const_fn bitvec + bitvec build fixes

src/bitboard.rs

@@ -1,4 +1,5 @@
 use crate::board::{BOARD_AREA, BOARD_SIZE};
+use const_fn::const_fn;
 use static_assertions::const_assert;
 
 // quick explanation for the dual-nature of [`BitBoard`]
@@ -11,7 +12,10 @@ use static_assertions::const_assert;
 use bitvec::prelude::*;
 
 #[cfg(feature = "bitvec")]
-pub type BitBoardInner = BitArr!(for BOARD_AREA, in u64, Lsb0);
+type BBBaseType = u64;
+
+#[cfg(feature = "bitvec")]
+pub type BitBoardInner = BitArr!(for BOARD_AREA, in BBBaseType, Lsb0);
 
 #[cfg(not(feature = "bitvec"))]
 pub type BitBoardInner = u64;
@@ -78,7 +82,8 @@ impl BitBoard {
         self.0.set(Self::get_index(row, col), value);
     }
 
-    // works on both `bitvec` and native
+    // works on both `bitvec` and native (const on native)
+    #[const_fn(cfg(not(feature = "bitvec")))]
     pub fn count(&self) -> usize {
         self.0.count_ones() as usize
     }