diff --git a/src/libcollections/string.rs b/src/libcollections/string.rs
index eedf4c2c11f..8ba5c6ffbf2 100644
--- a/src/libcollections/string.rs
+++ b/src/libcollections/string.rs
@@ -701,6 +701,12 @@ impl String {
     /// Violating these may cause problems like corrupting the allocator's
     /// internal datastructures.
     ///
+    /// The ownership of `ptr` is effectively transferred to the
+    /// `String` which may then deallocate, reallocate or change the
+    /// contents of memory pointed to by the pointer at will. Ensure
+    /// that nothing else uses the pointer after calling this
+    /// function.
+    ///
     /// # Examples
     ///
     /// Basic usage:
diff --git a/src/libcollections/vec.rs b/src/libcollections/vec.rs
index da56b21cf0c..98b5c4663cd 100644
--- a/src/libcollections/vec.rs
+++ b/src/libcollections/vec.rs
@@ -348,6 +348,12 @@ impl<T> Vec<T> {
     /// Violating these may cause problems like corrupting the allocator's
     /// internal datastructures.
     ///
+    /// The ownership of `ptr` is effectively transferred to the
+    /// `Vec<T>` which may then deallocate, reallocate or change the
+    /// contents of memory pointed to by the pointer at will. Ensure
+    /// that nothing else uses the pointer after calling this
+    /// function.
+    ///
     /// # Examples
     ///
     /// ```