`default_on_eof` does not always work when nested

[jeromegn]

I had been scratching my head at a default_on_eof not working (error: eof) and was finally able to reproduce.

#[derive(PartialEq, Debug, Readable, Writable)]
enum DerivedEnumWithNestedDefaultOnEof {
    First {
        b: u8,
        #[speedy(default_on_eof)]
        c: u8
    },
    Second {
        a: u8,
        nested: DerivedStructWithVecWithCountWithDefaultOnEof,
        e: RangeInclusive< u8 >
    },
    Third {
        d: u8
    }
}

#[derive(PartialEq, Debug, Readable, Writable)]
struct DerivedStructWithVecWithCountWithDefaultOnEof {
    length: u8,
    #[speedy(length = length, default_on_eof)]
    data: Vec< u8 >
}

// test

let deserialized: DerivedEnumWithNestedDefaultOnEof = Readable::read_from_buffer_with_ctx( Endianness::LittleEndian, &[1, 0, 0, 0, 0xAA, 2, 0, 0] ).unwrap();
assert_eq!( deserialized, DerivedEnumWithNestedDefaultOnEof::Second { a: 0xAA, nested: DerivedStructWithVecWithCountWithDefaultOnEof { length: 2, data: vec![] }, e: 0..=0 }  );

// thread 'test_derived_struct_with_default_on_eof' panicked at ...

It makes sense! Ultimately, putting a default_on_eof in a nested struct and then defining other mandatory fields on the parent struct is similar to adding a default_on_eof on a field that's not the last.

In this particular example (not the smallest repro) of a buffer being read from: &[1, 0, 0, 0, 0xAA, 2, 0, 0]:

• enum tag (Second)
• u8 (a: 0xAA)
• u8 (length: 2)
• Vec (data: vec![0, 0])
• EOF

The reader was expecting a RangeInclusive<u8> at this point, but there's nothing because it read the bytes into the Vec<u8>.

I'm not sure what the solution is here. I don't think there's any way for speedy to do the correct thing. I'm also not sure there's a way to disallow nesting a struct that has default_on_eof without it being the last field of the parent struct. If there is, that might throw a helpful error to the user.