std: Stabilize CString/OsString/PathBuf extra methods

Stabilizes:

* `CString::as_c_str`
* `CString::into_boxed_c_str`
* `CStr::into_c_string`
* `OsString::into_boxed_os_str`
* `OsStr::into_os_string`
* `PathBuf::into_boxed_path`
* `PathBuf::into_path_buf`

Closes #40380

src/libstd/ffi/c_str.rs

@@ -462,7 +462,7 @@ pub fn as_bytes_with_nul(&self) -> &[u8] {
     /// assert_eq!(c_str, CStr::from_bytes_with_nul(b"foo\0").unwrap());
     /// ```
     #[inline]
-    #[unstable(feature = "as_c_str", issue = "40380")]
+    #[stable(feature = "as_c_str", since = "1.20.0")]
     pub fn as_c_str(&self) -> &CStr {
         &*self
     }
@@ -482,7 +482,7 @@ pub fn as_c_str(&self) -> &CStr {
     /// let boxed = c_string.into_boxed_c_str();
     /// assert_eq!(&*boxed, CStr::from_bytes_with_nul(b"foo\0").unwrap());
     /// ```
-    #[unstable(feature = "into_boxed_c_str", issue = "40380")]
+    #[stable(feature = "into_boxed_c_str", since = "1.20.0")]
     pub fn into_boxed_c_str(self) -> Box<CStr> {
         unsafe { mem::transmute(self.into_inner()) }
     }
@@ -1009,7 +1009,7 @@ pub fn to_string_lossy(&self) -> Cow<str> {
     /// let boxed = c_string.into_boxed_c_str();
     /// assert_eq!(boxed.into_c_string(), CString::new("foo").unwrap());
     /// ```
-    #[unstable(feature = "into_boxed_c_str", issue = "40380")]
+    #[stable(feature = "into_boxed_c_str", since = "1.20.0")]
     pub fn into_c_string(self: Box<CStr>) -> CString {
         unsafe { mem::transmute(self) }
     }

src/libstd/ffi/os_str.rs

@@ -260,7 +260,7 @@ pub fn shrink_to_fit(&mut self) {
     ///
     /// let b: Box<OsStr> = s.into_boxed_os_str();
     /// ```
-    #[unstable(feature = "into_boxed_os_str", issue = "40380")]
+    #[stable(feature = "into_boxed_os_str", since = "1.20.0")]
     pub fn into_boxed_os_str(self) -> Box<OsStr> {
         unsafe { mem::transmute(self.inner.into_box()) }
     }
@@ -511,7 +511,7 @@ pub fn len(&self) -> usize {
     ///
     /// [`Box`]: ../boxed/struct.Box.html
     /// [`OsString`]: struct.OsString.html
-    #[unstable(feature = "into_boxed_os_str", issue = "40380")]
+    #[stable(feature = "into_boxed_os_str", since = "1.20.0")]
     pub fn into_os_string(self: Box<OsStr>) -> OsString {
         let inner: Box<Slice> = unsafe { mem::transmute(self) };
         OsString { inner: Buf::from_box(inner) }

src/libstd/path.rs

@@ -1327,7 +1327,7 @@ pub fn into_os_string(self) -> OsString {
     ///
     /// [`Box`]: ../../std/boxed/struct.Box.html
     /// [`Path`]: struct.Path.html
-    #[unstable(feature = "into_boxed_path", issue = "40380")]
+    #[stable(feature = "into_boxed_path", since = "1.20.0")]
     pub fn into_boxed_path(self) -> Box<Path> {
         unsafe { mem::transmute(self.inner.into_boxed_os_str()) }
     }
@@ -2300,7 +2300,7 @@ pub fn is_dir(&self) -> bool {
     ///
     /// [`Box`]: ../../std/boxed/struct.Box.html
     /// [`PathBuf`]: struct.PathBuf.html
-    #[unstable(feature = "into_boxed_path", issue = "40380")]
+    #[stable(feature = "into_boxed_path", since = "1.20.0")]
     pub fn into_path_buf(self: Box<Path>) -> PathBuf {
         let inner: Box<OsStr> = unsafe { mem::transmute(self) };
         PathBuf { inner: OsString::from(inner) }