[core] Address clippy suggestions; clean up unit tests

In coordinates.rs:

- Add some [must_use] decorators to some getters
- Rewrite some unit tests to remove the .expect() and use ? instead

core/src/coordinates.rs

@@ -59,6 +59,7 @@ macro_rules! range_bound_struct {
         }
 
         impl $type {
+            #[must_use]
             $vis fn new(x: $repr) -> Option<Self> {
                 if x < $max {
                     Some(Self(x))
@@ -73,14 +74,21 @@ macro_rules! range_bound_struct {
             ///
             /// This function does not perform any bounds checking. It should only be called when
             /// the input is already known to be within bounds, i.e. when `x >= Self::FIRST && x < Self::LAST`.
+            #[must_use]
             $vis unsafe fn new_unchecked(x: $repr) -> Self {
                 debug_assert!((Self::FIRST.0..=Self::LAST.0).contains(&x));
                 Self(x)
             }
 
+            #[must_use]
             $vis fn as_index(&self) -> &$repr {
                 &self.0
             }
+
+            #[must_use]
+            $vis fn iter(&self) -> impl Iterator<Item = Self> {
+                (Self::FIRST.0..=Self::LAST.0).map(Self)
+            }
         }
 
         impl From<$type> for $repr {
@@ -105,11 +113,13 @@ coordinate_enum!(
 );
 
 impl Direction {
+    #[must_use]
     pub fn to_offset(&self) -> i8 {
         const OFFSETS: [i8; 8] = [8, 9, 1, -7, -8, -9, -1, 7];
         OFFSETS[*self as usize]
     }
 
+    #[must_use]
     pub fn opposite(&self) -> Direction {
         const OPPOSITES: [Direction; 8] = [
             Direction::South,
@@ -228,7 +238,7 @@ impl Square {
 
     #[inline]
     pub fn file(self) -> File {
-        unsafe { File::new_unchecked((self as u8) & 0b000111) }
+        unsafe { File::new_unchecked((self as u8) & 0b000_00111) }
     }
 
     #[inline]
@@ -374,39 +384,45 @@ mod tests {
     }
 
     #[test]
-    fn good_algebraic_input() {
+    fn good_algebraic_input() -> Result<(), ParseSquareError> {
-        let sq = Square::from_algebraic_str("a4").expect("Failed to parse 'a4' square");
+        let sq = Square::from_algebraic_str("a4")?;
         assert_eq!(sq.file(), File(0));
         assert_eq!(sq.rank(), Rank(3));
 
-        let sq = Square::from_algebraic_str("B8").expect("Failed to parse 'B8' square");
+        let sq = Square::from_algebraic_str("B8")?;
         assert_eq!(sq.file(), File(1));
         assert_eq!(sq.rank(), Rank(7));
 
-        let sq = Square::from_algebraic_str("e4").expect("Failed to parse 'B8' square");
+        let sq = Square::from_algebraic_str("e4")?;
         assert_eq!(sq.file(), File(4));
         assert_eq!(sq.rank(), Rank(3));
+
+        Ok(())
     }
 
     #[test]
-    fn bad_algebraic_input() {
+    fn bad_algebraic_input() -> Result<(), ParseSquareError> {
-        Square::from_algebraic_str("a0").expect_err("Got valid Square for 'a0'");
+        Square::from_algebraic_str("a0")?;
-        Square::from_algebraic_str("j3").expect_err("Got valid Square for 'j3'");
+        Square::from_algebraic_str("j3")?;
-        Square::from_algebraic_str("a11").expect_err("Got valid Square for 'a11'");
+        Square::from_algebraic_str("a11")?;
-        Square::from_algebraic_str("b-1").expect_err("Got valid Square for 'b-1'");
+        Square::from_algebraic_str("b-1")?;
-        Square::from_algebraic_str("a 1").expect_err("Got valid Square for 'a 1'");
+        Square::from_algebraic_str("a 1")?;
-        Square::from_algebraic_str("").expect_err("Got valid Square for ''");
+        Square::from_algebraic_str("")?;
+
+        Ok(())
     }
 
     #[test]
-    fn from_index() {
+    fn from_index() -> Result<(), ()> {
-        let sq = Square::try_from(4u32).expect("Unable to get Square from index");
+        let sq = Square::try_from(4u32)?;
         assert_eq!(sq.file(), File(4));
         assert_eq!(sq.rank(), Rank(0));
 
-        let sq = Square::try_from(28u32).expect("Unable to get Square from index");
+        let sq = Square::try_from(28u32)?;
         assert_eq!(sq.file(), File(4));
         assert_eq!(sq.rank(), Rank(3));
+
+        Ok(())
     }
 
     #[test]