Add a proper Hash implementation for VecMap

Also re-add the previously deleted test with an additional test that would have
failed before, when the hash function depended on the capacity.

src/libcollections/vec_map.rs

@@ -21,6 +21,7 @@ use core::iter;
 use core::iter::{Enumerate, FilterMap};
 use core::mem::replace;
 
+use hash::{Hash, Writer};
 use {vec, slice};
 use vec::Vec;
 
@@ -81,6 +82,19 @@ impl<V:Clone> Clone for VecMap<V> {
     }
 }
 
+impl<S: Writer, V: Hash<S>> Hash<S> for VecMap<V> {
+    fn hash(&self, state: &mut S) {
+        // In order to not traverse the `VecMap` twice, count the elements
+        // during iteration.
+        let mut count: uint = 0;
+        for elt in self.iter() {
+            elt.hash(state);
+            count += 1;
+        }
+        count.hash(state);
+    }
+}
+
 impl<V> VecMap<V> {
     /// Creates an empty `VecMap`.
     ///
@@ -605,6 +619,7 @@ pub type MoveItems<V> =
 mod test_map {
     use std::prelude::*;
     use vec::Vec;
+    use hash::hash;
 
     use super::VecMap;
 
@@ -918,6 +933,28 @@ mod test_map {
         assert!(a < b && a <= b);
     }
 
+    #[test]
+    fn test_hash() {
+        let mut x = VecMap::new();
+        let mut y = VecMap::new();
+
+        assert!(hash(&x) == hash(&y));
+        x.insert(1, 'a');
+        x.insert(2, 'b');
+        x.insert(3, 'c');
+
+        y.insert(3, 'c');
+        y.insert(2, 'b');
+        y.insert(1, 'a');
+
+        assert!(hash(&x) == hash(&y));
+
+        x.insert(1000, 'd');
+        x.remove(&1000);
+
+        assert!(hash(&x) == hash(&y));
+    }
+
     #[test]
     fn test_from_iter() {
         let xs: Vec<(uint, char)> = vec![(1u, 'a'), (2, 'b'), (3, 'c'), (4, 'd'), (5, 'e')];