rollup merge of #19626: bluss/string-extend-str

Strings iterate to both char and &str, so it is natural it can also be extended or collected from an iterator of &str.

Apart from the trait implementations, `Extend<char>` is updated to use the iterator size hint, and the test added tests both the char and the &str versions of Extend and FromIterator.

src/libcollections/string.rs

@@ -729,15 +729,38 @@ impl FromIterator<char> for String {
     }
 }
 
+#[experimental = "waiting on FromIterator stabilization"]
+impl<'a> FromIterator<&'a str> for String {
+    fn from_iter<I:Iterator<&'a str>>(iterator: I) -> String {
+        let mut buf = String::new();
+        buf.extend(iterator);
+        buf
+    }
+}
+
 #[experimental = "waiting on Extend stabilization"]
 impl Extend<char> for String {
     fn extend<I:Iterator<char>>(&mut self, mut iterator: I) {
+        let (lower_bound, _) = iterator.size_hint();
+        self.reserve(lower_bound);
         for ch in iterator {
             self.push(ch)
         }
     }
 }
 
+#[experimental = "waiting on Extend stabilization"]
+impl<'a> Extend<&'a str> for String {
+    fn extend<I: Iterator<&'a str>>(&mut self, mut iterator: I) {
+        // A guess that at least one byte per iterator element will be needed.
+        let (lower_bound, _) = iterator.size_hint();
+        self.reserve(lower_bound);
+        for s in iterator {
+            self.push_str(s)
+        }
+    }
+}
+
 impl PartialEq for String {
     #[inline]
     fn eq(&self, other: &String) -> bool { PartialEq::eq(&**self, &**other) }
@@ -1307,6 +1330,27 @@ mod tests {
                "[[], [1], [1, 1]]");
     }
 
+    #[test]
+    fn test_from_iterator() {
+        let s = "ศไทย中华Việt Nam".to_string();
+        let t = "ศไทย中华";
+        let u = "Việt Nam";
+
+        let a: String = s.chars().collect();
+        assert_eq!(s, a);
+
+        let mut b = t.to_string();
+        b.extend(u.chars());
+        assert_eq!(s, b);
+
+        let c: String = vec![t, u].into_iter().collect();
+        assert_eq!(s, c);
+
+        let mut d = t.to_string();
+        d.extend(vec![u].into_iter());
+        assert_eq!(s, d);
+    }
+
     #[bench]
     fn bench_with_capacity(b: &mut Bencher) {
         b.iter(|| {