Add Deref and DerefMut implementations for PrimaryMap.

lib/entity/src/primary.rs

@@ -1,6 +1,6 @@
 //! Densely numbered entity references as mapping keys.
 use std::marker::PhantomData;
-use std::ops::{Index, IndexMut};
+use std::ops::{Deref, DerefMut, Index, IndexMut};
 use std::slice;
 use std::vec::Vec;
 use {EntityRef, Iter, IterMut, Keys};
@@ -145,6 +145,26 @@ where
     }
 }
 
+impl<K, V> Deref for PrimaryMap<K, V>
+where
+    K: EntityRef,
+{
+    type Target = [V];
+
+    fn deref(&self) -> &Self::Target {
+        &self.elems
+    }
+}
+
+impl<K, V> DerefMut for PrimaryMap<K, V>
+where
+    K: EntityRef,
+{
+    fn deref_mut(&mut self) -> &mut [V] {
+        &mut self.elems
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -322,4 +342,12 @@ mod tests {
         }
     }
 
+    #[test]
+    fn deref() {
+        let mut m = PrimaryMap::<E, isize>::new();
+        let _: &[isize] = m.as_ref();
+        let _: &mut [isize] = m.as_mut();
+        let _: &[isize] = &m;
+        let _: &mut [isize] = &mut m;
+    }
 }