Rollup merge of #49243 - murarth:stabilize-retain, r=BurntSushi

Stabilize method `String::retain`

Closes #43874

src/doc/unstable-book/src/library-features/string-retain.md

@@ -1,23 +0,0 @@
-# `string_retain`
-
-The tracking issue for this feature is: [#43874]
-
-[#43874]: https://github.com/rust-lang/rust/issues/43874
-
-------------------------
-
-Retains only the characters specified by the predicate.
-
-In other words, remove all characters `c` such that `f(c)` returns `false`.
-This method operates in place and preserves the order of the retained
-characters.
-
-```rust
-#![feature(string_retain)]
-
-let mut s = String::from("f_o_ob_ar");
-
-s.retain(|c| c != '_');
-
-assert_eq!(s, "foobar");
-```

src/liballoc/string.rs

@@ -1177,8 +1177,6 @@ impl String {
     /// # Examples
     ///
     /// ```
-    /// #![feature(string_retain)]
-    ///
     /// let mut s = String::from("f_o_ob_ar");
     ///
     /// s.retain(|c| c != '_');
@@ -1186,7 +1184,7 @@ impl String {
     /// assert_eq!(s, "foobar");
     /// ```
     #[inline]
-    #[unstable(feature = "string_retain", issue = "43874")]
+    #[stable(feature = "string_retain", since = "1.26.0")]
     pub fn retain<F>(&mut self, mut f: F)
         where F: FnMut(char) -> bool
     {