poll_token_derive: Calculate variant bits without sizeof_val

Calculate the number of bits necessary to represent the enum variant
using the next_power_of_two() and trailing_zeros() functions from the
primitive usize type.

Also add a test to ensure that the returned value is correct when there
is only one variant in the enum.

BUG=none
TEST=unit tests

Change-Id: Ibd15efd4f06e17a74489fee04ff19aca0dde68b2
Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/959624
Reviewed-by: Zach Reizner <zachr@chromium.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/sys_util/poll_token_derive/poll_token_derive.rs b/sys_util/poll_token_derive/poll_token_derive.rs
index 7af2de5..3506b4d 100644
--- a/sys_util/poll_token_derive/poll_token_derive.rs
+++ b/sys_util/poll_token_derive/poll_token_derive.rs
@@ -326,8 +326,7 @@ impl EnumModel {
         if self.variants.is_empty() {
             return 0;
         }
-        let variant_count = self.variants.len();
-        (mem::size_of_val(&variant_count) as u32 * 8) - (variant_count - 1).leading_zeros()
+        self.variants.len().next_power_of_two().trailing_zeros()
     }
 
     // Generates the function body for `as_raw_token`.