diff --git a/lib/cretonne/src/ir/heap.rs b/lib/cretonne/src/ir/heap.rs
index 7920ec344a..e2a5366a1a 100644
--- a/lib/cretonne/src/ir/heap.rs
+++ b/lib/cretonne/src/ir/heap.rs
@@ -25,6 +25,8 @@ pub struct HeapData {
 #[derive(Clone)]
 pub enum HeapBase {
     /// The heap base lives in a reserved register.
+    ///
+    /// This feature is not yet implemented.
     ReservedReg,
 
     /// The heap base is in a global variable.
diff --git a/lib/cretonne/src/isa/stack.rs b/lib/cretonne/src/isa/stack.rs
index e8a9b6981b..9e09a52808 100644
--- a/lib/cretonne/src/isa/stack.rs
+++ b/lib/cretonne/src/isa/stack.rs
@@ -68,6 +68,8 @@ pub enum StackBase {
     FP = 1,
 
     /// Use an explicit zone pointer in a general-purpose register.
+    ///
+    /// This feature is not yet implemented.
     Zone = 2,
 }