diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index ae75f2d4187ba..1722a7a1f358f 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -1135,8 +1135,10 @@ pub enum ProjectionElem<V, T> {
     ConstantIndex {
         /// index or -index (in Python terms), depending on from_end
         offset: u64,
-        /// The thing being indexed must be at least this long. For arrays this
-        /// is always the exact length.
+        /// The thing being indexed must be at least this long -- otherwise, the
+        /// projection is UB.
+        ///
+        /// For arrays this is always the exact length.
         min_length: u64,
         /// Counting backwards from end? This is always false when indexing an
         /// array.
diff --git a/compiler/stable_mir/src/mir/body.rs b/compiler/stable_mir/src/mir/body.rs
index 9f4db7e483335..742469a1c9325 100644
--- a/compiler/stable_mir/src/mir/body.rs
+++ b/compiler/stable_mir/src/mir/body.rs
@@ -768,8 +768,10 @@ pub enum ProjectionElem {
     ConstantIndex {
         /// index or -index (in Python terms), depending on from_end
         offset: u64,
-        /// The thing being indexed must be at least this long. For arrays this
-        /// is always the exact length.
+        /// The thing being indexed must be at least this long -- otherwise, the
+        /// projection is UB.
+        ///
+        /// For arrays this is always the exact length.
         min_length: u64,
         /// Counting backwards from end? This is always false when indexing an
         /// array.