auto merge of #19640 : aliblong/rust/power_of_two_reform, r=Gankro

The `is_power_of_two()` method of the `UnsignedInt` trait currently returns `true` for `self == 0`. Zero is not a power of two, assuming an integral exponent `k >= 0`. I've therefore moved this functionality to the new method `is_power_of_two_or_zero()` and reformed `is_power_of_two()` to return false for `self == 0`.

To illustrate the usefulness of the existence of both functions, consider `HashMap`. Its capacity must be zero or a power of two; conversely, it also requires a (non-zero) power of two for key and val alignment.

Also, added a small amount of documentation regarding #18604.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libstd/collections/hash/map.rs b/src/libstd/collections/hash/map.rs
index 04dd5afdfa2..6bfea7e3cb2 100644
--- a/src/libstd/collections/hash/map.rs
+++ b/src/libstd/collections/hash/map.rs
@@ -623,10 +623,10 @@ impl<K: Eq + Hash<S>, V, S, H: Hasher<S>> HashMap<K, V, H> {
     /// Resizes the internal vectors to a new capacity. It's your responsibility to:
     ///   1) Make sure the new capacity is enough for all the elements, accounting
     ///      for the load factor.
-    ///   2) Ensure new_capacity is a power of two.
+    ///   2) Ensure new_capacity is a power of two or zero.
     fn resize(&mut self, new_capacity: uint) {
         assert!(self.table.size() <= new_capacity);
-        assert!(new_capacity.is_power_of_two());
+        assert!(new_capacity.is_power_of_two() || new_capacity == 0);
 
         let mut old_table = replace(&mut self.table, RawTable::new(new_capacity));
         let old_size = old_table.size();