address review comments

arrow-buffer/src/util/bit_mask.rs

@@ -100,10 +100,12 @@ fn set_upto_64bits(
     } else {
         let len = std::cmp::min(len, 64 - std::cmp::max(read_shift, write_shift));
         let bytes = ceil(len + read_shift, 8);
-        // SAFETY: chunk gets masked, so it is safe
-        let chunk = unsafe { read_bytes_to_u64(data, read_byte, bytes) };
         let mask = u64::MAX >> (64 - len);
-        let chunk = (chunk >> read_shift) & mask;
+        // SAFETY: chunk gets masked, so it is safe
+        let chunk = unsafe {
+            let chunk = read_bytes_to_u64(data, read_byte, bytes);
+            (chunk >> read_shift) & mask
+        };
         let chunk = chunk << write_shift;
         let null_count = len - chunk.count_ones() as usize;
         let bytes = ceil(len + write_shift, 8);
@@ -119,7 +121,6 @@ fn set_upto_64bits(
 /// The caller must ensure all arguments are within the valid range.
 /// The caller must be aware `8 - count` bytes in the returned value are uninitialized.
 #[inline]
-#[cfg(not(miri))]
 unsafe fn read_bytes_to_u64(data: &[u8], offset: usize, count: usize) -> u64 {
     debug_assert!(count <= 8);
     let mut tmp = std::mem::MaybeUninit::<u64>::uninit();
@@ -131,13 +132,6 @@ unsafe fn read_bytes_to_u64(data: &[u8], offset: usize, count: usize) -> u64 {
     }
 }
 
-#[cfg(miri)]
-unsafe fn read_bytes_to_u64(data: &[u8], offset: usize, count: usize) -> u64 {
-    let mut arr = [0u8; 8];
-    arr[0..count].copy_from_slice(&data[offset..(offset + count)]);
-    u64::from_le_bytes(arr)
-}
-
 #[inline]
 fn write_u64_bytes(data: &mut [u8], offset: usize, chunk: u64) {
     // SAFETY: the caller must ensure `data` has `offset..(offset + 8)` range