diff --git a/compiler/rustc_index/src/bit_set.rs b/compiler/rustc_index/src/bit_set.rs
index 3ea1a52ae28..d730ef58deb 100644
--- a/compiler/rustc_index/src/bit_set.rs
+++ b/compiler/rustc_index/src/bit_set.rs
@@ -502,6 +502,7 @@ pub fn insert(&mut self, elem: T) -> bool {
                     };
                     #[cfg(not(feature = "nightly"))]
                     let mut words = {
+                        // FIXME: unconditionally use `Rc::new_zeroed` once it is stable (#63291).
                         let words = mem::MaybeUninit::<[Word; CHUNK_WORDS]>::zeroed();
                         // SAFETY: `words` can safely be all zeroes.
                         let words = unsafe { words.assume_init() };
@@ -567,6 +568,7 @@ pub fn remove(&mut self, elem: T) -> bool {
                     };
                     #[cfg(not(feature = "nightly"))]
                     let mut words = {
+                        // FIXME: unconditionally use `Rc::new_zeroed` once it is stable (#63291).
                         let words = mem::MaybeUninit::<[Word; CHUNK_WORDS]>::zeroed();
                         // SAFETY: `words` can safely be all zeroes.
                         let words = unsafe { words.assume_init() };