diff --git a/src/libcore/mem.rs b/src/libcore/mem.rs
index 9fed49903455c..31635ffa53c83 100644
--- a/src/libcore/mem.rs
+++ b/src/libcore/mem.rs
@@ -194,10 +194,12 @@ pub fn forget<T>(t: T) {
 /// u16 | 2
 /// u32 | 4
 /// u64 | 8
+/// u128 | 16
 /// i8 | 1
 /// i16 | 2
 /// i32 | 4
 /// i64 | 8
+/// i128 | 16
 /// f32 | 4
 /// f64 | 8
 /// char | 4