diff --git a/src/libstd/ffi/os_str.rs b/src/libstd/ffi/os_str.rs
index b1c6e7af693d..9b91bd0d41ef 100644
--- a/src/libstd/ffi/os_str.rs
+++ b/src/libstd/ffi/os_str.rs
@@ -348,6 +348,11 @@ impl OsString {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl From<String> for OsString {
+    /// Converts a [`String`] into a [`OsString`].
+    /// The conversion copies the data, and includes an allocation on the heap.
+    ///
+    /// [`String`]: ../string/struct.String.html
+    /// [`OsString`]: struct.OsString.html
     fn from(s: String) -> OsString {
         OsString { inner: Buf::from_string(s) }
     }
@@ -630,6 +635,10 @@ impl<'a> From<&'a OsStr> for Box<OsStr> {
 
 #[stable(feature = "os_string_from_box", since = "1.18.0")]
 impl From<Box<OsStr>> for OsString {
+    /// Converts a `Box<OsStr>` into a `OsString` without copying or allocating.
+    ///
+    /// [`Box`]: ../boxed/struct.Box.html
+    /// [`OsString`]: ../ffi/struct.OsString.html
     fn from(boxed: Box<OsStr>) -> OsString {
         boxed.into_os_string()
     }
@@ -637,6 +646,10 @@ impl From<Box<OsStr>> for OsString {
 
 #[stable(feature = "box_from_os_string", since = "1.20.0")]
 impl From<OsString> for Box<OsStr> {
+    /// Converts a [`OsString`] into a [`Box`]`<OsStr>` without copying or allocating.
+    ///
+    /// [`Box`]: ../boxed/struct.Box.html
+    /// [`OsString`]: ../ffi/struct.OsString.html
     fn from(s: OsString) -> Box<OsStr> {
         s.into_boxed_os_str()
     }
@@ -652,6 +665,10 @@ impl Clone for Box<OsStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<OsString> for Arc<OsStr> {
+    /// Converts a [`OsString`] into a [`Arc`]`<OsStr>` without copying or allocating.
+    ///
+    /// [`Arc`]: ../sync/struct.Arc.html
+    /// [`OsString`]: ../ffi/struct.OsString.html
     #[inline]
     fn from(s: OsString) -> Arc<OsStr> {
         let arc = s.inner.into_arc();
@@ -670,6 +687,10 @@ impl<'a> From<&'a OsStr> for Arc<OsStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<OsString> for Rc<OsStr> {
+    /// Converts a [`OsString`] into a [`Rc`]`<OsStr>` without copying or allocating.
+    ///
+    /// [`Rc`]: ../rc/struct.Rc.html
+    /// [`OsString`]: ../ffi/struct.OsString.html
     #[inline]
     fn from(s: OsString) -> Rc<OsStr> {
         let rc = s.inner.into_rc();