[board] Create const arrays for Shape that return &'static to its enum cases

board/src/piece.rs

@@ -2,6 +2,7 @@
 
 use crate::Square;
 use std::fmt;
+use std::slice::Iter;
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub enum Color {
@@ -33,16 +34,24 @@ pub enum Shape {
 }
 
 impl Shape {
-    pub fn iter() -> impl Iterator<Item = Shape> {
-        [
+    pub fn iter() -> Iter<'static, Shape> {
+        const ALL_SHAPES: [Shape; 6] = [
             Shape::Pawn,
             Shape::Knight,
             Shape::Bishop,
             Shape::Rook,
             Shape::Queen,
             Shape::King,
-        ]
-        .into_iter()
+        ];
+
+        ALL_SHAPES.iter()
+    }
+
+    pub fn promotable() -> Iter<'static, Shape> {
+        const PROMOTABLE_SHAPES: [Shape; 4] =
+            [Shape::Queen, Shape::Rook, Shape::Bishop, Shape::Knight];
+
+        PROMOTABLE_SHAPES.iter()
     }
 
     fn _ascii_representation(&self) -> char {

board/src/position/pieces.rs

@@ -11,7 +11,7 @@ pub struct Pieces<'a> {
 
     current_shape: Option<Shape>,
 
-    shape_iterator: Box<dyn Iterator<Item = Shape>>,
+    shape_iterator: Box<dyn Iterator<Item = &'static Shape>>,
     square_iterator: Option<Box<dyn Iterator<Item = Square>>>,
 }
 
@@ -41,7 +41,7 @@ impl<'a> Iterator for Pieces<'a> {
         let mut next_nonempty_bitboard: Option<&BitBoard> = None;
 
         while let Some(shape) = self.shape_iterator.next() {
-            let piece = Piece::new(self.color, shape);
+            let piece = Piece::new(self.color, *shape);
 
             let bitboard = self.position.bitboard_for_piece(piece);
             if bitboard.is_empty() {
@@ -49,7 +49,7 @@ impl<'a> Iterator for Pieces<'a> {
             }
 
             next_nonempty_bitboard = Some(bitboard);
-            current_shape = Some(shape);
+            current_shape = Some(*shape);
 
             break;
         }

board/src/position/position.rs

@@ -129,7 +129,7 @@ impl Position {
     pub fn piece_on_square(&self, sq: Square) -> Option<PlacedPiece> {
         for color in Color::iter() {
             for shape in Shape::iter() {
-                let piece = Piece::new(color, shape);
+                let piece = Piece::new(color, *shape);
                 let bb = self.bitboard_for_piece(piece);
                 if bb.has_piece_at(&sq) {
                     return Some(PlacedPiece::new(piece, sq));