,
                                 });
 
                             if let Some(all_facts) = all_facts {