png: Use transmute instead of align_to

zune-png/src/decoder.rs

@@ -27,7 +27,7 @@ use crate::filters::de_filter::{
 use crate::options::default_chunk_handler;
 use crate::utils::{
     add_alpha, convert_be_to_target_endian_u16, expand_bits_to_byte, expand_palette, expand_trns,
-    is_le
+    is_le, convert_u16_to_u8_slice
 };
 
 /// A palette entry.
@@ -777,26 +777,8 @@ impl<T: ZReaderTrait> PngDecoder<T> {
         {
             &mut out_u8
         } else {
-            #[inline(always)]
-            fn align_to_mut(slice: &mut [u16]) -> (&mut [u16], &mut [u8], &mut [u16]) {
-                // this is safe for two different reasons:
-                // - the crate compiles with "bytemuck" feature on, proving that we satisfy slice's trait requirements for align_to_mut to be safe
-                // - align_to_mut from u16 slice to u8 slice is, in fact, safe
-                #[cfg(not(feature = "bytemuck"))]
-                {
-                    unsafe { slice.align_to_mut::<u8>() }
-                }
-
-                #[cfg(feature = "bytemuck")]
-                {
-                    bytemuck::pod_align_to_mut::<u16, u8>(&mut out_u16)
-                }
-            }
-            let (a, b, c) = align_to_mut(&mut out_u16);
+            let b = convert_u16_to_u8_slice(&mut out_u16);
 
-            // a and c should be empty since we do not expect slop bytes on either edge
-            assert!(a.is_empty());
-            assert!(c.is_empty());
             assert_eq!(b.len(), new_len * 2); // length should be twice that of u8
             b
         };

zune-png/src/utils.rs

@@ -350,3 +350,14 @@ pub(crate) fn add_alpha(input: &[u8], output: &mut [u8], colorspace: PngColor, d
         (a, b) => panic!("Unknown combination of depth {a:?} and color type for expand alpha {b:?}")
     }
 }
+
+pub fn convert_u16_to_u8_slice(slice: &mut [u16]) -> &mut [u8] {
+    // Converting a u16 slice to a u8 slice is always correct because
+    // the alignment of the target is smaller.
+    unsafe {
+        std::slice::from_raw_parts_mut(
+            slice.as_ptr() as *mut u8,
+            slice.len().checked_mul(2).unwrap(),
+        )
+    }
+}