Regarding the safety of `chunks_mut(...).remove_alias()`

[koenichiwa]

Hi I had a question regarding the safety of remove_alias. The safety explanation is a bit hard for me to understand. Especially this part:

You must consume the adapted iterator in a loop that does not allow multiple yielded items to exist in the same scope. Each yielded item must have a completely non-overlapping lifetime from all the others.

I kinda think you mean that it's unsafe to have multiple chuncks pointing to overlapping parts of the same data, but I'm not sure. Could you help me understand it a bit better?

In my project I need a fixed size array of nibbles, which is one of the reasons I'm using bit_vec in my project. I tried to make a simple example of how I use it:

use bitvec::prelude::{BitArray, BitSlice, Lsb0};

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Nibble(BitArray<u8, Lsb0>);

impl Nibble {
    pub const BITS: usize = 4;
    pub const MAX: u8 = 2u8.pow(Self::BITS as u32);

    pub fn new(value: u8) -> Self {
        assert!(value < Self::MAX);
        Nibble(BitArray::new(value))
    }

    pub fn as_bitslice(&self) -> &BitSlice<u8, Lsb0> {
        &self.0[0..Self::BITS]
    }
}

fn example(nibbles: &[Nibble; 40]) -> BitArray<[u8; 20], Lsb0> {
    let mut result = BitArray::ZERO;
    nibbles.iter()
        .map(Nibble::as_bitslice)
        .zip( unsafe { result.chunks_mut(Nibble::BITS).remove_alias() } )
        .for_each(|(incoming, outgoing)|{
            outgoing.copy_from_bitslice(incoming);
    });
    result
}

Is this safe? Is this an XY problem and should I use a different method?

[koenichiwa]

I think it might've been an XY problem. I'm still hoping for the explanation regarding safety though. Just out of curiosity. Maybe you have an even better solution.

I solved it with something like:

use bitvec::prelude::{BitArray, BitSlice, Lsb0, BitVec};

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Nibble(BitArray<u8, Lsb0>);

impl Nibble {
    pub const ZERO: Self = Self(BitArray::ZERO);
    pub const BITS: usize = 4;
    pub const MAX: u8 = 2u8.pow(Self::BITS as u32);

    pub fn new(value: u8) -> Self {
        assert!(value < Self::MAX, "{value} is too high");
        Nibble(BitArray::new(value))
    }

    pub fn as_bitslice(&self) -> &BitSlice<u8, Lsb0> {
        &self.0[0..Self::BITS]
    }
}

impl FromIterator<Nibble> for BitVec<u8, Lsb0> {
    fn from_iter<T: IntoIterator<Item = Nibble>>(iter: T) -> Self {
        iter.into_iter().fold(BitVec::<u8, Lsb0>::EMPTY, |mut acc, nibble| {
            acc.extend_from_bitslice(nibble.as_bitslice());
            acc
        })
    }
}

#[cfg(test)]
mod test {
    use bitvec::prelude::{BitArray,Lsb0, BitVec};
    use super::Nibble;

    #[test]
    fn example() {
        let mut nibbles: [Nibble; Nibble::MAX as usize] = [Nibble::ZERO; Nibble::MAX as usize];
        for index in 0..nibbles.len() {
            nibbles[index] = Nibble::new(index as u8);
        }

        let mut result: BitArray<[u8; 8], Lsb0> = BitArray::ZERO;
        result.copy_from_bitslice(nibbles.iter().copied().collect::<BitVec<u8, Lsb0>>().as_bitslice());
        eprintln!("BitVec contents: {result}");

        result.chunks(4).enumerate().for_each(|(index, chunk)| {
            let vec: BitVec<u8, Lsb0> = BitVec::from_element(index as u8);
            let expected = &vec.as_bitslice()[0..Nibble::BITS];
            assert_eq!(chunk, expected);
        });
    }
}