compute live loans during liveness

Record all the loans flowing into the regions in a value's type as live
at the given locations where the value is live. This will then be used
to compute when a given loan goes out of scope in the polonius
out-of-scope precomputer.

Author: lqd

compiler/rustc_borrowck/src/dataflow.rs

@@ -1,7 +1,8 @@
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
-use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
-use rustc_index::bit_set::{BitSet, SparseBitMatrix};
+use rustc_data_structures::fx::FxIndexMap;
+use rustc_data_structures::graph::WithSuccessors;
+use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::{
     self, BasicBlock, Body, CallReturnPlaces, Location, Place, TerminatorEdges,
 };
@@ -13,8 +14,6 @@ use rustc_mir_dataflow::{self, fmt::DebugWithContext, GenKill};
 use rustc_mir_dataflow::{Analysis, Direction, Results};
 use std::fmt;
 
-use crate::constraints::ConstraintSccIndex;
-use crate::region_infer::values::PointIndex;
 use crate::{places_conflict, BorrowSet, PlaceConflictBias, PlaceExt, RegionInferenceContext};
 
 /// A tuple with named fields that can hold either the results or the transient state of the
@@ -246,65 +245,17 @@ struct PoloniusOutOfScopePrecomputer<'a, 'tcx> {
     body: &'a Body<'tcx>,
     regioncx: &'a RegionInferenceContext<'tcx>,
 
-    sccs_live_at_all_points: FxHashSet<ConstraintSccIndex>,
-    live_sccs_per_point: SparseBitMatrix<PointIndex, ConstraintSccIndex>,
-
-    reachability: BitSet<ConstraintSccIndex>,
-    reachability_stack: Vec<ConstraintSccIndex>,
-
     loans_out_of_scope_at_location: FxIndexMap<Location, Vec<BorrowIndex>>,
 }
 
 impl<'a, 'tcx> PoloniusOutOfScopePrecomputer<'a, 'tcx> {
     fn new(body: &'a Body<'tcx>, regioncx: &'a RegionInferenceContext<'tcx>) -> Self {
-        let sccs = regioncx.constraint_sccs();
-        let num_sccs = sccs.num_sccs();
-
-        // Compute the list of SCCs that are live at all points, as it will be used for all the
-        // loan scopes we'll compute.
-        // FIXME: they're surely already available somewhere.
-        let sccs_live_at_all_points: FxHashSet<_> = regioncx
-            .regions()
-            .filter(|&r| {
-                use rustc_infer::infer::{NllRegionVariableOrigin, RegionVariableOrigin};
-                let origin = regioncx.var_infos[r].origin;
-                let live_at_all_points = matches!(
-                    origin,
-                    RegionVariableOrigin::Nll(
-                        NllRegionVariableOrigin::Placeholder(_)
-                            | NllRegionVariableOrigin::FreeRegion
-                    )
-                );
-                live_at_all_points
-            })
-            .map(|r| sccs.scc(r))
-            .collect();
-
-        // Pre-compute the set of live SCCs per point
-        let liveness = regioncx.liveness_values();
-        let mut live_sccs_per_point = SparseBitMatrix::new(num_sccs);
-
-        for region in liveness.rows() {
-            let scc = sccs.scc(region);
-            if sccs_live_at_all_points.contains(&scc) {
-                continue;
-            }
-            for location in liveness.get_elements(region) {
-                let point = liveness.point_from_location(location);
-                live_sccs_per_point.insert(point, scc);
-            }
-        }
-
         Self {
             visited: BitSet::new_empty(body.basic_blocks.len()),
             visit_stack: vec![],
             body,
             regioncx,
             loans_out_of_scope_at_location: FxIndexMap::default(),
-            sccs_live_at_all_points,
-            live_sccs_per_point,
-            reachability: BitSet::new_empty(num_sccs),
-            reachability_stack: vec![],
         }
     }
 }
@@ -317,66 +268,50 @@ impl<'tcx> PoloniusOutOfScopePrecomputer<'_, 'tcx> {
         &mut self,
         loan_idx: BorrowIndex,
         issuing_region: RegionVid,
-        first_location: Location,
+        loan_issued_at: Location,
     ) {
-        // Let's precompute the reachability set of the issuing region, via reachability on the
-        // condensation graph. We can also early return when reaching regions that outlive free
-        // regions via member constraints. (The `OutOfScopePrecomputer` wouldn't be called on a
-        // region that outlives free regions via outlives constraints.)
-
         let sccs = self.regioncx.constraint_sccs();
-
         let issuing_region_scc = sccs.scc(issuing_region);
-        self.reachability_stack.push(issuing_region_scc);
-        self.reachability.insert(issuing_region_scc);
 
-        while let Some(scc) = self.reachability_stack.pop() {
-            // Handle successors of this SCC:
-            //
+        // We first handle the cases where the loan doesn't go out of scope, depending on the issuing
+        // region's successors.
+        for scc in sccs.depth_first_search(issuing_region_scc) {
             // 1. Via member constraints
             //
-            // The issuing region can flow into the choice regions here, and they are either:
+            // The issuing region can flow into the choice regions, and they are either:
             // - placeholders or free regions themselves,
             // - or also transitively outlive a free region.
             //
             // That is to say, if there are member constraints here, the loan escapes the function
             // and cannot go out of scope. We can early return.
-            //
+            if self.regioncx.scc_has_member_constraints(scc) {
+                return;
+            }
+
             // 2. Via regions that are live at all points: placeholders and free regions.
             //
             // If the issuing region outlives such a region, its loan escapes the function and
             // cannot go out of scope. We can early return.
-            if self.regioncx.scc_has_member_constraints(scc)
-                || self.sccs_live_at_all_points.contains(&scc)
-            {
-                self.reachability_stack.clear();
-                self.reachability.clear();
+            if self.regioncx.scc_is_live_at_all_points(scc) {
                 return;
             }
-
-            // 3. Via outlives successors, which we want to record and traverse: we add them to the
-            //    worklist stack
-            for &succ_scc in sccs.successors(scc) {
-                if self.reachability.insert(succ_scc) {
-                    self.reachability_stack.push(succ_scc);
-                }
-            }
         }
 
-        let first_block = first_location.block;
+        let first_block = loan_issued_at.block;
         let first_bb_data = &self.body.basic_blocks[first_block];
 
         // The first block we visit is the one where the loan is issued, starting from the statement
-        // where the loan is issued: at `first_location`.
-        let first_lo = first_location.statement_index;
+        // where the loan is issued: at `loan_issued_at`.
+        let first_lo = loan_issued_at.statement_index;
         let first_hi = first_bb_data.statements.len();
 
-        if let Some(kill_location) = self.loan_kill_location(first_block, first_lo, first_hi) {
+        if let Some(kill_location) =
+            self.loan_kill_location(loan_idx, loan_issued_at, first_block, first_lo, first_hi)
+        {
             debug!("loan {:?} gets killed at {:?}", loan_idx, kill_location);
             self.loans_out_of_scope_at_location.entry(kill_location).or_default().push(loan_idx);
 
             // The loan dies within the first block, we're done and can early return.
-            self.reachability.clear();
             return;
         }
 
@@ -393,7 +328,9 @@ impl<'tcx> PoloniusOutOfScopePrecomputer<'_, 'tcx> {
         while let Some(block) = self.visit_stack.pop() {
             let bb_data = &self.body[block];
             let num_stmts = bb_data.statements.len();
-            if let Some(kill_location) = self.loan_kill_location(block, 0, num_stmts) {
+            if let Some(kill_location) =
+                self.loan_kill_location(loan_idx, loan_issued_at, block, 0, num_stmts)
+            {
                 debug!("loan {:?} gets killed at {:?}", loan_idx, kill_location);
                 self.loans_out_of_scope_at_location
                     .entry(kill_location)
@@ -413,35 +350,44 @@ impl<'tcx> PoloniusOutOfScopePrecomputer<'_, 'tcx> {
         }
 
         self.visited.clear();
-        self.reachability.clear();
         assert!(self.visit_stack.is_empty(), "visit stack should be empty");
-        assert!(self.reachability_stack.is_empty(), "reachability stack should be empty");
     }
 
     /// Returns the lowest statement in `start..=end`, where the loan goes out of scope, if any.
     /// This is the statement where the issuing region can't reach any of the regions that are live
     /// at this point.
-    fn loan_kill_location(&self, block: BasicBlock, start: usize, end: usize) -> Option<Location> {
+    fn loan_kill_location(
+        &self,
+        loan_idx: BorrowIndex,
+        loan_issued_at: Location,
+        block: BasicBlock,
+        start: usize,
+        end: usize,
+    ) -> Option<Location> {
         for statement_index in start..=end {
             let location = Location { block, statement_index };
 
-            // Check whether the issuing region can reach local regions that are live at this
-            // point.
+            // Check whether the issuing region can reach local regions that are live at this point:
+            // - a loan is always live at its issuing location because it can reach the issuing
+            // region, which is always live at this location.
+            if location == loan_issued_at {
+                continue;
+            }
+
+            // - the loan goes out of scope at `location` if it's not contained within any regions
+            // live at this point.
             //
-            // FIXME: if the issuing region `i` can reach a live region `r` at point `p`, and
-            // `r` is live at point `q`, then it's guaranteed that `i` would reach `r` at point
-            // `q`. Reachability is location-insensitive, and we could take advantage of
-            // that, by jumping to a further point than the next statement. We can jump to the
-            // furthest point within the block where `r` is live.
-            let point = self.regioncx.liveness_values().point_from_location(location);
-            if let Some(live_sccs) = self.live_sccs_per_point.row(point) {
-                if live_sccs.iter().any(|live_scc| self.reachability.contains(live_scc)) {
-                    continue;
-                }
+            // FIXME: if the issuing region `i` can reach a live region `r` at point `p`, and `r` is
+            // live at point `q`, then it's guaranteed that `i` would reach `r` at point `q`.
+            // Reachability is location-insensitive, and we could take advantage of that, by jumping
+            // to a further point than just the next statement: we can jump to the furthest point
+            // within the block where `r` is live.
+            if self.regioncx.is_loan_live_at(loan_idx, location) {
+                continue;
             }
 
-            // No live region is reachable from the issuing region: the loan is killed at this point
-            // and goes out of scope.
+            // No live region is reachable from the issuing region: the loan is killed at this
+            // point.
             return Some(location);
         }
 

compiler/rustc_borrowck/src/nll.rs

@@ -182,22 +182,26 @@ pub(crate) fn compute_regions<'cx, 'tcx>(
     let elements = &Rc::new(RegionValueElements::new(&body));
 
     // Run the MIR type-checker.
-    let MirTypeckResults { constraints, universal_region_relations, opaque_type_values } =
-        type_check::type_check(
-            infcx,
-            param_env,
-            body,
-            promoted,
-            &universal_regions,
-            location_table,
-            borrow_set,
-            &mut all_facts,
-            flow_inits,
-            move_data,
-            elements,
-            upvars,
-            polonius_input,
-        );
+    let MirTypeckResults {
+        constraints,
+        universal_region_relations,
+        opaque_type_values,
+        live_loans,
+    } = type_check::type_check(
+        infcx,
+        param_env,
+        body,
+        promoted,
+        &universal_regions,
+        location_table,
+        borrow_set,
+        &mut all_facts,
+        flow_inits,
+        move_data,
+        elements,
+        upvars,
+        polonius_input,
+    );
 
     if let Some(all_facts) = &mut all_facts {
         let _prof_timer = infcx.tcx.prof.generic_activity("polonius_fact_generation");
@@ -275,6 +279,7 @@ pub(crate) fn compute_regions<'cx, 'tcx>(
         type_tests,
         liveness_constraints,
         elements,
+        live_loans,
     );
 
     // Generate various additional constraints.

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -7,6 +7,7 @@ use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_data_structures::graph::scc::Sccs;
 use rustc_errors::Diagnostic;
 use rustc_hir::def_id::CRATE_DEF_ID;
+use rustc_index::bit_set::SparseBitMatrix;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_infer::infer::outlives::test_type_match;
 use rustc_infer::infer::region_constraints::{GenericKind, VarInfos, VerifyBound, VerifyIfEq};
@@ -21,6 +22,7 @@ use rustc_middle::traits::ObligationCauseCode;
 use rustc_middle::ty::{self, RegionVid, Ty, TyCtxt, TypeFoldable, TypeVisitableExt};
 use rustc_span::Span;
 
+use crate::dataflow::BorrowIndex;
 use crate::{
     constraints::{
         graph::NormalConstraintGraph, ConstraintSccIndex, OutlivesConstraint, OutlivesConstraintSet,
@@ -30,8 +32,8 @@ use crate::{
     nll::PoloniusOutput,
     region_infer::reverse_sccs::ReverseSccGraph,
     region_infer::values::{
-        LivenessValues, PlaceholderIndices, RegionElement, RegionValueElements, RegionValues,
-        ToElementIndex,
+        LivenessValues, PlaceholderIndices, PointIndex, RegionElement, RegionValueElements,
+        RegionValues, ToElementIndex,
     },
     type_check::{free_region_relations::UniversalRegionRelations, Locations},
     universal_regions::UniversalRegions,
@@ -119,6 +121,9 @@ pub struct RegionInferenceContext<'tcx> {
     /// Information about how the universally quantified regions in
     /// scope on this function relate to one another.
     universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
+
+    /// The set of loans that are live at a given point in the CFG, when using `-Zpolonius=next`.
+    live_loans: SparseBitMatrix<PointIndex, BorrowIndex>,
 }
 
 /// Each time that `apply_member_constraint` is successful, it appends
@@ -330,6 +335,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         type_tests: Vec<TypeTest<'tcx>>,
         liveness_constraints: LivenessValues<RegionVid>,
         elements: &Rc<RegionValueElements>,
+        live_loans: SparseBitMatrix<PointIndex, BorrowIndex>,
     ) -> Self {
         debug!("universal_regions: {:#?}", universal_regions);
         debug!("outlives constraints: {:#?}", outlives_constraints);
@@ -383,6 +389,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             type_tests,
             universal_regions,
             universal_region_relations,
+            live_loans,
         };
 
         result.init_free_and_bound_regions();
@@ -2285,15 +2292,35 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         self.constraint_sccs.as_ref()
     }
 
-    /// Access to the liveness values.
-    pub(crate) fn liveness_values(&self) -> &LivenessValues<RegionVid> {
-        &self.liveness_constraints
-    }
-
     /// Returns whether the given SCC has any member constraints.
     pub(crate) fn scc_has_member_constraints(&self, scc: ConstraintSccIndex) -> bool {
         self.member_constraints.indices(scc).next().is_some()
     }
+
+    /// Returns whether the given SCC is live at all points: whether the representative is a
+    /// placeholder or a free region.
+    pub(crate) fn scc_is_live_at_all_points(&self, scc: ConstraintSccIndex) -> bool {
+        // FIXME: there must be a cleaner way to find this information. At least, when
+        // higher-ranked subtyping is abstracted away from the borrowck main path, we'll only
+        // need to check whether this is a universal region.
+        let representative = self.scc_representatives[scc];
+        let origin = self.var_infos[representative].origin;
+        let live_at_all_points = matches!(
+            origin,
+            RegionVariableOrigin::Nll(
+                NllRegionVariableOrigin::Placeholder(_) | NllRegionVariableOrigin::FreeRegion
+            )
+        );
+        live_at_all_points
+    }
+
+    /// Returns whether the `loan_idx` is live at the given `location`: whether its issuing
+    /// region is contained within the type of a variable that is live at this point.
+    /// Note: for now, the sets of live loans is only available when using `-Zpolonius=next`.
+    pub(crate) fn is_loan_live_at(&self, loan_idx: BorrowIndex, location: Location) -> bool {
+        let point = self.liveness_constraints.point_from_location(location);
+        self.live_loans.contains(point, loan_idx)
+    }
 }
 
 impl<'tcx> RegionDefinition<'tcx> {