diff --git a/src/libcollections/string.rs b/src/libcollections/string.rs
index 3a304c6292930..4ea8c565169c9 100644
--- a/src/libcollections/string.rs
+++ b/src/libcollections/string.rs
@@ -1567,6 +1567,7 @@ impl_eq! { Cow<'a, str>, String }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Default for String {
+    /// Create an empty `String`.
     #[inline]
     fn default() -> String {
         String::new()
diff --git a/src/libcore/sync/atomic.rs b/src/libcore/sync/atomic.rs
index 75ddd2021a8f7..3c10c5431c23c 100644
--- a/src/libcore/sync/atomic.rs
+++ b/src/libcore/sync/atomic.rs
@@ -95,6 +95,7 @@ pub struct AtomicBool {
 #[cfg(target_has_atomic = "8")]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Default for AtomicBool {
+    /// Create an `AtomicBool` initialized to `false`.
     fn default() -> Self {
         Self::new(false)
     }