diff --git a/src/librustc_mir/borrow_check/borrow_set.rs b/src/librustc_mir/borrow_check/borrow_set.rs
index ef9af7bace96f..b4299fbc5a1fe 100644
--- a/src/librustc_mir/borrow_check/borrow_set.rs
+++ b/src/librustc_mir/borrow_check/borrow_set.rs
@@ -3,9 +3,8 @@ use crate::borrow_check::path_utils::allow_two_phase_borrow;
 use crate::borrow_check::place_ext::PlaceExt;
 use crate::dataflow::indexes::BorrowIndex;
 use crate::dataflow::move_paths::MoveData;
-use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
 use rustc_index::bit_set::BitSet;
-use rustc_index::vec::IndexVec;
 use rustc_middle::mir::traversal;
 use rustc_middle::mir::visit::{MutatingUseContext, NonUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{self, Body, Local, Location};
@@ -15,14 +14,11 @@ use std::ops::Index;
 
 crate struct BorrowSet<'tcx> {
     /// The fundamental map relating bitvector indexes to the borrows
-    /// in the MIR.
-    crate borrows: IndexVec<BorrowIndex, BorrowData<'tcx>>,
-
-    /// Each borrow is also uniquely identified in the MIR by the
-    /// `Location` of the assignment statement in which it appears on
-    /// the right hand side; we map each such location to the
-    /// corresponding `BorrowIndex`.
-    crate location_map: FxHashMap<Location, BorrowIndex>,
+    /// in the MIR. Each borrow is also uniquely identified in the MIR
+    /// by the `Location` of the assignment statement in which it
+    /// appears on the right hand side. Thus the location is the map
+    /// key, and its position in the map corresponds to `BorrowIndex`.
+    crate location_map: FxIndexMap<Location, BorrowData<'tcx>>,
 
     /// Locations which activate borrows.
     /// NOTE: a given location may activate more than one borrow in the future
@@ -40,7 +36,7 @@ impl<'tcx> Index<BorrowIndex> for BorrowSet<'tcx> {
     type Output = BorrowData<'tcx>;
 
     fn index(&self, index: BorrowIndex) -> &BorrowData<'tcx> {
-        &self.borrows[index]
+        &self.location_map[index.as_usize()]
     }
 }
 
@@ -129,7 +125,6 @@ impl<'tcx> BorrowSet<'tcx> {
         let mut visitor = GatherBorrows {
             tcx,
             body: &body,
-            idx_vec: IndexVec::new(),
             location_map: Default::default(),
             activation_map: Default::default(),
             local_map: Default::default(),
@@ -146,7 +141,6 @@ impl<'tcx> BorrowSet<'tcx> {
         }
 
         BorrowSet {
-            borrows: visitor.idx_vec,
             location_map: visitor.location_map,
             activation_map: visitor.activation_map,
             local_map: visitor.local_map,
@@ -157,13 +151,32 @@ impl<'tcx> BorrowSet<'tcx> {
     crate fn activations_at_location(&self, location: Location) -> &[BorrowIndex] {
         self.activation_map.get(&location).map(|activations| &activations[..]).unwrap_or(&[])
     }
+
+    crate fn len(&self) -> usize {
+        self.location_map.len()
+    }
+
+    crate fn indices(&self) -> impl Iterator<Item = BorrowIndex> {
+        BorrowIndex::from_usize(0)..BorrowIndex::from_usize(self.len())
+    }
+
+    crate fn iter_enumerated(&self) -> impl Iterator<Item = (BorrowIndex, &BorrowData<'tcx>)> {
+        self.indices().zip(self.location_map.values())
+    }
+
+    crate fn get_index_of(&self, location: &Location) -> Option<BorrowIndex> {
+        self.location_map.get_index_of(location).map(BorrowIndex::from)
+    }
+
+    crate fn contains(&self, location: &Location) -> bool {
+        self.location_map.contains_key(location)
+    }
 }
 
 struct GatherBorrows<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     body: &'a Body<'tcx>,
-    idx_vec: IndexVec<BorrowIndex, BorrowData<'tcx>>,
-    location_map: FxHashMap<Location, BorrowIndex>,
+    location_map: FxIndexMap<Location, BorrowData<'tcx>>,
     activation_map: FxHashMap<Location, Vec<BorrowIndex>>,
     local_map: FxHashMap<mir::Local, FxHashSet<BorrowIndex>>,
 
@@ -203,8 +216,8 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'tcx> {
                 borrowed_place: *borrowed_place,
                 assigned_place: *assigned_place,
             };
-            let idx = self.idx_vec.push(borrow);
-            self.location_map.insert(location, idx);
+            let (idx, _) = self.location_map.insert_full(location, borrow);
+            let idx = BorrowIndex::from(idx);
 
             self.insert_as_pending_if_two_phase(location, assigned_place, kind, idx);
 
@@ -224,7 +237,7 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'tcx> {
         //
         //     TMP = &mut place
         if let Some(&borrow_index) = self.pending_activations.get(temp) {
-            let borrow_data = &mut self.idx_vec[borrow_index];
+            let borrow_data = &mut self.location_map[borrow_index.as_usize()];
 
             // Watch out: the use of TMP in the borrow itself
             // doesn't count as an activation. =)
@@ -265,8 +278,7 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'tcx> {
         if let mir::Rvalue::Ref(region, kind, ref place) = *rvalue {
             // double-check that we already registered a BorrowData for this
 
-            let borrow_index = self.location_map[&location];
-            let borrow_data = &self.idx_vec[borrow_index];
+            let borrow_data = &self.location_map[&location];
             assert_eq!(borrow_data.reserve_location, location);
             assert_eq!(borrow_data.kind, kind);
             assert_eq!(borrow_data.region, region.to_region_vid());
@@ -316,7 +328,7 @@ impl<'a, 'tcx> GatherBorrows<'a, 'tcx> {
         // Consider the borrow not activated to start. When we find an activation, we'll update
         // this field.
         {
-            let borrow_data = &mut self.idx_vec[borrow_index];
+            let borrow_data = &mut self.location_map[borrow_index.as_usize()];
             borrow_data.activation_location = TwoPhaseActivation::NotActivated;
         }
 
@@ -332,7 +344,7 @@ impl<'a, 'tcx> GatherBorrows<'a, 'tcx> {
                        at borrow_index: {:?} with associated data {:?}",
                 temp,
                 old_index,
-                self.idx_vec[old_index]
+                self.location_map[old_index.as_usize()]
             );
         }
     }
diff --git a/src/librustc_mir/borrow_check/constraint_generation.rs b/src/librustc_mir/borrow_check/constraint_generation.rs
index e0420d974fbdf..33b09dcb888ed 100644
--- a/src/librustc_mir/borrow_check/constraint_generation.rs
+++ b/src/librustc_mir/borrow_check/constraint_generation.rs
@@ -217,7 +217,7 @@ impl<'cx, 'cg, 'tcx> ConstraintGeneration<'cx, 'cg, 'tcx> {
                             let places_conflict = places_conflict::places_conflict(
                                 self.infcx.tcx,
                                 self.body,
-                                self.borrow_set.borrows[borrow_index].borrowed_place,
+                                self.borrow_set[borrow_index].borrowed_place,
                                 place,
                                 places_conflict::PlaceConflictBias::NoOverlap,
                             );
diff --git a/src/librustc_mir/borrow_check/invalidation.rs b/src/librustc_mir/borrow_check/invalidation.rs
index fd8f17718e795..2de2124dc5e0e 100644
--- a/src/librustc_mir/borrow_check/invalidation.rs
+++ b/src/librustc_mir/borrow_check/invalidation.rs
@@ -166,8 +166,8 @@ impl<'cx, 'tcx> Visitor<'tcx> for InvalidationGenerator<'cx, 'tcx> {
                 // Invalidate all borrows of local places
                 let borrow_set = self.borrow_set.clone();
                 let resume = self.location_table.start_index(resume.start_location());
-                for i in borrow_set.borrows.indices() {
-                    if borrow_of_local_data(borrow_set.borrows[i].borrowed_place) {
+                for (i, data) in borrow_set.iter_enumerated() {
+                    if borrow_of_local_data(data.borrowed_place) {
                         self.all_facts.invalidates.push((resume, i));
                     }
                 }
@@ -178,8 +178,8 @@ impl<'cx, 'tcx> Visitor<'tcx> for InvalidationGenerator<'cx, 'tcx> {
                 // Invalidate all borrows of local places
                 let borrow_set = self.borrow_set.clone();
                 let start = self.location_table.start_index(location);
-                for i in borrow_set.borrows.indices() {
-                    if borrow_of_local_data(borrow_set.borrows[i].borrowed_place) {
+                for (i, data) in borrow_set.iter_enumerated() {
+                    if borrow_of_local_data(data.borrowed_place) {
                         self.all_facts.invalidates.push((start, i));
                     }
                 }
@@ -369,7 +369,7 @@ impl<'cx, 'tcx> InvalidationGenerator<'cx, 'tcx> {
         let tcx = self.tcx;
         let body = self.body;
         let borrow_set = self.borrow_set.clone();
-        let indices = self.borrow_set.borrows.indices();
+        let indices = self.borrow_set.indices();
         each_borrow_involving_path(
             self,
             tcx,
diff --git a/src/librustc_mir/borrow_check/mod.rs b/src/librustc_mir/borrow_check/mod.rs
index 76cc03fa60909..6e211b42a052e 100644
--- a/src/librustc_mir/borrow_check/mod.rs
+++ b/src/librustc_mir/borrow_check/mod.rs
@@ -1131,11 +1131,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 (
                     Reservation(WriteKind::MutableBorrow(bk)),
                     BorrowKind::Shallow | BorrowKind::Shared,
-                ) if {
-                    tcx.migrate_borrowck() && this.borrow_set.location_map.contains_key(&location)
-                } =>
-                {
-                    let bi = this.borrow_set.location_map[&location];
+                ) if { tcx.migrate_borrowck() && this.borrow_set.contains(&location) } => {
+                    let bi = this.borrow_set.get_index_of(&location).unwrap();
                     debug!(
                         "recording invalid reservation of place: {:?} with \
                          borrow index {:?} as warning",
diff --git a/src/librustc_mir/borrow_check/nll.rs b/src/librustc_mir/borrow_check/nll.rs
index f6b3be59d9576..66a17cba6bb01 100644
--- a/src/librustc_mir/borrow_check/nll.rs
+++ b/src/librustc_mir/borrow_check/nll.rs
@@ -206,7 +206,7 @@ pub(in crate::borrow_check) fn compute_regions<'cx, 'tcx>(
         //   the `borrow_set`, their `BorrowIndex` are synthesized as the universal region index
         //   added to the existing number of loans, as if they succeeded them in the set.
         //
-        let borrow_count = borrow_set.borrows.len();
+        let borrow_count = borrow_set.len();
         debug!(
             "compute_regions: polonius placeholders, num_universals={}, borrow_count={}",
             universal_regions.len(),
diff --git a/src/librustc_mir/borrow_check/type_check/mod.rs b/src/librustc_mir/borrow_check/type_check/mod.rs
index bc5c144cd742c..ff98de5475ecf 100644
--- a/src/librustc_mir/borrow_check/type_check/mod.rs
+++ b/src/librustc_mir/borrow_check/type_check/mod.rs
@@ -2469,11 +2469,11 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         // example).
         if let Some(all_facts) = all_facts {
             let _prof_timer = self.infcx.tcx.prof.generic_activity("polonius_fact_generation");
-            if let Some(borrow_index) = borrow_set.location_map.get(&location) {
+            if let Some(borrow_index) = borrow_set.get_index_of(&location) {
                 let region_vid = borrow_region.to_region_vid();
                 all_facts.borrow_region.push((
                     region_vid,
-                    *borrow_index,
+                    borrow_index,
                     location_table.mid_index(location),
                 ));
             }
diff --git a/src/librustc_mir/dataflow/impls/borrows.rs b/src/librustc_mir/dataflow/impls/borrows.rs
index dfca270396de9..7e7b7f2cc76fa 100644
--- a/src/librustc_mir/dataflow/impls/borrows.rs
+++ b/src/librustc_mir/dataflow/impls/borrows.rs
@@ -136,9 +136,9 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
         borrow_set: &Rc<BorrowSet<'tcx>>,
     ) -> Self {
         let mut borrows_out_of_scope_at_location = FxHashMap::default();
-        for (borrow_index, borrow_data) in borrow_set.borrows.iter_enumerated() {
+        for (borrow_index, borrow_data) in borrow_set.iter_enumerated() {
             let borrow_region = borrow_data.region.to_region_vid();
-            let location = borrow_set.borrows[borrow_index].reserve_location;
+            let location = borrow_data.reserve_location;
 
             precompute_borrows_out_of_scope(
                 body,
@@ -160,7 +160,7 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
     }
 
     pub fn location(&self, idx: BorrowIndex) -> &Location {
-        &self.borrow_set.borrows[idx].reserve_location
+        &self.borrow_set[idx].reserve_location
     }
 
     /// Add all borrows to the kill set, if those borrows are out of scope at `location`.
@@ -216,7 +216,7 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
             places_conflict(
                 self.tcx,
                 self.body,
-                self.borrow_set.borrows[i].borrowed_place,
+                self.borrow_set[i].borrowed_place,
                 place,
                 PlaceConflictBias::NoOverlap,
             )
@@ -232,7 +232,7 @@ impl<'tcx> dataflow::AnalysisDomain<'tcx> for Borrows<'_, 'tcx> {
     const NAME: &'static str = "borrows";
 
     fn bits_per_block(&self, _: &mir::Body<'tcx>) -> usize {
-        self.borrow_set.borrows.len() * 2
+        self.borrow_set.len() * 2
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut BitSet<Self::Idx>) {
@@ -271,11 +271,11 @@ impl<'tcx> dataflow::GenKillAnalysis<'tcx> for Borrows<'_, 'tcx> {
                     ) {
                         return;
                     }
-                    let index = self.borrow_set.location_map.get(&location).unwrap_or_else(|| {
+                    let index = self.borrow_set.get_index_of(&location).unwrap_or_else(|| {
                         panic!("could not find BorrowIndex for location {:?}", location);
                     });
 
-                    trans.gen(*index);
+                    trans.gen(index);
                 }
 
                 // Make sure there are no remaining borrows for variables