iterator: add an Extendable trait

src/libstd/iterator.rs

@@ -32,6 +32,12 @@ pub trait FromIterator<A, T: Iterator<A>> {
     fn from_iterator(iterator: &mut T) -> Self;
 }
 
+/// A type growable from an `Iterator` implementation
+pub trait Extendable<A, T: Iterator<A>>: FromIterator<A, T> {
+    /// Extend a container with the elements yielded by an iterator
+    fn extend(&mut self, iterator: &mut T);
+}
+
 /// An interface for dealing with "external iterators". These types of iterators
 /// can be resumed at any time as all state is stored internally as opposed to
 /// being located on the call stack.

src/libstd/vec.rs

@@ -2224,7 +2224,7 @@ impl<T> Iterator<T> for VecConsumeRevIterator<T> {
 }
 
 impl<A, T: Iterator<A>> FromIterator<A, T> for ~[A] {
-    pub fn from_iterator(iterator: &mut T) -> ~[A] {
+    fn from_iterator(iterator: &mut T) -> ~[A] {
         let (lower, _) = iterator.size_hint();
         let mut xs = with_capacity(lower);
         for iterator.advance |x| {
@@ -2234,6 +2234,17 @@ impl<A, T: Iterator<A>> FromIterator<A, T> for ~[A] {
     }
 }
 
+impl<A, T: Iterator<A>> Extendable<A, T> for ~[A] {
+    fn extend(&mut self, iterator: &mut T) {
+        let (lower, _) = iterator.size_hint();
+        let len = self.len();
+        self.reserve(len + lower);
+        for iterator.advance |x| {
+            self.push(x);
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use option::{None, Option, Some};