Statically assert zero-cost invariants

Since some or all of those invariants will come in handy to ensure we
use as little memory as possible, to maximize the speed of the move
generation later on.

flake.nix

@@ -144,6 +144,8 @@
         seer = naersk-lib.buildPackage {
           src = self;
 
+          doCheck = true;
+
           passthru = {
             inherit my-rust;
           };

src/board/bitboard/mod.rs

@@ -1,4 +1,6 @@
 use super::Square;
+use crate::utils::static_assert;
+
 mod iterator;
 use iterator::*;
 
@@ -63,6 +65,9 @@ impl Bitboard {
     }
 }
 
+// Ensure zero-cost (at least size-wise) wrapping.
+static_assert!(std::mem::size_of::<Bitboard>() == std::mem::size_of::<u64>());
+
 impl Default for Bitboard {
     fn default() -> Self {
         Self::EMPTY

src/board/file.rs

@@ -1,4 +1,5 @@
 use super::Bitboard;
+use crate::utils::static_assert;
 
 /// An enum representing a singular file on a chess board (i.e: the columns).
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
@@ -70,6 +71,9 @@ impl File {
     }
 }
 
+// Ensure that niche-optimization is in effect.
+static_assert!(std::mem::size_of::<Option<File>>() == std::mem::size_of::<File>());
+
 #[cfg(test)]
 mod test {
     use super::*;

src/board/rank.rs

@@ -1,4 +1,5 @@
 use super::Bitboard;
+use crate::utils::static_assert;
 
 /// An enum representing a singular rank on a chess board (i.e: the rows).
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
@@ -70,6 +71,9 @@ impl Rank {
     }
 }
 
+// Ensure that niche-optimization is in effect.
+static_assert!(std::mem::size_of::<Option<Rank>>() == std::mem::size_of::<Rank>());
+
 #[cfg(test)]
 mod test {
     use super::*;

src/board/square.rs

@@ -1,4 +1,5 @@
 use super::{Bitboard, File, Rank};
+use crate::utils::static_assert;
 
 /// Represent a square on a chessboard. Defined in the same order as the
 /// [Bitboard](crate::board::Bitboard) squares.
@@ -179,6 +180,9 @@ impl std::ops::Sub<Bitboard> for Square {
     }
 }
 
+// Ensure that niche-optimization is in effect.
+static_assert!(std::mem::size_of::<Option<Square>>() == std::mem::size_of::<Square>());
+
 #[cfg(test)]
 mod test {
     use super::*;

src/lib.rs

@@ -1 +1,2 @@
 pub mod board;
+pub mod utils;

src/utils/mod.rs

@@ -0,0 +1,2 @@
+pub mod static_assert;
+pub use static_assert::*;

src/utils/static_assert.rs

@@ -0,0 +1,28 @@
+//! Static assert.
+
+/// Assert a condition at compile-time.
+///
+/// See [RFC 2790] for potential addition into Rust itself.
+///
+/// [RFC 2790]: https://github.com/rust-lang/rfcs/issues/2790
+///
+/// # Examples
+///
+/// ```
+/// use seer::utils::static_assert;
+///
+/// static_assert!(42 > 0);
+/// ```
+#[macro_export]
+macro_rules! static_assert {
+    ($condition:expr) => {
+        // Based on the latest one in `rustc`'s one before it was [removed].
+        //
+        // [removed]: https://github.com/rust-lang/rust/commit/c2dad1c6b9f9636198d7c561b47a2974f5103f6d
+        #[allow(dead_code)]
+        const _: () = [()][!($condition) as usize];
+    };
+}
+
+// I want it namespaced, even though it is exported to the root of the crate by `#[macro_export]`.
+pub use static_assert;