diff --git a/compiler/rustc_middle/src/mir/traversal.rs b/compiler/rustc_middle/src/mir/traversal.rs
index 9321044c14b7..049cdaab7831 100644
--- a/compiler/rustc_middle/src/mir/traversal.rs
+++ b/compiler/rustc_middle/src/mir/traversal.rs
@@ -296,7 +296,7 @@ pub fn reverse_postorder<'a, 'tcx>(
 /// of `if <T as Trait>::CONST`, as well as [`NullOp::UbChecks`].
 ///
 /// [`NullOp::UbChecks`]: rustc_middle::mir::NullOp::UbChecks
-fn reachable_blocks<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) -> BitSet<BasicBlock> {
+pub fn reachable_blocks<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) -> BitSet<BasicBlock> {
     let body = tcx.instance_mir(instance.def);
     let mut visitor = MonoReachable {
         body,
diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 396b5670e3f8..d83b1d7fe374 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -672,7 +672,7 @@ struct MirUsedCollector<'a, 'tcx> {
     visiting_call_terminator: bool,
     /// Set of functions for which it is OK to move large data into.
     skip_move_check_fns: Option<Vec<DefId>>,
-    reachable_blocks: Option<&'tcx BitSet<mir::BasicBlock>>,
+    reachable_blocks: Option<BitSet<mir::BasicBlock>>,
 }
 
 impl<'a, 'tcx> MirUsedCollector<'a, 'tcx> {
@@ -836,6 +836,7 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
     fn visit_basic_block_data(&mut self, block: mir::BasicBlock, data: &mir::BasicBlockData<'tcx>) {
         if self
             .reachable_blocks
+            .as_ref()
             .expect("we should only walk blocks with CollectionMode::UsedItems")
             .contains(block)
         {
@@ -1414,6 +1415,7 @@ fn collect_items_of_instance<'tcx>(
     // added to `used_items` in a hash set, which can efficiently query in the
     // `body.mentioned_items` loop below without even having to monomorphize the item.
     let mut used_mentioned_items = FxHashSet::<MentionedItem<'tcx>>::default();
+
     let mut collector = MirUsedCollector {
         tcx,
         body,
@@ -1424,7 +1426,7 @@ fn collect_items_of_instance<'tcx>(
         visiting_call_terminator: false,
         skip_move_check_fns: None,
         reachable_blocks: if mode == CollectionMode::UsedItems {
-            Some(tcx.reachable_blocks(instance))
+            Some(rustc_middle::mir::traversal::reachable_blocks(tcx, instance))
         } else {
             None
         },