Improve test_new_aligned_sized to catch more bugs (google#511)

In google#506, a bug was discovered which `test_new_aligned_sized` did not
previously catch. This commit improves that test to catch that bug.

src/lib.rs

@@ -4196,7 +4196,8 @@ mod tests {
  let mut buf = Align::<[u8; 8], AU64>::default();
  // `buf.t` should be aligned to 8, so this should always succeed.
  test_new_helper(Ref::<_, AU64>::new(&mut buf.t[..]).unwrap());
- buf.t = [0xFFu8; 8];
+ let ascending: [u8; 8] = (0..8).collect::<Vec<_>>().try_into().unwrap();
+ buf.t = ascending;
  test_new_helper(Ref::<_, AU64>::new_zeroed(&mut buf.t[..]).unwrap());
  {
  // In a block so that `r` and `suffix` don't live too long.
@@ -4206,7 +4207,7 @@ mod tests {
  test_new_helper(r);
  }
  {
- buf.t = [0xFFu8; 8];
+ buf.t = ascending;
  let (r, suffix) = Ref::<_, AU64>::new_from_prefix_zeroed(&mut buf.t[..]).unwrap();
  assert!(suffix.is_empty());
  test_new_helper(r);
@@ -4218,46 +4219,54 @@ mod tests {
  test_new_helper(r);
  }
  {
- buf.t = [0xFFu8; 8];
+ buf.t = ascending;
  let (prefix, r) = Ref::<_, AU64>::new_from_suffix_zeroed(&mut buf.t[..]).unwrap();
  assert!(prefix.is_empty());
  test_new_helper(r);
  }
 
- // A buffer with alignment 8 and length 16.
- let mut buf = Align::<[u8; 16], AU64>::default();
+ // A buffer with alignment 8 and length 24. We choose this length very
+ // intentionally: if we instead used length 16, then the prefix and
+ // suffix lengths would be identical. In the past, we used length 16,
+ // which resulted in this test failing to discover the bug uncovered in
+ // #506.
+ let mut buf = Align::<[u8; 24], AU64>::default();
  // `buf.t` should be aligned to 8 and have a length which is a multiple
  // of `size_of::<AU64>()`, so this should always succeed.
- test_new_helper_slice(Ref::<_, [AU64]>::new_slice(&mut buf.t[..]).unwrap(), 2);
- buf.t = [0xFFu8; 16];
- test_new_helper_slice(Ref::<_, [AU64]>::new_slice_zeroed(&mut buf.t[..]).unwrap(), 2);
+ test_new_helper_slice(Ref::<_, [AU64]>::new_slice(&mut buf.t[..]).unwrap(), 3);
+ let ascending: [u8; 24] = (0..24).collect::<Vec<_>>().try_into().unwrap();
+ // 16 ascending bytes followed by 8 zeros.
+ let mut ascending_prefix = ascending;
+ ascending_prefix[16..].copy_from_slice(&[0, 0, 0, 0, 0, 0, 0, 0]);
+ // 8 zeros followed by 16 ascending bytes.
+ let mut ascending_suffix = ascending;
+ ascending_suffix[..8].copy_from_slice(&[0, 0, 0, 0, 0, 0, 0, 0]);
+ test_new_helper_slice(Ref::<_, [AU64]>::new_slice_zeroed(&mut buf.t[..]).unwrap(), 3);
 
  {
- buf.set_default();
- buf.t[8..].fill(0xFF);
+ buf.t = ascending_suffix;
  let (r, suffix) = Ref::<_, [AU64]>::new_slice_from_prefix(&mut buf.t[..], 1).unwrap();
- assert_eq!(suffix, [0xFF; 8]);
+ assert_eq!(suffix, &ascending[8..]);
  test_new_helper_slice(r, 1);
  }
  {
- buf.t = [0xFFu8; 16];
+ buf.t = ascending_suffix;
  let (r, suffix) =
  Ref::<_, [AU64]>::new_slice_from_prefix_zeroed(&mut buf.t[..], 1).unwrap();
- assert_eq!(suffix, [0xFF; 8]);
+ assert_eq!(suffix, &ascending[8..]);
  test_new_helper_slice(r, 1);
  }
  {
- buf.set_default();
- buf.t[..8].fill(0xFF);
+ buf.t = ascending_prefix;
  let (prefix, r) = Ref::<_, [AU64]>::new_slice_from_suffix(&mut buf.t[..], 1).unwrap();
- assert_eq!(prefix, [0xFF; 8]);
+ assert_eq!(prefix, &ascending[..16]);
  test_new_helper_slice(r, 1);
  }
  {
- buf.t = [0xFFu8; 16];
+ buf.t = ascending_prefix;
  let (prefix, r) =
  Ref::<_, [AU64]>::new_slice_from_suffix_zeroed(&mut buf.t[..], 1).unwrap();
- assert_eq!(prefix, [0xFF; 8]);
+ assert_eq!(prefix, &ascending[..16]);
  test_new_helper_slice(r, 1);
  }
  }