Update the stable attributes to use the current nightly version number

src/liballoc/string.rs

@@ -2239,7 +2239,7 @@ impl<'a> From<String> for Cow<'a, str> {
     }
 }
 
-#[stable(feature = "cow_from_string_ref", since = "1.28.0")]
+#[stable(feature = "cow_from_string_ref", since = "1.27.0")]
 impl<'a> From<&'a String> for Cow<'a, str> {
     #[inline]
     fn from(s: &'a String) -> Cow<'a, str> {

src/liballoc/vec.rs

@@ -2285,7 +2285,7 @@ impl<'a, T: Clone> From<Vec<T>> for Cow<'a, [T]> {
     }
 }
 
-#[stable(feature = "cow_from_vec_ref", since = "1.28.0")]
+#[stable(feature = "cow_from_vec_ref", since = "1.27.0")]
 impl<'a, T: Clone> From<&'a Vec<T>> for Cow<'a, [T]> {
     fn from(v: &'a Vec<T>) -> Cow<'a, [T]> {
         Cow::Borrowed(v.as_slice())

src/libstd/ffi/c_str.rs

@@ -682,7 +682,7 @@ impl Borrow<CStr> for CString {
     fn borrow(&self) -> &CStr { self }
 }
 
-#[stable(feature = "cstring_from_cow_cstr", since = "1.28.0")]
+#[stable(feature = "cstring_from_cow_cstr", since = "1.27.0")]
 impl<'a> From<Cow<'a, CStr>> for CString {
     #[inline]
     fn from(s: Cow<'a, CStr>) -> Self {
@@ -714,23 +714,23 @@ impl From<CString> for Box<CStr> {
     }
 }
 
-#[stable(feature = "cow_from_cstr", since = "1.28.0")]
+#[stable(feature = "cow_from_cstr", since = "1.27.0")]
 impl<'a> From<CString> for Cow<'a, CStr> {
     #[inline]
     fn from(s: CString) -> Cow<'a, CStr> {
         Cow::Owned(s)
     }
 }
 
-#[stable(feature = "cow_from_cstr", since = "1.28.0")]
+#[stable(feature = "cow_from_cstr", since = "1.27.0")]
 impl<'a> From<&'a CStr> for Cow<'a, CStr> {
     #[inline]
     fn from(s: &'a CStr) -> Cow<'a, CStr> {
         Cow::Borrowed(s)
     }
 }
 
-#[stable(feature = "cow_from_cstr", since = "1.28.0")]
+#[stable(feature = "cow_from_cstr", since = "1.27.0")]
 impl<'a> From<&'a CString> for Cow<'a, CStr> {
     #[inline]
     fn from(s: &'a CString) -> Cow<'a, CStr> {

src/libstd/ffi/os_str.rs

@@ -664,31 +664,31 @@ impl<'a> From<&'a OsStr> for Rc<OsStr> {
     }
 }
 
-#[stable(feature = "cow_from_osstr", since = "1.28.0")]
+#[stable(feature = "cow_from_osstr", since = "1.27.0")]
 impl<'a> From<OsString> for Cow<'a, OsStr> {
     #[inline]
     fn from(s: OsString) -> Cow<'a, OsStr> {
         Cow::Owned(s)
     }
 }
 
-#[stable(feature = "cow_from_osstr", since = "1.28.0")]
+#[stable(feature = "cow_from_osstr", since = "1.27.0")]
 impl<'a> From<&'a OsStr> for Cow<'a, OsStr> {
     #[inline]
     fn from(s: &'a OsStr) -> Cow<'a, OsStr> {
         Cow::Borrowed(s)
     }
 }
 
-#[stable(feature = "cow_from_osstr", since = "1.28.0")]
+#[stable(feature = "cow_from_osstr", since = "1.27.0")]
 impl<'a> From<&'a OsString> for Cow<'a, OsStr> {
     #[inline]
     fn from(s: &'a OsString) -> Cow<'a, OsStr> {
         Cow::Borrowed(s.as_os_str())
     }
 }
 
-#[stable(feature = "osstring_from_cow_osstr", since = "1.28.0")]
+#[stable(feature = "osstring_from_cow_osstr", since = "1.27.0")]
 impl<'a> From<Cow<'a, OsStr>> for OsString {
     #[inline]
     fn from(s: Cow<'a, OsStr>) -> Self {

src/libstd/path.rs

@@ -1532,15 +1532,15 @@ impl<'a> From<PathBuf> for Cow<'a, Path> {
     }
 }
 
-#[stable(feature = "cow_from_pathbuf_ref", since = "1.28.0")]
+#[stable(feature = "cow_from_pathbuf_ref", since = "1.27.0")]
 impl<'a> From<&'a PathBuf> for Cow<'a, Path> {
     #[inline]
     fn from(p: &'a PathBuf) -> Cow<'a, Path> {
         Cow::Borrowed(p.as_path())
     }
 }
 
-#[stable(feature = "pathbuf_from_cow_path", since = "1.28.0")]
+#[stable(feature = "pathbuf_from_cow_path", since = "1.27.0")]
 impl<'a> From<Cow<'a, Path>> for PathBuf {
     #[inline]
     fn from(p: Cow<'a, Path>) -> Self {