Composing structs with top level attributes

[DevonKS]

Hi,

I'm trying to compose 2 structs where both structs use the DekuRead derive macro and the top level struct has a endian attribute. However when I try to do this I get an error.

Here is an example code snippet:

use deku::{DekuContainerRead, DekuRead};

#[derive(Debug, PartialEq, Eq, Clone, DekuRead)]
struct Foo {
    a: u16,
}

#[derive(Debug, PartialEq, Eq, Clone, DekuRead)]
#[deku(endian = "little")]
struct Bar {
    f: Foo,
    b: u16,
}

fn main() {
    let bs = [0x02, 0x01, 0x01, 0x02];
    let (_, b) = Bar::from_bytes((&bs, 0)).unwrap();
    println!("{:?}", b);
}

The error I get is: expected (), found Endian. arguments to this function are incorrect

It looks like the issue is that the generate impl of DekuReader for Bar calls the generated from_reader_with_ctx of Foo and passes Endian::Little as the second arg but it expects ().

I tried adding #[deku(endian = "little")] to Foo so that maybe the generated from_reader_with_ctx would expect an Endian as the second arg but that didn't work.

My expectation is that setting endian = "little" on the top level struct would flow down and any reading of struct fields would use this context.

Is my expectation correct? and how would I achieve something like this?

[sharksforarms]

Hi @DevonKS , we have an example of this pattern here: https://docs.rs/deku/latest/deku/attributes/index.html#endian

// Note: The endian is passed as a context argument to sub-types

// Example:

#[deku(endian = "endian", ctx = "endian: deku::ctx::Endian")] // context passed from `DekuTest` top-level endian
struct Child {
    field_a: u16
}

#[deku(endian = "little")] // top-level, defaults to system endianness
struct DekuTest {
    #[deku(endian = "big")] // field-level override
    field_be: u16,
    field_default: u16, // defaults to top-level

    // because a top-level endian is specified,
    // it is passed as a context
    field_child: Child,
}

let data: &[u8] = &[0xAB, 0xCD, 0xAB, 0xCD, 0xEF, 0xBE];

let value = DekuTest::try_from(data).unwrap();

assert_eq!(
    DekuTest {
       field_be: 0xABCD,
       field_default: 0xCDAB,
       field_child: Child { field_a: 0xBEEF }
    },
    value
);

let value: Vec<u8> = value.try_into().unwrap();
assert_eq!(&*data, value);

[DevonKS]

Ah great, this is exactly what I need. Thanks!