br(count = ...) conflicts with br(args(...))

[Holzhaus]

Apparently it's not possible to to pass a arguments to a Vec of BinRead objects.

Here's an example:

use binrw::{binrw, BinRead};

#[binrw]
#[derive(Debug)]
#[brw(little)]
struct Data {
    version: u16,
    #[br(temp)]
    #[bw(calc = items.len().try_into().unwrap())]
    num_items: u32,
    #[br(count = num_items, args(version))]
    items: Vec<Item>,
}

#[binrw]
#[derive(Debug)]
#[brw(little)]
#[br(import(version: u16))]
struct Item {
    field1: u32,
    #[br(if(version >= 5, 0))]
    field2: u64,
}

fn main() {
    let path = std::env::args().nth(1).expect("no path given");
    let mut reader = std::fs::File::open(&path).expect("failed to open file");
    let data = Data::read(&mut reader).expect("failed to parse file");
    println!("{:#04x?}", data);
}

This results in the following error:

error[E0308]: mismatched types
 --> src/bin/example.rs:3:1
  |
3 | #[binrw]
  | ^^^^^^^^
  | |
  | expected struct `VecArgs`, found tuple
  | expected due to this
  |
  = note: expected struct `VecArgs<(u16,)>`
              found tuple `(u16,)`
note: return type inferred to be `VecArgs<(u16,)>` here
 --> src/bin/example.rs:3:1
  |
3 | #[binrw]
  | ^^^^^^^^
  = note: this error originates in the attribute macro `binrw` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0308`.
error: could not compile `rekordcrate` due to previous error

[kitlith]

The thing that jumps out at me, is that you're using #[br(args(...))]: this means you're using positional (tuple) arguments instead of named arguments (struct), which is what vecs use. I'd be somewhat surprised if that worked without count being present (i.e. the problem isn't with count), but maybe something was done to make that work in the absense of count. (it's been awhile since I touched named arguments)

In general, #[br(count = ...)] is shorthand for #[br(args { count: ... })]. iirc inner arguments are typically passed through #[br(args { inner: ...})]. I believe these are compatible with each other, but I'd just use #[br(args { count: ..., inner: ...})].

Does this help?

[Holzhaus]

Thanks, this does indeed work:

use binrw::{binrw, BinRead};

#[binrw]
#[derive(Debug)]
#[brw(little)]
struct Data {
    version: u16,
    #[br(temp)]
    #[bw(calc = items.len().try_into().unwrap())]
    num_items: u32,
    #[br(args{ count: num_items.try_into().unwrap(), inner: (version,) })]
    items: Vec<Item>,
}

#[binrw]
#[derive(Debug)]
#[brw(little)]
#[br(import(version: u16))]
struct Item {
    field1: u32,
    #[br(if(version >= 5, 0))]
    field2: u64,
}

fn main() {
    let path = std::env::args().nth(1).expect("no path given");
    let mut reader = std::fs::File::open(&path).expect("failed to open file");
    let data = Data::read(&mut reader).expect("failed to parse file");
    println!("{:#04x?}", data);
}

But it feels like I'm hacking around some implementation detail of the library, so it would be nice if this would receive a proper fix.

[Holzhaus]

Btw, I just saw that this workaround actually documented in 38f731b.

[csnover]

There is now an explicit error message explaining the problem and what to do instead. Thanks for your report!