Support nested array

[fengyc]

The code like below could not compile:

    #[derive(Debug, EtherCrabWireReadWrite)]
    #[wire(bits = 104)]
    struct Check {
        #[wire(bits = 32)]
        foo: u32,
        #[wire(bits = 24)]
        control: Inner,
        #[wire(bits = 48)]
        status: [Inner; 2],
    }

    #[derive(Debug, Copy, Clone, EtherCrabWireReadWrite)]
    #[wire(bits = 24)]
    struct Inner {
        #[wire(bits = 1)]
        yes: bool,
        #[wire(bits = 1)]
        no: bool,
        #[wire(pre_skip = 6, bits = 16)]
        stuff: u16,
    }

The error:

error[E0277]: the trait bound `[Inner; 2]: EtherCrabWireWrite` is not satisfied
  --> src/ethercat.rs:40:14
   |
40 |     control: [Inner; 2],
   |              ^^^^^^^^^^ the trait `EtherCrabWireWrite` is not implemented for `[Inner; 2]`
   |
   = help: the following other types implement trait `EtherCrabWireWrite`:
             &[u8]
             [u8; N]

Is it possible to support nested array? for example, replace the impl for [u8; N] with:

impl<const N: usize, T> EtherCrabWireWrite for [T; N]
where
    T: EtherCrabWireWrite,
{
    fn pack_to_slice_unchecked<'buf>(&self, buf: &'buf mut [u8]) -> &'buf [u8] {
        if N == 0 {
            return &mut buf[0..0];
        }
        let chunk_size = self[0].packed_len();
        let buf = &mut buf[0..self.packed_len()];
        let mut chunk = &mut buf[..];
        for i in 0..N {
            let _ = EtherCrabWireWrite::pack_to_slice_unchecked(&self[i], chunk);
            chunk = &mut chunk[chunk_size..];
        }
        buf
    }

    fn packed_len(&self) -> usize {
        if N == 0 {
            return 0;
        }
        N * self[0].packed_len()
    }
}

[jamwaffles]

Thanks for the issue! This is related to #321 in that it's not possible to automatically compute the nested field bit size, however the consensus in that PR is to just document this behaviour and support nested types (in that case, generics too) with the caveat the user must give the correct bit size.

[jamwaffles]

Hey @fengyc with #360, this should now be possible. Would you mind giving that branch a test? You can patch your Cargo.toml like this:

[patch.crates-io]
ethercrab = { git = "https://github.com/ethercrab-rs/ethercrab.git", branch = "wire-write-for-arrays" }

I'll merge that branch in a couple of days, but don't worry if you don't get round to it. We can fix forward :)

Thanks for your patience on this!