diff --git a/src/librustc_mir/borrow_check/borrow_set.rs b/src/librustc_mir/borrow_check/borrow_set.rs
index a25c3afd63c2..1fae97566d0a 100644
--- a/src/librustc_mir/borrow_check/borrow_set.rs
+++ b/src/librustc_mir/borrow_check/borrow_set.rs
@@ -35,7 +35,7 @@ crate struct BorrowSet<'tcx> {
     /// NOTE: A given location may activate more than one borrow in the future
     /// when more general two-phase borrow support is introduced, but for now we
     /// only need to store one borrow index
-    crate activation_map: FxHashMap<Location, FxHashSet<BorrowIndex>>,
+    crate activation_map: FxHashMap<Location, Vec<BorrowIndex>>,
 
     /// Every borrow has a region; this maps each such regions back to
     /// its borrow-indexes.
@@ -131,7 +131,7 @@ struct GatherBorrows<'a, 'gcx: 'tcx, 'tcx: 'a> {
     mir: &'a Mir<'tcx>,
     idx_vec: IndexVec<BorrowIndex, BorrowData<'tcx>>,
     location_map: FxHashMap<Location, BorrowIndex>,
-    activation_map: FxHashMap<Location, FxHashSet<BorrowIndex>>,
+    activation_map: FxHashMap<Location, Vec<BorrowIndex>>,
     region_map: FxHashMap<Region<'tcx>, FxHashSet<BorrowIndex>>,
     local_map: FxHashMap<mir::Local, FxHashSet<BorrowIndex>>,
     region_span_map: FxHashMap<RegionKind, Span>,
@@ -230,8 +230,8 @@ impl<'a, 'gcx, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'gcx, 'tcx> {
                     borrow_data.activation_location = Some(location);
                     self.activation_map
                         .entry(location)
-                        .or_insert(FxHashSet())
-                        .insert(borrow_index);
+                        .or_insert(Vec::new())
+                        .push(borrow_index);
                 }
 
                 None => {}
diff --git a/src/librustc_mir/borrow_check/mod.rs b/src/librustc_mir/borrow_check/mod.rs
index 2e64626e2ea4..dd76b883d63d 100644
--- a/src/librustc_mir/borrow_check/mod.rs
+++ b/src/librustc_mir/borrow_check/mod.rs
@@ -1267,36 +1267,30 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         // Two-phase borrow support: For each activation that is newly
         // generated at this statement, check if it interferes with
         // another borrow.
-        let domain = flow_state.borrows.operator();
-        let data = domain.borrows();
-        flow_state.borrows.each_gen_bit(|gen| {
-            if gen.is_activation() {
-                let borrow_index = gen.borrow_index();
-                let borrow = &data[borrow_index];
-                // currently the flow analysis registers
-                // activations for both mutable and immutable
-                // borrows. So make sure we are talking about a
-                // mutable borrow before we check it.
-                match borrow.kind {
-                    BorrowKind::Shared => return,
-                    BorrowKind::Unique | BorrowKind::Mut { .. } => {}
-                }
+        let borrows = flow_state.borrows.operator();
+        for &borrow_index in borrows.activations_at_location(location) {
+            let borrow = &borrows.borrows()[borrow_index];
 
-                self.access_place(
-                    ContextKind::Activation.new(location),
-                    (&borrow.borrowed_place, span),
-                    (
-                        Deep,
-                        Activation(WriteKind::MutableBorrow(borrow.kind), borrow_index),
-                    ),
-                    LocalMutationIsAllowed::No,
-                    flow_state,
-                );
-                // We do not need to call `check_if_path_or_subpath_is_moved`
-                // again, as we already called it when we made the
-                // initial reservation.
-            }
-        });
+            // only mutable borrows should be 2-phase
+            assert!(match borrow.kind {
+                BorrowKind::Shared => false,
+                BorrowKind::Unique | BorrowKind::Mut { .. } => true,
+            });
+
+            self.access_place(
+                ContextKind::Activation.new(location),
+                (&borrow.borrowed_place, span),
+                (
+                    Deep,
+                    Activation(WriteKind::MutableBorrow(borrow.kind), borrow_index),
+                ),
+                LocalMutationIsAllowed::No,
+                flow_state,
+            );
+            // We do not need to call `check_if_path_or_subpath_is_moved`
+            // again, as we already called it when we made the
+            // initial reservation.
+        }
     }
 }
 
diff --git a/src/librustc_mir/dataflow/impls/borrows.rs b/src/librustc_mir/dataflow/impls/borrows.rs
index 25f1278d7fa6..190d08e83de4 100644
--- a/src/librustc_mir/dataflow/impls/borrows.rs
+++ b/src/librustc_mir/dataflow/impls/borrows.rs
@@ -156,6 +156,13 @@ impl<'a, 'gcx, 'tcx> Borrows<'a, 'gcx, 'tcx> {
         }
     }
 
+    crate fn activations_at_location(&self, location: Location) -> &[BorrowIndex] {
+        self.borrow_set.activation_map
+            .get(&location)
+            .map(|activations| &activations[..])
+            .unwrap_or(&[])
+    }
+
     /// Performs the activations for a given location
     fn perform_activations_at_location(&self,
                                        sets: &mut BlockSets<ReserveOrActivateIndex>,