Auto merge of #113218 - lqd:polonius-scopes, r=jackh726

Compute NLL loan scopes using the polonius model

For a *location-insensitive* analysis (that is, without expressiveness improvements for users yet), this PR implements loans going out of scope using reachability and liveness, rather than checking if the issuing region's values contain a given CFG point. This is equivalent to NLL scopes and computes the same data.

r? `@matthewjasper`

A couple of notes:
- there are some assumptions about SCC representatives, placeholders, free regions, and member constraints that I believe hold, and they're documented in the code
- this passes all the UI tests with `-Zpolonius=next` -- the perf is [not terrible](https://github.com/rust-lang/rust/pull/112432#issuecomment-1749685862) and there are a bunch of ways to improve it in the future.
- there's a fixme left, hopefully Matthew you know a clean way to get the information it mentions.

compiler/rustc_borrowck/src/dataflow.rs

@@ -1,6 +1,7 @@
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
 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,
@@ -222,6 +223,7 @@ impl<'tcx> OutOfScopePrecomputer<'_, 'tcx> {
     }
 }
 
+// This is `pub` because it's used by unstable external borrowck data users, see `consumers.rs`.
 pub fn calculate_borrows_out_of_scope_at_location<'tcx>(
     body: &Body<'tcx>,
     regioncx: &RegionInferenceContext<'tcx>,
@@ -238,15 +240,196 @@ pub fn calculate_borrows_out_of_scope_at_location<'tcx>(
     prec.borrows_out_of_scope_at_location
 }
 
+struct PoloniusOutOfScopePrecomputer<'a, 'tcx> {
+    visited: BitSet<mir::BasicBlock>,
+    visit_stack: Vec<mir::BasicBlock>,
+    body: &'a Body<'tcx>,
+    regioncx: &'a RegionInferenceContext<'tcx>,
+
+    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 {
+        Self {
+            visited: BitSet::new_empty(body.basic_blocks.len()),
+            visit_stack: vec![],
+            body,
+            regioncx,
+            loans_out_of_scope_at_location: FxIndexMap::default(),
+        }
+    }
+}
+
+impl<'tcx> PoloniusOutOfScopePrecomputer<'_, 'tcx> {
+    /// Loans are in scope while they are live: whether they are contained within any live region.
+    /// In the location-insensitive analysis, a loan will be contained in a region if the issuing
+    /// region can reach it in the subset graph. So this is a reachability problem.
+    fn precompute_loans_out_of_scope(
+        &mut self,
+        loan_idx: BorrowIndex,
+        issuing_region: RegionVid,
+        loan_issued_at: Location,
+    ) {
+        let sccs = self.regioncx.constraint_sccs();
+        let issuing_region_scc = sccs.scc(issuing_region);
+
+        // 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, 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_is_live_at_all_points(scc) {
+                return;
+            }
+        }
+
+        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 `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(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.
+            return;
+        }
+
+        // The loan is not dead. Add successor BBs to the work list, if necessary.
+        for succ_bb in first_bb_data.terminator().successors() {
+            if self.visited.insert(succ_bb) {
+                self.visit_stack.push(succ_bb);
+            }
+        }
+
+        // We may end up visiting `first_block` again. This is not an issue: we know at this point
+        // that the loan is not killed in the `first_lo..=first_hi` range, so checking the
+        // `0..first_lo` range and the `0..first_hi` range gives the same result.
+        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(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)
+                    .or_default()
+                    .push(loan_idx);
+
+                // The loan dies within this block, so we don't need to visit its successors.
+                continue;
+            }
+
+            // Add successor BBs to the work list, if necessary.
+            for succ_bb in bb_data.terminator().successors() {
+                if self.visited.insert(succ_bb) {
+                    self.visit_stack.push(succ_bb);
+                }
+            }
+        }
+
+        self.visited.clear();
+        assert!(self.visit_stack.is_empty(), "visit 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,
+        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:
+            // - 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 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.
+            return Some(location);
+        }
+
+        None
+    }
+}
+
 impl<'a, 'tcx> Borrows<'a, 'tcx> {
     pub fn new(
         tcx: TyCtxt<'tcx>,
         body: &'a Body<'tcx>,
-        nonlexical_regioncx: &'a RegionInferenceContext<'tcx>,
+        regioncx: &'a RegionInferenceContext<'tcx>,
         borrow_set: &'a BorrowSet<'tcx>,
     ) -> Self {
-        let borrows_out_of_scope_at_location =
-            calculate_borrows_out_of_scope_at_location(body, nonlexical_regioncx, borrow_set);
+        let mut borrows_out_of_scope_at_location =
+            calculate_borrows_out_of_scope_at_location(body, regioncx, borrow_set);
+
+        // The in-tree polonius analysis computes loans going out of scope using the set-of-loans
+        // model, and makes sure they're identical to the existing computation of the set-of-points
+        // model.
+        if tcx.sess.opts.unstable_opts.polonius.is_next_enabled() {
+            let mut polonius_prec = PoloniusOutOfScopePrecomputer::new(body, regioncx);
+            for (loan_idx, loan_data) in borrow_set.iter_enumerated() {
+                let issuing_region = loan_data.region;
+                let issued_location = loan_data.reserve_location;
+
+                polonius_prec.precompute_loans_out_of_scope(
+                    loan_idx,
+                    issuing_region,
+                    issued_location,
+                );
+            }
+
+            assert_eq!(
+                borrows_out_of_scope_at_location, polonius_prec.loans_out_of_scope_at_location,
+                "the loans out of scope must be the same as the borrows out of scope"
+            );
+
+            borrows_out_of_scope_at_location = polonius_prec.loans_out_of_scope_at_location;
+        }
+
         Borrows { tcx, body, borrow_set, borrows_out_of_scope_at_location }
     }
 
@@ -333,6 +516,13 @@ impl<'tcx> rustc_mir_dataflow::AnalysisDomain<'tcx> for Borrows<'_, 'tcx> {
     }
 }
 
+/// Forward dataflow computation of the set of borrows that are in scope at a particular location.
+/// - we gen the introduced loans
+/// - we kill loans on locals going out of (regular) scope
+/// - we kill the loans going out of their region's NLL scope: in NLL terms, the frontier where a
+///   region stops containing the CFG points reachable from the issuing location.
+/// - we also kill loans of conflicting places when overwriting a shared path: e.g. borrows of
+///   `a.b.c` when `a` is overwritten.
 impl<'tcx> rustc_mir_dataflow::GenKillAnalysis<'tcx> for Borrows<'_, 'tcx> {
     type Idx = BorrowIndex;
 

compiler/rustc_borrowck/src/facts.rs

@@ -41,7 +41,8 @@ pub(crate) trait AllFactsExt {
 impl AllFactsExt for AllFacts {
     /// Return
     fn enabled(tcx: TyCtxt<'_>) -> bool {
-        tcx.sess.opts.unstable_opts.nll_facts || tcx.sess.opts.unstable_opts.polonius
+        tcx.sess.opts.unstable_opts.nll_facts
+            || tcx.sess.opts.unstable_opts.polonius.is_legacy_enabled()
     }
 
     fn write_to_dir(

compiler/rustc_borrowck/src/nll.rs

@@ -169,10 +169,11 @@ pub(crate) fn compute_regions<'cx, 'tcx>(
     upvars: &[Upvar<'tcx>],
     consumer_options: Option<ConsumerOptions>,
 ) -> NllOutput<'tcx> {
+    let is_polonius_legacy_enabled = infcx.tcx.sess.opts.unstable_opts.polonius.is_legacy_enabled();
     let polonius_input = consumer_options.map(|c| c.polonius_input()).unwrap_or_default()
-        || infcx.tcx.sess.opts.unstable_opts.polonius;
+        || is_polonius_legacy_enabled;
     let polonius_output = consumer_options.map(|c| c.polonius_output()).unwrap_or_default()
-        || infcx.tcx.sess.opts.unstable_opts.polonius;
+        || is_polonius_legacy_enabled;
     let mut all_facts =
         (polonius_input || AllFacts::enabled(infcx.tcx)).then_some(AllFacts::default());
 
@@ -181,22 +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");
@@ -274,6 +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();
@@ -683,7 +690,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         // In Polonius mode, the errors about missing universal region relations are in the output
         // and need to be emitted or propagated. Otherwise, we need to check whether the
         // constraints were too strong, and if so, emit or propagate those errors.
-        if infcx.tcx.sess.opts.unstable_opts.polonius {
+        if infcx.tcx.sess.opts.unstable_opts.polonius.is_legacy_enabled() {
             self.check_polonius_subset_errors(
                 outlives_requirements.as_mut(),
                 &mut errors_buffer,
@@ -2279,6 +2286,41 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         }
         None
     }
+
+    /// Access to the SCC constraint graph.
+    pub(crate) fn constraint_sccs(&self) -> &Sccs<RegionVid, ConstraintSccIndex> {
+        self.constraint_sccs.as_ref()
+    }
+
+    /// 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> {

compiler/rustc_borrowck/src/region_infer/values.rs

@@ -176,6 +176,11 @@ impl<N: Idx> LivenessValues<N> {
     pub(crate) fn region_value_str(&self, r: N) -> String {
         region_value_str(self.get_elements(r).map(RegionElement::Location))
     }
+
+    #[inline]
+    pub(crate) fn point_from_location(&self, location: Location) -> PointIndex {
+        self.elements.point_from_location(location)
+    }
 }
 
 /// Maps from `ty::PlaceholderRegion` values that are used in the rest of

compiler/rustc_borrowck/src/type_check/liveness/trace.rs

@@ -1,10 +1,11 @@
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
-use rustc_index::bit_set::HybridBitSet;
+use rustc_data_structures::graph::WithSuccessors;
+use rustc_index::bit_set::{HybridBitSet, SparseBitMatrix};
 use rustc_index::interval::IntervalSet;
 use rustc_infer::infer::canonical::QueryRegionConstraints;
 use rustc_middle::mir::{BasicBlock, Body, ConstraintCategory, Local, Location};
 use rustc_middle::traits::query::DropckOutlivesResult;
-use rustc_middle::ty::{Ty, TyCtxt, TypeVisitable, TypeVisitableExt};
+use rustc_middle::ty::{RegionVid, Ty, TyCtxt, TypeVisitable, TypeVisitableExt};
 use rustc_span::DUMMY_SP;
 use rustc_trait_selection::traits::query::type_op::outlives::DropckOutlives;
 use rustc_trait_selection::traits::query::type_op::{TypeOp, TypeOpOutput};
@@ -14,6 +15,7 @@ use rustc_mir_dataflow::impls::MaybeInitializedPlaces;
 use rustc_mir_dataflow::move_paths::{HasMoveData, MoveData, MovePathIndex};
 use rustc_mir_dataflow::ResultsCursor;
 
+use crate::dataflow::BorrowIndex;
 use crate::{
     region_infer::values::{self, PointIndex, RegionValueElements},
     type_check::liveness::local_use_map::LocalUseMap,
@@ -50,6 +52,33 @@ pub(super) fn trace<'mir, 'tcx>(
 
     let local_use_map = &LocalUseMap::build(&relevant_live_locals, elements, body);
 
+    // When using `-Zpolonius=next`, compute the set of loans that can reach a given region.
+    let num_loans = typeck.borrowck_context.borrow_set.len();
+    let mut inflowing_loans = SparseBitMatrix::new(num_loans);
+    if typeck.tcx().sess.opts.unstable_opts.polonius.is_next_enabled() {
+        let borrowck_context = &typeck.borrowck_context;
+        let borrow_set = &borrowck_context.borrow_set;
+        let constraint_set = &borrowck_context.constraints.outlives_constraints;
+
+        let num_region_vars = typeck.infcx.num_region_vars();
+        let graph = constraint_set.graph(num_region_vars);
+        let region_graph =
+            graph.region_graph(&constraint_set, borrowck_context.universal_regions.fr_static);
+
+        // Traverse each issuing region's constraints, and record the loan as flowing into the
+        // outlived region.
+        for (loan, issuing_region_data) in borrow_set.iter_enumerated() {
+            for succ in region_graph.depth_first_search(issuing_region_data.region) {
+                // We don't need to mention that a loan flows into its issuing region.
+                if succ == issuing_region_data.region {
+                    continue;
+                }
+
+                inflowing_loans.insert(succ, loan);
+            }
+        }
+    };
+
     let cx = LivenessContext {
         typeck,
         body,
@@ -58,6 +87,7 @@ pub(super) fn trace<'mir, 'tcx>(
         local_use_map,
         move_data,
         drop_data: FxIndexMap::default(),
+        inflowing_loans,
     };
 
     let mut results = LivenessResults::new(cx);
@@ -95,6 +125,9 @@ struct LivenessContext<'me, 'typeck, 'flow, 'tcx> {
     /// Index indicating where each variable is assigned, used, or
     /// dropped.
     local_use_map: &'me LocalUseMap,
+
+    /// Set of loans that flow into a given region, when using `-Zpolonius=next`.
+    inflowing_loans: SparseBitMatrix<RegionVid, BorrowIndex>,
 }
 
 struct DropData<'tcx> {
@@ -486,7 +519,13 @@ impl<'tcx> LivenessContext<'_, '_, '_, 'tcx> {
     ) {
         debug!("add_use_live_facts_for(value={:?})", value);
 
-        Self::make_all_regions_live(self.elements, &mut self.typeck, value, live_at)
+        Self::make_all_regions_live(
+            self.elements,
+            &mut self.typeck,
+            value,
+            live_at,
+            &self.inflowing_loans,
+        );
     }
 
     /// Some variable with type `live_ty` is "drop live" at `location`
@@ -537,7 +576,13 @@ impl<'tcx> LivenessContext<'_, '_, '_, 'tcx> {
         // All things in the `outlives` array may be touched by
         // the destructor and must be live at this point.
         for &kind in &drop_data.dropck_result.kinds {
-            Self::make_all_regions_live(self.elements, &mut self.typeck, kind, live_at);
+            Self::make_all_regions_live(
+                self.elements,
+                &mut self.typeck,
+                kind,
+                live_at,
+                &self.inflowing_loans,
+            );
 
             polonius::add_drop_of_var_derefs_origin(&mut self.typeck, dropped_local, &kind);
         }
@@ -548,6 +593,7 @@ impl<'tcx> LivenessContext<'_, '_, '_, 'tcx> {
         typeck: &mut TypeChecker<'_, 'tcx>,
         value: impl TypeVisitable<TyCtxt<'tcx>>,
         live_at: &IntervalSet<PointIndex>,
+        inflowing_loans: &SparseBitMatrix<RegionVid, BorrowIndex>,
     ) {
         debug!("make_all_regions_live(value={:?})", value);
         debug!(
@@ -556,15 +602,35 @@ impl<'tcx> LivenessContext<'_, '_, '_, 'tcx> {
         );
 
         let tcx = typeck.tcx();
+        let borrowck_context = &mut typeck.borrowck_context;
+
+        // When using `-Zpolonius=next`, we want to record the loans that flow into this value's
+        // regions as being live at the given `live_at` points: this will be used to compute the
+        // location where a loan goes out of scope.
+        let num_loans = borrowck_context.borrow_set.len();
+        let mut value_loans = HybridBitSet::new_empty(num_loans);
+
         tcx.for_each_free_region(&value, |live_region| {
-            let live_region_vid =
-                typeck.borrowck_context.universal_regions.to_region_vid(live_region);
-            typeck
-                .borrowck_context
+            let live_region_vid = borrowck_context.universal_regions.to_region_vid(live_region);
+
+            borrowck_context
                 .constraints
                 .liveness_constraints
                 .add_elements(live_region_vid, live_at);
+
+            // There can only be inflowing loans for this region when we are using
+            // `-Zpolonius=next`.
+            if let Some(inflowing) = inflowing_loans.row(live_region_vid) {
+                value_loans.union(inflowing);
+            }
         });
+
+        // Record the loans reaching the value.
+        if !value_loans.is_empty() {
+            for point in live_at.iter() {
+                borrowck_context.live_loans.union_row(point, &value_loans);
+            }
+        }
     }
 
     fn compute_drop_data(

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -14,6 +14,7 @@ use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::lang_items::LangItem;
+use rustc_index::bit_set::SparseBitMatrix;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_infer::infer::canonical::QueryRegionConstraints;
 use rustc_infer::infer::outlives::env::RegionBoundPairs;
@@ -50,6 +51,8 @@ use rustc_mir_dataflow::impls::MaybeInitializedPlaces;
 use rustc_mir_dataflow::move_paths::MoveData;
 use rustc_mir_dataflow::ResultsCursor;
 
+use crate::dataflow::BorrowIndex;
+use crate::region_infer::values::PointIndex;
 use crate::session_diagnostics::{MoveUnsized, SimdShuffleLastConst};
 use crate::{
     borrow_set::BorrowSet,
@@ -163,6 +166,9 @@ pub(crate) fn type_check<'mir, 'tcx>(
 
     debug!(?normalized_inputs_and_output);
 
+    // When using `-Zpolonius=next`, liveness will record the set of live loans per point.
+    let mut live_loans = SparseBitMatrix::new(borrow_set.len());
+
     let mut borrowck_context = BorrowCheckContext {
         universal_regions,
         location_table,
@@ -170,6 +176,7 @@ pub(crate) fn type_check<'mir, 'tcx>(
         all_facts,
         constraints: &mut constraints,
         upvars,
+        live_loans: &mut live_loans,
     };
 
     let mut checker = TypeChecker::new(
@@ -240,7 +247,7 @@ pub(crate) fn type_check<'mir, 'tcx>(
         })
         .collect();
 
-    MirTypeckResults { constraints, universal_region_relations, opaque_type_values }
+    MirTypeckResults { constraints, universal_region_relations, opaque_type_values, live_loans }
 }
 
 fn translate_outlives_facts(typeck: &mut TypeChecker<'_, '_>) {
@@ -855,12 +862,21 @@ struct BorrowCheckContext<'a, 'tcx> {
     borrow_set: &'a BorrowSet<'tcx>,
     pub(crate) constraints: &'a mut MirTypeckRegionConstraints<'tcx>,
     upvars: &'a [Upvar<'tcx>],
+
+    /// The set of loans that are live at a given point in the CFG, filled in by `liveness::trace`,
+    /// when using `-Zpolonius=next`.
+    pub(crate) live_loans: &'a mut SparseBitMatrix<PointIndex, BorrowIndex>,
 }
 
+/// Holder struct for passing results from MIR typeck to the rest of the non-lexical regions
+/// inference computation.
 pub(crate) struct MirTypeckResults<'tcx> {
     pub(crate) constraints: MirTypeckRegionConstraints<'tcx>,
     pub(crate) universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
     pub(crate) opaque_type_values: FxIndexMap<OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>>,
+
+    /// The set of loans that are live at a given point in the CFG, when using `-Zpolonius=next`.
+    pub(crate) live_loans: SparseBitMatrix<PointIndex, BorrowIndex>,
 }
 
 /// A collection of region constraints that must be satisfied for the

compiler/rustc_interface/src/tests.rs

@@ -9,6 +9,7 @@ use rustc_session::config::DebugInfo;
 use rustc_session::config::Input;
 use rustc_session::config::InstrumentXRay;
 use rustc_session::config::LinkSelfContained;
+use rustc_session::config::Polonius;
 use rustc_session::config::TraitSolver;
 use rustc_session::config::{build_configuration, build_session_options, to_crate_config};
 use rustc_session::config::{
@@ -814,7 +815,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(panic_abort_tests, true);
     tracked!(panic_in_drop, PanicStrategy::Abort);
     tracked!(plt, Some(true));
-    tracked!(polonius, true);
+    tracked!(polonius, Polonius::Legacy);
     tracked!(precise_enum_drop_elaboration, false);
     tracked!(print_fuel, Some("abc".to_string()));
     tracked!(profile, true);

compiler/rustc_session/src/config.rs

@@ -3172,6 +3172,7 @@ impl PpMode {
 /// we have an opt-in scheme here, so one is hopefully forced to think about
 /// how the hash should be calculated when adding a new command-line argument.
 pub(crate) mod dep_tracking {
+    use super::Polonius;
     use super::{
         BranchProtection, CFGuard, CFProtection, CrateType, DebugInfo, DebugInfoCompression,
         ErrorOutputType, InstrumentCoverage, InstrumentXRay, LinkerPluginLto, LocationDetail,
@@ -3281,6 +3282,7 @@ pub(crate) mod dep_tracking {
         OomStrategy,
         LanguageIdentifier,
         TraitSolver,
+        Polonius,
     );
 
     impl<T1, T2> DepTrackingHash for (T1, T2)
@@ -3419,3 +3421,35 @@ impl DumpMonoStatsFormat {
         }
     }
 }
+
+/// `-Zpolonius` values, enabling the borrow checker polonius analysis, and which version: legacy,
+/// or future prototype.
+#[derive(Clone, Copy, PartialEq, Hash, Debug)]
+pub enum Polonius {
+    /// The default value: disabled.
+    Off,
+
+    /// Legacy version, using datalog and the `polonius-engine` crate. Historical value for `-Zpolonius`.
+    Legacy,
+
+    /// In-tree prototype, extending the NLL infrastructure.
+    Next,
+}
+
+impl Default for Polonius {
+    fn default() -> Self {
+        Polonius::Off
+    }
+}
+
+impl Polonius {
+    /// Returns whether the legacy version of polonius is enabled
+    pub fn is_legacy_enabled(&self) -> bool {
+        matches!(self, Polonius::Legacy)
+    }
+
+    /// Returns whether the "next" version of polonius is enabled
+    pub fn is_next_enabled(&self) -> bool {
+        matches!(self, Polonius::Next)
+    }
+}

compiler/rustc_session/src/options.rs

@@ -414,6 +414,7 @@ mod desc {
         "one of supported split dwarf modes (`split` or `single`)";
     pub const parse_link_self_contained: &str = "one of: `y`, `yes`, `on`, `n`, `no`, `off`, or a list of enabled (`+` prefix) and disabled (`-` prefix) \
         components: `crto`, `libc`, `unwind`, `linker`, `sanitizers`, `mingw`";
+    pub const parse_polonius: &str = "either no value or `legacy` (the default), or `next`";
     pub const parse_stack_protector: &str =
         "one of (`none` (default), `basic`, `strong`, or `all`)";
     pub const parse_branch_protection: &str =
@@ -471,6 +472,21 @@ mod parse {
         }
     }
 
+    /// Parses whether polonius is enabled, and if so, which version.
+    pub(crate) fn parse_polonius(slot: &mut Polonius, v: Option<&str>) -> bool {
+        match v {
+            Some("legacy") | None => {
+                *slot = Polonius::Legacy;
+                true
+            }
+            Some("next") => {
+                *slot = Polonius::Next;
+                true
+            }
+            _ => false,
+        }
+    }
+
     /// Use this for any string option that has a static default.
     pub(crate) fn parse_string(slot: &mut String, v: Option<&str>) -> bool {
         match v {
@@ -1659,7 +1675,7 @@ options! {
         "whether to use the PLT when calling into shared libraries;
         only has effect for PIC code on systems with ELF binaries
         (default: PLT is disabled if full relro is enabled on x86_64)"),
-    polonius: bool = (false, parse_bool, [TRACKED],
+    polonius: Polonius = (Polonius::default(), parse_polonius, [TRACKED],
         "enable polonius-based borrow-checker (default: no)"),
     polymorphize: bool = (false, parse_bool, [TRACKED],
           "perform polymorphization analysis"),