Fix test for target with pointer width < 8 bytes

The alignment tests relied on the fact that the pointer width is 8
bytes, which is not the case for i686.
Fix the alignment tests so that they work on targets with pointer
width of 4 or even 2 bytes.

Fixes #21

.github/workflows/ci.yml

@@ -76,7 +76,7 @@ jobs:
 
     - name: Run tests
       # TODO(#21): Fix `test_new_error` on i686 and re-enable it here.
-      run: cargo +${{ matrix.channel }} test --target ${{ matrix.target }} --features "${{ matrix.features }}" --verbose -- ${{ contains(matrix.target, 'i686') && '--skip test_new_error' || '' }}
+      run: cargo +${{ matrix.channel }} test --target ${{ matrix.target }} --features "${{ matrix.features }}" --verbose
       # Only run tests when targetting x86 (32- or 64-bit) - we're executing on
       # x86_64, so we can't run tests for any non-x86 target.
       if: ${{ contains(matrix.target, 'x86_64') || contains(matrix.target, 'i686') }}

src/lib.rs

@@ -2768,26 +2768,27 @@ mod tests {
 
         // Fail because the alignment is insufficient.
 
-        // A buffer with an alignment of 8.
-        let mut buf = AlignedBuffer::<u64, [u8; 12]>::default();
-        // Slicing from 4, we get a buffer with size 8 (so the length check
-        // should succeed) but an alignment of only 4, which is insufficient.
-        assert!(LayoutVerified::<_, u64>::new(&buf.buf[4..]).is_none());
-        assert!(LayoutVerified::<_, u64>::new_zeroed(&mut buf.buf[4..]).is_none());
-        assert!(LayoutVerified::<_, u64>::new_from_prefix(&buf.buf[4..]).is_none());
-        assert!(LayoutVerified::<_, u64>::new_from_prefix_zeroed(&mut buf.buf[4..]).is_none());
-        assert!(LayoutVerified::<_, [u64]>::new_slice(&buf.buf[4..]).is_none());
-        assert!(LayoutVerified::<_, [u64]>::new_slice_zeroed(&mut buf.buf[4..]).is_none());
-        assert!(LayoutVerified::<_, [u64]>::new_slice_from_prefix(&buf.buf[4..], 1).is_none());
-        assert!(LayoutVerified::<_, [u64]>::new_slice_from_prefix_zeroed(&mut buf.buf[4..], 1)
+        // A buffer with an alignment of 8. An odd buffer size is chosen so that the last byte of 
+        // the buffer has odd alignment.
+        let mut buf = AlignedBuffer::<u64, [u8; 13]>::default();
+        // Slicing from 1, we get a buffer with size 12 (so the length check should succeed) but an
+        // alignment of only 1, which is insufficient.
+        assert!(LayoutVerified::<_, u64>::new(&buf.buf[1..]).is_none());
+        assert!(LayoutVerified::<_, u64>::new_zeroed(&mut buf.buf[1..]).is_none());
+        assert!(LayoutVerified::<_, u64>::new_from_prefix(&buf.buf[1..]).is_none());
+        assert!(LayoutVerified::<_, u64>::new_from_prefix_zeroed(&mut buf.buf[1..]).is_none());
+        assert!(LayoutVerified::<_, [u64]>::new_slice(&buf.buf[1..]).is_none());
+        assert!(LayoutVerified::<_, [u64]>::new_slice_zeroed(&mut buf.buf[1..]).is_none());
+        assert!(LayoutVerified::<_, [u64]>::new_slice_from_prefix(&buf.buf[1..], 1).is_none());
+        assert!(LayoutVerified::<_, [u64]>::new_slice_from_prefix_zeroed(&mut buf.buf[1..], 1)
             .is_none());
-        assert!(LayoutVerified::<_, [u64]>::new_slice_from_suffix(&buf.buf[4..], 1).is_none());
-        assert!(LayoutVerified::<_, [u64]>::new_slice_from_suffix_zeroed(&mut buf.buf[4..], 1)
+        assert!(LayoutVerified::<_, [u64]>::new_slice_from_suffix(&buf.buf[1..], 1).is_none());
+        assert!(LayoutVerified::<_, [u64]>::new_slice_from_suffix_zeroed(&mut buf.buf[1..], 1)
             .is_none());
-        // Slicing from 4 should be unnecessary because
-        // `new_from_suffix[_zeroed]` use the suffix of the slice.
-        assert!(LayoutVerified::<_, u64>::new_from_suffix(&buf.buf[..]).is_none());
-        assert!(LayoutVerified::<_, u64>::new_from_suffix_zeroed(&mut buf.buf[..]).is_none());
+        // Slicing is unnecessary here because `new_from_suffix[_zeroed]` use the suffix of the 
+        // slice, which has odd alignment
+        assert!(LayoutVerified::<_, u64>::new_from_suffix(&buf.buf[1..]).is_none());
+        assert!(LayoutVerified::<_, u64>::new_from_suffix_zeroed(&mut buf.buf[1..]).is_none());
 
         // Fail due to arithmetic overflow.