diff --git a/src/libstd/ffi/os_str.rs b/src/libstd/ffi/os_str.rs
index 7b8bf42e0a74a..41bdd9c51d458 100644
--- a/src/libstd/ffi/os_str.rs
+++ b/src/libstd/ffi/os_str.rs
@@ -205,6 +205,12 @@ impl OsString {
         self.inner.reserve_exact(additional)
     }
 
+    /// Shrinks the capacity of the `OsString` to match its length.
+    #[unstable(feature = "osstring_shrink_to_fit", issue = "40421")]
+    pub fn shrink_to_fit(&mut self) {
+        self.inner.shrink_to_fit()
+    }
+
     /// Converts this `OsString` into a boxed `OsStr`.
     #[unstable(feature = "into_boxed_os_str", issue = "0")]
     pub fn into_boxed_os_str(self) -> Box<OsStr> {
diff --git a/src/libstd/sys/redox/os_str.rs b/src/libstd/sys/redox/os_str.rs
index 0f967863899cb..474d59eed83d1 100644
--- a/src/libstd/sys/redox/os_str.rs
+++ b/src/libstd/sys/redox/os_str.rs
@@ -83,6 +83,11 @@ impl Buf {
         self.inner.reserve_exact(additional)
     }
 
+    #[inline]
+    pub fn shrink_to_fit(&mut self) {
+        self.inner.shrink_to_fit()
+    }
+
     pub fn as_slice(&self) -> &Slice {
         unsafe { mem::transmute(&*self.inner) }
     }
diff --git a/src/libstd/sys/unix/os_str.rs b/src/libstd/sys/unix/os_str.rs
index 938bcfc6d162e..c27599ec0206f 100644
--- a/src/libstd/sys/unix/os_str.rs
+++ b/src/libstd/sys/unix/os_str.rs
@@ -83,6 +83,11 @@ impl Buf {
         self.inner.reserve_exact(additional)
     }
 
+    #[inline]
+    pub fn shrink_to_fit(&mut self) {
+        self.inner.shrink_to_fit()
+    }
+
     pub fn as_slice(&self) -> &Slice {
         unsafe { mem::transmute(&*self.inner) }
     }
diff --git a/src/libstd/sys/windows/os_str.rs b/src/libstd/sys/windows/os_str.rs
index 04e45dcf54963..b02b06e1ef2e1 100644
--- a/src/libstd/sys/windows/os_str.rs
+++ b/src/libstd/sys/windows/os_str.rs
@@ -89,6 +89,10 @@ impl Buf {
         self.inner.reserve_exact(additional)
     }
 
+    pub fn shrink_to_fit(&mut self) {
+        self.inner.shrink_to_fit()
+    }
+
     #[inline]
     pub fn into_box(self) -> Box<Slice> {
         unsafe { mem::transmute(self.inner.into_box()) }
diff --git a/src/libstd/sys_common/wtf8.rs b/src/libstd/sys_common/wtf8.rs
index 1d61181a4ee0f..b486d4ffda3fd 100644
--- a/src/libstd/sys_common/wtf8.rs
+++ b/src/libstd/sys_common/wtf8.rs
@@ -236,6 +236,11 @@ impl Wtf8Buf {
         self.bytes.reserve_exact(additional)
     }
 
+    #[inline]
+    pub fn shrink_to_fit(&mut self) {
+        self.bytes.shrink_to_fit()
+    }
+
     /// Returns the number of bytes that this string buffer can hold without reallocating.
     #[inline]
     pub fn capacity(&self) -> usize {