Have `seq!` trace the field name when parsing each field

[epage]

seq! will trace the struct name for the whole thing but we can also trace the name of the individual fields (maybe also the tuple index?). This would automatically provide more context for the user as they go which can help with complex seq! calls like https://github.com/MIDAS-rs/midasio/blob/cc2ed1e54f8fc40eb7bfabd79d490ec4e8ba38b2/src/event.rs#L115-L139

[DJDuque]

I don't think I understand what you are suggesting here. How would the seq! you quoted look like? Is this to help avoiding all the chained flat_maps?

In case it is to help with the chained flat_maps, a nice thing to have could be _temporary variables (prefixed with _ that don't make it into the struct, but can be referenced in later fields).

So the above seq! could look something like:

 pub(crate) fn event_view<'a>(
    endian: Endianness,
) -> impl Parser<&'a [u8], EventView<'a>, ContextError> {
    seq! {EventView {
        event_id: u16(endian),
        trigger_mask: u16(endian),
        serial_number: u32(endian),
        timestamp: u32(endian),
        _event_size: u32(endian).verify(|&n| n >= 8),
        _banks_size: u32(endian).verify(&n| n == _event_size - 8),
        bank_views: dispatch! {u32(endian);
                1 => length_and_then(success(_banks_size), repeat_till0(padded_bank(bank_16_view(endian)), eof)),
                17 => length_and_then(success(_banks_size), repeat_till0(padded_bank(bank_32_view(endian)), eof)),
                49 => length_and_then(success(_banks_size), repeat_till0(padded_bank(bank_32a_view(endian)), eof)),
                _ => fail,
            }.map(|(bank_views, _)| bank_views),
    }}
}

[epage]

That is a separate concern and if you want to find a way to solve it, I would recommend opening a separate issue.

winnow has debug tracing for parsers built in. You can do cargo test -F winnow/debug -- test foo -- bar and it will show you each parse operation (that has a trace combinator call) attempt the parse with the result.

See https://docs.rs/winnow/latest/winnow/trace/index.html for an example of what this looks like.

seq! inserts a trace for the struct as a whole but the proposal here is to add a trace on each field so the user automatically gets names for the operations, making it easier to follow what is happening.