auto merge of #13614 : cgaebel/rust/master, r=brson

We previously allocated 3x for every HashMap creation and resize. This patch reduces it to 1x.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/libstd/num/mod.rs b/src/libstd/num/mod.rs
index 12befed743a..b21a80cfd23 100644
--- a/src/libstd/num/mod.rs
+++ b/src/libstd/num/mod.rs
@@ -311,6 +311,12 @@ pub fn next_power_of_two<T: Unsigned + Int>(n: T) -> T {
     tmp + one()
 }
 
+// Returns `true` iff `n == 2^k` for some k.
+#[inline]
+pub fn is_power_of_two<T: Unsigned + Int>(n: T) -> bool {
+    (n - one()) & n == zero()
+}
+
 /// Returns the smallest power of 2 greater than or equal to `n`. If the next
 /// power of two is greater than the type's maximum value, `None` is returned,
 /// otherwise the power of 2 is wrapped in `Some`.