Merge remote-tracking branch 'origin/master' into azure-pipelines

1 files changed, 24 insertions, 2 deletions

diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index a3e2098695f..e74d37c1c2b 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -1200,8 +1200,8 @@ impl String {
     /// 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.
+    /// This method operates in place, visiting each character exactly once in the
+    /// original order, and preserves the order of the retained characters.
     ///
     /// # Examples
     ///
@@ -1212,6 +1212,16 @@ impl String {
     ///
     /// assert_eq!(s, "foobar");
     /// ```
+    ///
+    /// The exact order may be useful for tracking external state, like an index.
+    ///
+    /// ```
+    /// let mut s = String::from("abcde");
+    /// let keep = [false, true, true, false, true];
+    /// let mut i = 0;
+    /// s.retain(|_| (keep[i], i += 1).0);
+    /// assert_eq!(s, "bce");
+    /// ```
     #[inline]
     #[stable(feature = "string_retain", since = "1.26.0")]
     pub fn retain<F>(&mut self, mut f: F)
@@ -2179,6 +2189,14 @@ impl From<&str> for String {
     }
 }
 
+#[stable(feature = "from_ref_string", since = "1.35.0")]
+impl From<&String> for String {
+    #[inline]
+    fn from(s: &String) -> String {
+        s.clone()
+    }
+}
+
 // note: test pulls in libstd, which causes errors here
 #[cfg(not(test))]
 #[stable(feature = "string_from_box", since = "1.18.0")]
@@ -2367,6 +2385,10 @@ impl Iterator for Drain<'_> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.iter.size_hint()
     }
+    #[inline]
+    fn last(mut self) -> Option<char> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "drain", since = "1.6.0")]