Decouple SCC annotations from SCCs

This rewires SCC annotations to have them be a separate,
visitor-type data structure. It was broken out of #130227,
which needed them to be able to remove unused annotations
after computation without recomputing the SCCs themselves.

As a drive-by it also removes some redundant code from
the hot loop in SCC construction for a performance improvement.

Author: amandasystems

compiler/rustc_borrowck/src/constraints/mod.rs

@@ -7,7 +7,7 @@ use rustc_middle::ty::{RegionVid, TyCtxt, VarianceDiagInfo};
 use rustc_span::Span;
 use tracing::{debug, instrument};
 
-use crate::region_infer::{ConstraintSccs, RegionDefinition, RegionTracker};
+use crate::region_infer::{AnnotatedSccs, ConstraintSccs, RegionDefinition, SccAnnotations};
 use crate::type_check::Locations;
 use crate::universal_regions::UniversalRegions;
 
@@ -61,12 +61,14 @@ impl<'tcx> OutlivesConstraintSet<'tcx> {
         &self,
         static_region: RegionVid,
         definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
-    ) -> ConstraintSccs {
+    ) -> AnnotatedSccs {
         let constraint_graph = self.graph(definitions.len());
         let region_graph = &constraint_graph.region_graph(self, static_region);
-        ConstraintSccs::new_with_annotation(&region_graph, |r| {
-            RegionTracker::new(r, &definitions[r])
-        })
+        let mut annotation_visitor = SccAnnotations::new(definitions);
+        (
+            ConstraintSccs::new_with_annotation(&region_graph, &mut annotation_visitor),
+            annotation_visitor.scc_to_annotation,
+        )
     }
 
     /// This method handles Universe errors by rewriting the constraint
@@ -79,12 +81,12 @@ impl<'tcx> OutlivesConstraintSet<'tcx> {
     /// eventually go away.
     ///
     /// For a more precise definition, see the documentation for
-    /// [`RegionTracker::has_incompatible_universes()`].
+    /// [`crate::region_infer::RegionTracker`].
     ///
     /// This edge case used to be handled during constraint propagation
     /// by iterating over the strongly connected components in the constraint
     /// graph while maintaining a set of bookkeeping mappings similar
-    /// to what is stored in `RegionTracker` and manually adding 'sttaic as
+    /// to what is stored in `RegionTracker` and manually adding 'static as
     /// needed.
     ///
     /// It was rewritten as part of the Polonius project with the goal of moving
@@ -108,9 +110,9 @@ impl<'tcx> OutlivesConstraintSet<'tcx> {
         &mut self,
         universal_regions: &UniversalRegions<'tcx>,
         definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
-    ) -> ConstraintSccs {
+    ) -> AnnotatedSccs {
         let fr_static = universal_regions.fr_static;
-        let sccs = self.compute_sccs(fr_static, definitions);
+        let (sccs, annotations) = self.compute_sccs(fr_static, definitions);
 
         // Changed to `true` if we added any constraints to `self` and need to
         // recompute SCCs.
@@ -124,7 +126,7 @@ impl<'tcx> OutlivesConstraintSet<'tcx> {
                 continue;
             }
 
-            let annotation = sccs.annotation(scc);
+            let annotation = annotations[scc];
 
             // If this SCC participates in a universe violation,
             // e.g. if it reaches a region with a universe smaller than
@@ -154,7 +156,7 @@ impl<'tcx> OutlivesConstraintSet<'tcx> {
             self.compute_sccs(fr_static, definitions)
         } else {
             // If we didn't add any back-edges; no more work needs doing
-            sccs
+            (sccs, annotations)
         }
     }
 }

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -45,12 +45,13 @@ mod reverse_sccs;
 
 pub(crate) mod values;
 
-pub(crate) type ConstraintSccs = Sccs<RegionVid, ConstraintSccIndex, RegionTracker>;
+pub(crate) type ConstraintSccs = Sccs<RegionVid, ConstraintSccIndex>;
+pub(crate) type AnnotatedSccs = (ConstraintSccs, IndexVec<ConstraintSccIndex, RegionTracker>);
 
 /// An annotation for region graph SCCs that tracks
-/// the values of its elements.
+/// the values of its elements. This annotates a single SCC.
 #[derive(Copy, Debug, Clone)]
-pub struct RegionTracker {
+pub(crate) struct RegionTracker {
     /// The largest universe of a placeholder reached from this SCC.
     /// This includes placeholders within this SCC.
     max_placeholder_universe_reached: UniverseIndex,
@@ -95,6 +96,31 @@ impl scc::Annotation for RegionTracker {
     }
 }
 
+/// A Visitor for SCC annotation construction.
+pub(crate) struct SccAnnotations<'d, 'tcx, A: scc::Annotation> {
+    pub(crate) scc_to_annotation: IndexVec<ConstraintSccIndex, A>,
+    definitions: &'d IndexVec<RegionVid, RegionDefinition<'tcx>>,
+}
+
+impl<'d, 'tcx, A: scc::Annotation> SccAnnotations<'d, 'tcx, A> {
+    pub(crate) fn new(definitions: &'d IndexVec<RegionVid, RegionDefinition<'tcx>>) -> Self {
+        Self { scc_to_annotation: IndexVec::new(), definitions }
+    }
+}
+
+impl scc::Annotations<RegionVid, ConstraintSccIndex, RegionTracker>
+    for SccAnnotations<'_, '_, RegionTracker>
+{
+    fn new(&self, element: RegionVid) -> RegionTracker {
+        RegionTracker::new(element, &self.definitions[element])
+    }
+
+    fn annotate_scc(&mut self, scc: ConstraintSccIndex, annotation: RegionTracker) {
+        let idx = self.scc_to_annotation.push(annotation);
+        assert!(idx == scc);
+    }
+}
+
 impl RegionTracker {
     pub(crate) fn new(rvid: RegionVid, definition: &RegionDefinition<'_>) -> Self {
         let (representative_is_placeholder, representative_is_existential) = match definition.origin
@@ -166,6 +192,8 @@ pub struct RegionInferenceContext<'tcx> {
     /// compute the values of each region.
     constraint_sccs: ConstraintSccs,
 
+    scc_annotations: IndexVec<ConstraintSccIndex, RegionTracker>,
+
     /// Reverse of the SCC constraint graph --  i.e., an edge `A -> B` exists if
     /// `B: A`. This is used to compute the universal regions that are required
     /// to outlive a given SCC. Computed lazily.
@@ -428,7 +456,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             .map(|info| RegionDefinition::new(info.universe, info.origin))
             .collect();
 
-        let constraint_sccs =
+        let (constraint_sccs, scc_annotations) =
             outlives_constraints.add_outlives_static(&universal_regions, &definitions);
         let constraints = Frozen::freeze(outlives_constraints);
         let constraint_graph = Frozen::freeze(constraints.graph(definitions.len()));
@@ -455,6 +483,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             constraints,
             constraint_graph,
             constraint_sccs,
+            scc_annotations,
             rev_scc_graph: None,
             member_constraints,
             member_constraints_applied: Vec::new(),
@@ -752,6 +781,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         debug!(value = ?self.scc_values.region_value_str(scc_a));
     }
 
+    fn scc_annotations(&self) -> &IndexVec<ConstraintSccIndex, RegionTracker> {
+        &self.scc_annotations
+    }
+
     /// Invoked for each `R0 member of [R1..Rn]` constraint.
     ///
     /// `scc` is the SCC containing R0, and `choice_regions` are the
@@ -793,7 +826,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
 
         // If the member region lives in a higher universe, we currently choose
         // the most conservative option by leaving it unchanged.
-        if !self.constraint_sccs().annotation(scc).min_universe().is_root() {
+        if !self.scc_annotations()[scc].min_universe().is_root() {
             return;
         }
 
@@ -869,8 +902,8 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// in `scc_a`. Used during constraint propagation, and only once
     /// the value of `scc_b` has been computed.
     fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
-        let a_annotation = self.constraint_sccs().annotation(scc_a);
-        let b_annotation = self.constraint_sccs().annotation(scc_b);
+        let a_annotation = self.scc_annotations()[scc_a];
+        let b_annotation = self.scc_annotations()[scc_b];
         let a_universe = a_annotation.min_universe();
 
         // If scc_b's declared universe is a subset of
@@ -986,7 +1019,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             "lower_bound = {:?} r_scc={:?} universe={:?}",
             lower_bound,
             r_scc,
-            self.constraint_sccs.annotation(r_scc).min_universe()
+            self.scc_annotations()[r_scc].min_universe()
         );
         // If the type test requires that `T: 'a` where `'a` is a
         // placeholder from another universe, that effectively requires
@@ -1467,7 +1500,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// The minimum universe of any variable reachable from this
     /// SCC, inside or outside of it.
     fn scc_universe(&self, scc: ConstraintSccIndex) -> UniverseIndex {
-        self.constraint_sccs().annotation(scc).min_universe()
+        self.scc_annotations()[scc].min_universe()
     }
 
     /// Checks the final value for the free region `fr` to see if it
@@ -2214,7 +2247,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// they *must* be equal (though not having the same repr does not
     /// mean they are unequal).
     fn scc_representative(&self, scc: ConstraintSccIndex) -> RegionVid {
-        self.constraint_sccs.annotation(scc).representative
+        self.scc_annotations()[scc].representative
     }
 
     pub(crate) fn liveness_constraints(&self) -> &LivenessValues {