diff --git a/compiler/rustc_index/src/interval.rs b/compiler/rustc_index/src/interval.rs
index 503470f896d09..34f541a8cc639 100644
--- a/compiler/rustc_index/src/interval.rs
+++ b/compiler/rustc_index/src/interval.rs
@@ -17,7 +17,7 @@ mod tests;
 /// first value of the following element.
 #[derive(Debug, Clone)]
 pub struct IntervalSet<I> {
-    // Start, end
+    // Start, end (both inclusive)
     map: SmallVec<[(u32, u32); 2]>,
     domain: usize,
     _data: PhantomData<I>,