How to use optional arguments with default values?

[jyio]

Greetings! binrw is an amazing package. I'm still pretty green, but already getting some good use out of it. Thank you for sharing it with the world.

Now, I'd like to have a sort of padding field that's written with a default value during serialization, and allow the option to override it. I'm trying to use the import and calc directives for this, but the optional argument is always zero by default, instead of whatever else I specify as default. What am I missing?

Example:

use binrw::{binrw, BinWrite, BinWriterExt};

#[binrw]
struct TestX {
	#[bw(calc = 42)]
	value: u8,
}

#[binrw]
#[brw(import { substitute: u8 = 42 })]
struct TestY {
	#[bw(calc = substitute)]
	value: u8,
}

fn main() -> std::io::Result<()> {
	let inst = TestX {};

	let mut bufcursor = binrw::io::Cursor::new(Vec::new());
	bufcursor.write_le(&inst).unwrap();
	println!("TestX #1: {:?}", bufcursor.into_inner());

	let inst = TestY {};

	let mut bufcursor = binrw::io::Cursor::new(Vec::new());
	bufcursor.write_le(&inst).unwrap();
	println!("TestY #1: {:?}", bufcursor.into_inner());

	let mut bufcursor = binrw::io::Cursor::new(Vec::new());
	inst.write_options(&mut bufcursor, binrw::Endian::NATIVE, TestYBinWriteArgs { substitute: 255 });
	println!("TestY #2: {:?}", bufcursor.into_inner());

	Ok(())
}

What I get:

TestX #1: [42]
TestY #1: [0]
TestY #2: [255]

What I expect:

TestX #1: [42]
TestY #1: [42]
TestY #2: [255]

I could work around this by implementing a default constructor, but this leaves the field in the struct, and I'd like to hide it if possible. The other workaround is to use this TestYBinWriteArgs struct created by the macros, but reaching behind an otherwise beautiful abstraction feels wrong to me. Is this the only way?