Auto merge of #305 - nnethercote:add-is_empty-checks, r=Amanieu

Don't hash the key when searching in an empty table.

In rustc, approximately one third of all non-modifying lookups are on an
empty table!

src/map.rs

@@ -1096,8 +1096,12 @@ where
  K: Borrow<Q>,
  Q: Hash + Eq,
  {
- let hash = make_hash::<K, Q, S>(&self.hash_builder, k);
- self.table.get(hash, equivalent_key(k))
+ if self.table.is_empty() {
+ None
+ } else {
+ let hash = make_hash::<K, Q, S>(&self.hash_builder, k);
+ self.table.get(hash, equivalent_key(k))
+ }
  }
 
  /// Returns the key-value pair corresponding to the supplied key, with a mutable reference to value.
@@ -1204,8 +1208,12 @@ where
  K: Borrow<Q>,
  Q: Hash + Eq,
  {
- let hash = make_hash::<K, Q, S>(&self.hash_builder, k);
- self.table.get_mut(hash, equivalent_key(k))
+ if self.table.is_empty() {
+ None
+ } else {
+ let hash = make_hash::<K, Q, S>(&self.hash_builder, k);
+ self.table.get_mut(hash, equivalent_key(k))
+ }
  }
 
  /// Attempts to get mutable references to `N` values in the map at once.