iter: Special case .fold() for several adaptors

Chain and FlatMap can do something interesting here where they convert
folds into sequences of folds on their inner iterators. This generally
results in better code.

Also implement .fold() specifically for .map() and .cloned() so that any
inner fold improvements are available through map and cloned.

Author: bluss

src/libcore/iter/mod.rs

@@ -399,6 +399,12 @@ impl<'a, I, T: 'a> Iterator for Cloned<I>
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.it.size_hint()
     }
+
+    fn fold<Acc, F>(self, init: Acc, mut f: F) -> Acc
+        where F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.it.fold(init, move |acc, elt| f(acc, elt.clone()))
+    }
 }
 
 #[stable(feature = "iter_cloned", since = "1.1.0")]
@@ -544,6 +550,25 @@ impl<A, B> Iterator for Chain<A, B> where
         }
     }
 
+    fn fold<Acc, F>(self, init: Acc, mut f: F) -> Acc
+        where F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        let mut accum = init;
+        match self.state {
+            ChainState::Both | ChainState::Front => {
+                accum = self.a.fold(accum, &mut f);
+            }
+            _ => { }
+        }
+        match self.state {
+            ChainState::Both | ChainState::Back => {
+                accum = self.b.fold(accum, &mut f);
+            }
+            _ => { }
+        }
+        accum
+    }
+
     #[inline]
     fn nth(&mut self, mut n: usize) -> Option<A::Item> {
         match self.state {
@@ -939,6 +964,13 @@ impl<B, I: Iterator, F> Iterator for Map<I, F> where F: FnMut(I::Item) -> B {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.iter.size_hint()
     }
+
+    fn fold<Acc, G>(self, init: Acc, mut g: G) -> Acc
+        where G: FnMut(Acc, Self::Item) -> Acc,
+    {
+        let mut f = self.f;
+        self.iter.fold(init, move |acc, elt| g(acc, f(elt)))
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1688,6 +1720,22 @@ impl<I: Iterator, U: IntoIterator, F> Iterator for FlatMap<I, U, F>
         }
     }
 
+    fn fold<Acc, G>(self, init: Acc, mut f: G) -> Acc
+        where G: FnMut(Acc, Self::Item) -> Acc,
+    {
+        let mut accum = init;
+        if let Some(iter) = self.frontiter {
+            accum = iter.fold(accum, &mut f);
+        }
+        for iter in self.iter.map(self.f) {
+            accum = iter.into_iter().fold(accum, &mut f);
+        }
+        if let Some(iter) = self.backiter {
+            accum = iter.fold(accum, &mut f);
+        }
+        accum
+    }
+
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
         let (flo, fhi) = self.frontiter.as_ref().map_or((0, Some(0)), |it| it.size_hint());