Conditionally parse past the end of the file

[lfrancke]

I have an optional field right at the end of my file.
Some files can have it, others can end earlier. When doing that I always get an error that the stream is empty (or similar, basically an EOF).

I'd like deku to attempt to read a byte and if that exists continue as normal, if not then end it right there. I wasn't able to find a combination of attributes that works here but there's a good chance I might be missing something obvious.

Thank you!

[sharksforarms]

The cond attribute is what you're looking for. You can check deku::rest which provides the remaining data to be read, here's an example:

use deku::prelude::*;
use std::convert::{TryFrom, TryInto};

#[derive(Debug, PartialEq, DekuRead, DekuWrite)]
struct DekuTest {
    field_a: u8,
    #[deku(cond = "!deku::rest.is_empty()")]
    field_b: Option<u8>,
}

fn main() {
    {
        let test_data: &[u8] = [
            0xAB,
        ]
            .as_ref();

        let test_deku = DekuTest::try_from(test_data).unwrap();

        assert_eq!(
            DekuTest {
                field_a: 0xAB,
                field_b: None,
            },
            test_deku
            );

        let test_deku: Vec<u8> = test_deku.try_into().unwrap();
        assert_eq!(test_data.to_vec(), test_deku);
    }

    {
        let test_data: &[u8] = [
            0xAB,
            0xBA,
        ]
            .as_ref();

        let test_deku = DekuTest::try_from(test_data).unwrap();

        assert_eq!(
            DekuTest {
                field_a: 0xAB,
                field_b: Some(0xBA),
            },
            test_deku
            );

        let test_deku: Vec<u8> = test_deku.try_into().unwrap();
        assert_eq!(test_data.to_vec(), test_deku);
    }
}

[lfrancke]

Ahhh, again, thank you!
I did not think of using the rest here, makes total sense.