Reading a variable, external amount of data into a struct at runtime

[pbar1]

Let's say I've got a struct that looks like this:

#[binrw]
pub struct MyPacket {
    #[br(/* this can vary at runtime */)]
    #[bw(try_calc(u32::try_from(payload.len())))]
    pub length: u32,

    #[br(count = length)]
    pub payload: Vec<u8>,
}

How would I go about setting length for the purposes of BinRead if it can vary at runtime according to external conditions (say the program's state) rather than in relation to any of the other fields of MyPacket?

I can't make use of something like until_eof helper because this is part of a stream.

[kitlith]

You can add arguments to the struct using #[br(import(...)) and similar, to be given using #[br(args(...)] or BinRead::read_options(...)