Add method to extract single flags

Added the `BitFlags::to_flag` method that converts `BitFlags<T>` to
`T` if there is exactly one flag that is set.

This is semantically equivalent to `flags.iter().exactly_one()` with
`itertools`, but without the dependency and faster.

Author: chitoyuu

src/lib.rs

@@ -230,10 +230,16 @@ pub mod _internal {
         + Copy
         + Clone
     {
+        #[doc(hidden)]
+        fn is_power_of_two(self) -> bool;
     }
 
     for_each_uint! { $ty $hide_docs =>
-        impl BitFlagNum for $ty {}
+        impl BitFlagNum for $ty {
+            fn is_power_of_two(self) -> bool {
+                <$ty>::is_power_of_two(self)
+            }
+        }
     }
 
     // Re-export libcore so the macro doesn't inject "extern crate" downstream.
@@ -268,6 +274,8 @@ pub mod _internal {
     }
 }
 
+use _internal::BitFlagNum;
+
 // Internal debug formatting implementations
 mod formatting;
 
@@ -555,6 +563,16 @@ where
         self.val == T::EMPTY
     }
 
+    /// Returns the flag that is set if there is exactly one.
+    #[inline(always)]
+    pub fn to_flag(self) -> Option<T> {
+        if self.val.is_power_of_two() {
+            Some(unsafe { core::mem::transmute_copy(&self.val) })
+        } else {
+            None
+        }
+    }
+
     /// Returns the underlying bitwise value.
     ///
     /// ```

test_suite/common.rs

@@ -72,6 +72,10 @@ fn test_foo() {
     assert_eq!((Test::A ^ Test::B), Test::A | Test::B);
 
     assert_eq!(BitFlags::<Default6>::default(), Default6::B | Default6::C);
+
+    assert_eq!(BitFlags::<Test>::empty().to_flag(), None);
+    assert_eq!(BitFlags::<Test>::from(Test::B).to_flag(), Some(Test::B));
+    assert_eq!((Test::A | Test::C).to_flag(), None);
 }
 
 #[test]