Fix BitVec on Big-Endian Systems

[Alexhuszagh]

Relevant Code Failing

let u16b = u16::from_ne_bytes(0x1234u16.to_le_bytes());
bytes[5 ..][.. 14].store_le(u16b);
assert_eq!(bytes[5 ..][.. 14].load_le::<u16>(), 0x1234u16);

Result

---- fields::tests::lsb0 stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `13330`,
 right: `4660`', src/fields.rs:999:3

---- fields::tests::msb0 stdout ----
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `13330`,
 right: `4660`', src/fields.rs:1073:3

On big-endian PPC, the above code cails, but on little-endian PPC, it passes.

[myrrlyn]

Awesome. I've always been afraid these were subtly incorrect and I just didn't have a machine on which to test them.

[Alexhuszagh]

Some further debugging with a lot of printlns shows that it's store_le that's failing:

println!("0x1234u16.to_le_bytes()={:?}", 0x1234u16.to_le_bytes());
// 0x1234u16.to_le_bytes()=[52, 18]

let u16b = u16::from_ne_bytes(0x1234u16.to_le_bytes());
println!("u16b={:?}", u16b);
// u16b=13330

bytes[..16].store_le(u16b);
println!("bytes[..16]={:?}", bytes[..16]);
// ACTUAL:   bytes[..16]=BitSlice<Lsb0, u8> [01001000, 00101100]
// EXPECTED: bytes[..16]=BitSlice<Lsb0, u8> [00101100, 01001000]

Now, I"m not sure if this is a real issue, or faulty assumptions during unittesting, since to_le_bytes() properly switches the byteorder of the integer from big-endian to little-endian, and store_le properly switches the byteorder again, assuming it's storing a big-endian value as little-endian.

[Alexhuszagh]

The more I look at this, the more I believe this is simply a logic bug in the testing suite. The following works, as expected:

let u16b = u16::from_ne_bytes(0x1234u16.to_ne_bytes());
bytes[..16].store_le(u16b);
assert_eq!(bytes[..16].load_le::<u16>(), 0x1234u16);

[Alexhuszagh]

On big-endian, one more issue:

let bits: &BitSlice<Local, usize> = bits![1, 0, 1, 1, 0, 1, 0, 0, 1, 0];
println!("bits={:?}", bits);
// bits=BitSlice<Msb0, usize> [0000000000]

The bits macro is therefore broken on big-endian architectures.

[obeah]

In my case (little-endian) bitvec! makes wrong initialization after 48th bit.
Probably completely different problem, just found this issue while searching for solution.

let bits = bitvec![0, 1, 0, 1, 1, 0, 1, 1, 0, 0,
                   0, 0, 0, 0, 0, 0, 1, 1, 0, 0,
                   0, 0, 0, 0, 0, 0, 1, 1, 0, 0,
                   0, 1, 0, 1, 1, 0, 1, 1, 0, 0,
                   0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                   0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                   0, 0, 0, 0, 0, 0, 0, 0, 0, 0];

println!("{:?}", bits);
// BitVec<Lsb0, usize> [
//    0b0101101100000000110000000011000101011011000000001100000000110001,
//    0b000000,
//]

[myrrlyn]

Thank you @obeah for this issue report.

The behavior is now fixed on trunk. I will cherry-pick it down into a 0.17.3 patch release later today or this week.