std::iterator: Introduce trait ExactSizeHint

The trait `ExactSizeHint` is introduced to solve a few small niggles:

* We can't reverse (`.invert()`) an enumeration iterator
* for a vector, we have `v.iter().position(f)` but `v.rposition(f)`.
* We can't reverse `Zip` even if both iterators are from vectors

`ExactSizeHint` is an empty trait that is intended to indicate that an
iterator, for example `VecIterator`, knows its exact finite size and
reports it correctly using `.size_hint()`. Only adaptors that preserve
this at all times, can expose this trait further. (Where here we say
finite for fitting in uint).

src/libstd/iterator.rs

@@ -616,6 +616,26 @@ pub trait RandomAccessIterator<A>: Iterator<A> {
     fn idx(&self, index: uint) -> Option<A>;
 }
 
+/// An iterator that knows its exact length
+///
+/// This trait is a helper for iterators like the vector iterator, so that
+/// it can support double-ended enumeration.
+///
+/// `Iterator::size_hint` *must* return the exact size of the iterator.
+/// Note that the size must fit in `uint`.
+pub trait ExactSizeHint {}
+
+// All adaptors that preserve the size of the wrapped iterator are fine
+// Adaptors that may overflow in `size_hint` are not, i.e. `Chain`.
+impl<T: ExactSizeHint> ExactSizeHint for Enumerate<T> {}
+impl<'self, A, T: ExactSizeHint> ExactSizeHint for Inspect<'self, A, T> {}
+impl<T: ExactSizeHint> ExactSizeHint for Invert<T> {}
+impl<'self, A, B, T: ExactSizeHint> ExactSizeHint for Map<'self, A, B, T> {}
+impl<A, T: ExactSizeHint> ExactSizeHint for Peekable<A, T> {}
+impl<T: ExactSizeHint> ExactSizeHint for Skip<T> {}
+impl<T: ExactSizeHint> ExactSizeHint for Take<T> {}
+impl<T: ExactSizeHint, U: ExactSizeHint> ExactSizeHint for Zip<T, U> {}
+
 /// An double-ended iterator with the direction inverted
 #[deriving(Clone)]
 pub struct Invert<T> {
@@ -1094,6 +1114,21 @@ impl<A, T: Iterator<A>> Iterator<(uint, A)> for Enumerate<T> {
     }
 }
 
+impl<A, T: DoubleEndedIterator<A> + ExactSizeHint> DoubleEndedIterator<(uint, A)>
+for Enumerate<T> {
+    #[inline]
+    fn next_back(&mut self) -> Option<(uint, A)> {
+        match self.iter.next_back() {
+            Some(a) => {
+                let (len, _) = self.iter.size_hint();
+                let ret = Some((self.count + len, a));
+                ret
+            }
+            _ => None
+        }
+    }
+}
+
 impl<A, T: RandomAccessIterator<A>> RandomAccessIterator<(uint, A)> for Enumerate<T> {
     #[inline]
     fn indexable(&self) -> uint {

src/libstd/vec.rs

@@ -2319,6 +2319,9 @@ iterator!{impl VecIterator -> &'self T}
 double_ended_iterator!{impl VecIterator -> &'self T}
 pub type RevIterator<'self, T> = Invert<VecIterator<'self, T>>;
 
+impl<'self, T> ExactSizeHint for VecIterator<'self, T> {}
+impl<'self, T> ExactSizeHint for VecMutIterator<'self, T> {}
+
 impl<'self, T> Clone for VecIterator<'self, T> {
     fn clone(&self) -> VecIterator<'self, T> { *self }
 }