Privatize Rc::is_unique

[unstable, deprecated since 1.15.0]

Author: cuviper

src/doc/unstable-book/src/SUMMARY.md

@@ -155,7 +155,6 @@
     - [io_error_internals](library-features/io-error-internals.md)
     - [io](library-features/io.md)
     - [ip](library-features/ip.md)
-    - [is_unique](library-features/is-unique.md)
     - [iter_rfind](library-features/iter-rfind.md)
     - [libstd_io_internals](library-features/libstd-io-internals.md)
     - [libstd_sys_internals](library-features/libstd-sys-internals.md)

src/doc/unstable-book/src/library-features/is-unique.md

@@ -488,11 +488,7 @@ impl<T: ?Sized> Rc<T> {
     ///
     /// [weak]: struct.Weak.html
     #[inline]
-    #[unstable(feature = "is_unique", reason = "uniqueness has unclear meaning",
-               issue = "28356")]
-    #[rustc_deprecated(since = "1.15.0",
-                       reason = "too niche; use `strong_count` and `weak_count` instead")]
-    pub fn is_unique(this: &Self) -> bool {
+    fn is_unique(this: &Self) -> bool {
         Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
     }