Add PodFaceMap for use by the light propagation chart.

Motivation: I’m planning a new revision of the light propagation chart,
which will need more `FaceMap`s, so this makes it more convenient.

all-is-cubes/build.rs

@@ -147,14 +147,7 @@ mod chart_schema {
             let face_cosines = face_cosines.map(|_, cosine| (cosine * 255.0).round() as u8);
             Self {
                 direction: direction.map(TargetEndian::from).into(),
-                face_cosines: [
-                    face_cosines.nx,
-                    face_cosines.ny,
-                    face_cosines.nz,
-                    face_cosines.px,
-                    face_cosines.py,
-                    face_cosines.pz,
-                ],
+                face_cosines: face_cosines.into(),
                 _padding: [0; 2],
             }
         }

all-is-cubes/src/space/light/chart/local.rs

@@ -6,16 +6,7 @@ use crate::space::light::chart;
 
 impl chart::OneRay {
     pub fn face_cosines(&self) -> FaceMap<f32> {
-        let [nx, ny, nz, px, py, pz] = self.face_cosines;
-        FaceMap {
-            nx,
-            ny,
-            nz,
-            px,
-            py,
-            pz,
-        }
-        .map(|_, byte| f32::from(byte) / 255.0f32)
+        FaceMap::from(self.face_cosines).map(|_, byte| f32::from(byte) / 255.0f32)
     }
 }
 

all-is-cubes/src/space/light/chart/shared.rs

@@ -4,7 +4,7 @@
 //! This is the most efficient way I could think of to store and transfer the
 //! pre-computed light ray chart data.
 
-use all_is_cubes_base::math::Face7;
+use all_is_cubes_base::math::{Face7, FaceMap};
 
 // conditionally defined to be equal to f32 except in the build script
 use super::TargetEndian;
@@ -19,7 +19,7 @@ pub(crate) struct OneRay {
 
     /// `FaceMap` data which stores the cosine (rescaled to 0-255)
     /// between each face normal and this ray.
-    pub face_cosines: [u8; 6],
+    pub face_cosines: PodFaceMap<u8>,
 
     /// Guaranteed zero padding to make up a multiple of 4 bytes.
     pub _padding: [u8; 2],
@@ -106,6 +106,33 @@ impl From<Face7> for Face7Safe {
     }
 }
 
+/// Variant of [`FaceMap`] that can implement `bytemuck::Pod` because it is `repr(packed)`.
+/// (The regular `FaceMap` could too, *unsafely*, but I don't want to do that unsafe yet.)
+#[derive(Clone, Copy, Debug, bytemuck::Pod, bytemuck::Zeroable)]
+#[repr(C, packed)]
+pub(crate) struct PodFaceMap<T> {
+    nx: T,
+    ny: T,
+    nz: T,
+    pz: T,
+    py: T,
+    px: T,
+}
+impl<T> From<FaceMap<T>> for PodFaceMap<T> {
+    #[rustfmt::skip]
+    fn from(value: FaceMap<T>) -> Self {
+        let FaceMap { nx, ny, nz, px, py, pz } = value;
+        Self { nx, ny, nz, pz, py, px }
+    }
+}
+impl<T> From<PodFaceMap<T>> for FaceMap<T> {
+    #[rustfmt::skip]
+    fn from(value: PodFaceMap<T>) -> Self {
+        let PodFaceMap { nx, ny, nz, pz, py, px } = value;
+        FaceMap { nx, ny, nz, px, py, pz }
+    }
+}
+
 // Note: Most of the methods are either only used for reading or only used for writing,
 // so they're defined in the respective crates to reduce complications like what they depend on,
 // how `TargetEndian` is defined, and dead code warnings.