Improved the use of Extend in a few Iterator methods

src/libcore/iter/traits/iterator.rs

@@ -1504,34 +1504,44 @@ pub trait Iterator {
     /// assert_eq!(odd, vec![1, 3]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn partition<B, F>(self, f: F) -> (B, B) where
+    fn partition<B, F>(self, mut predicate: F) -> (B, B)
+    where
         Self: Sized,
         B: Default + Extend<Self::Item>,
-        F: FnMut(&Self::Item) -> bool
+        F: FnMut(&Self::Item) -> bool,
     {
+        let mut left: B = Default::default();
+        let mut right: B = Default::default();
+        let mut fused = self.fuse();
+
         #[inline]
-        fn extend<'a, T, B: Extend<T>>(
-            mut f: impl FnMut(&T) -> bool + 'a,
-            left: &'a mut B,
-            right: &'a mut B,
-        ) -> impl FnMut(T) + 'a {
-            move |x| {
-                if f(&x) {
-                    left.extend(Some(x));
+        fn extend_rhs<'a, A, T: Extend<A>, P: FnMut(&A) -> bool + 'a>(
+            rhs: &'a mut T,
+            mut predicate: P,
+        ) -> impl FnMut(A) -> Option<A> + 'a {
+            move |item| {
+                if predicate(&item) {
+                    Some(item)
                 } else {
-                    right.extend(Some(x));
+                    rhs.extend(Some(item));
+                    None
                 }
             }
         }
 
-        let mut left: B = Default::default();
-        let mut right: B = Default::default();
+        left.extend((&mut fused).filter_map(extend_rhs(&mut right, &mut predicate)));
 
-        self.for_each(extend(f, &mut left, &mut right));
+        // left.extend may not have fully consumed self.
+        right.extend(fused.filter(move |item| !predicate(item)));
+
+        // right.extend may not have fully consumed self, but in that case we
+        // assume that we don't care about the remaining elements of `self`,
+        // since both `left` and `right` finsished being extended
 
         (left, right)
     }
 
+
     /// Reorder the elements of this iterator *in-place* according to the given predicate,
     /// such that all those that return `true` precede all those that return `false`.
     /// Returns the number of `true` elements found.
@@ -2373,29 +2383,41 @@ pub trait Iterator {
     /// assert_eq!(right, [2, 4]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB) where
+    fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
+    where
         FromA: Default + Extend<A>,
         FromB: Default + Extend<B>,
-        Self: Sized + Iterator<Item=(A, B)>,
+        Self: Sized + Iterator<Item = (A, B)>,
     {
-        fn extend<'a, A, B>(
-            ts: &'a mut impl Extend<A>,
-            us: &'a mut impl Extend<B>,
-        ) -> impl FnMut((A, B)) + 'a {
-            move |(t, u)| {
-                ts.extend(Some(t));
-                us.extend(Some(u));
+        let mut lhs = FromA::default();
+        let mut rhs = FromB::default();
+        let mut fused = self.fuse();
+
+        #[inline]
+        fn extend_rhs<'a, A, B, T: Extend<B>>(rhs: &'a mut T) -> impl FnMut((A, B)) -> A + 'a {
+            move |(a, b)| {
+                rhs.extend(Some(b));
+                a
             }
         }
 
-        let mut ts: FromA = Default::default();
-        let mut us: FromB = Default::default();
+        #[inline]
+        fn second<A, B>((_a, b): (A, B)) -> B {
+            b
+        }
+
+        lhs.extend((&mut fused).map(extend_rhs(&mut rhs)));
 
-        self.for_each(extend(&mut ts, &mut us));
+        // lhs.extend may not have fully consumed the iterator
+        rhs.extend((&mut fused).map(second));
 
-        (ts, us)
+        // rhs.extend may not have fully consumed the iterator
+        fused.for_each(drop);
+
+        (lhs, rhs)
     }
 
+
     /// Creates an iterator which copies all of its elements.
     ///
     /// This is useful when you have an iterator over `&T`, but you need an