iterator: add a FromIterator trait

This is able to take advantage of the lower bound from the size hint.

src/libstd/iterator.rs

@@ -27,6 +27,12 @@ use ops::{Add, Mul};
 use cmp::Ord;
 use clone::Clone;
 
+/// Conversion from an `Iterator`
+pub trait FromIterator<A, T: Iterator<A>> {
+    /// Build a container with elements from an external iterator.
+    pub fn from_iterator(iterator: &mut T) -> Self;
+}
+
 /// 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.
@@ -931,7 +937,7 @@ mod tests {
     #[test]
     fn test_counter_from_iter() {
         let mut it = Counter::new(0, 5).take_(10);
-        let xs: ~[int] = iter::FromIter::from_iter::<int, ~[int]>(|f| it.advance(f));
+        let xs: ~[int] = FromIterator::from_iterator(&mut it);
         assert_eq!(xs, ~[0, 5, 10, 15, 20, 25, 30, 35, 40, 45]);
     }
 

src/libstd/vec.rs

@@ -19,7 +19,7 @@ use cmp::{Eq, Ord, TotalEq, TotalOrd, Ordering, Less, Equal, Greater};
 use clone::Clone;
 use old_iter::BaseIter;
 use old_iter;
-use iterator::{Iterator, IteratorUtil};
+use iterator::{FromIterator, Iterator, IteratorUtil};
 use iter::FromIter;
 use kinds::Copy;
 use libc;
@@ -2511,6 +2511,18 @@ impl<T> FromIter<T> for ~[T]{
     }
 }
 
+#[cfg(not(stage0))]
+impl<A, T: Iterator<A>> FromIterator<A, T> for ~[A] {
+    pub fn from_iterator(iterator: &mut T) -> ~[A] {
+        let (lower, _) = iterator.size_hint();
+        let mut xs = with_capacity(lower.get_or_zero());
+        for iterator.advance |x| {
+            xs.push(x);
+        }
+        xs
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use option::{None, Option, Some};