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).

1 files changed, 3 insertions, 0 deletions

diff --git a/src/libstd/vec.rs b/src/libstd/vec.rs
index 12aebe20161..6ddb2a72286 100644
--- a/src/libstd/vec.rs
+++ b/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 }
 }