Macro to DRY sync and async code?

help
1

Suppose that I want to write two functions,

use futures::AsyncRead;
use std::io::Read;

async fn decode_variable_async<R: AsyncRead + Unpin + Send>(reader: &mut R) -> Result<u64>;
fn decode_variable<R: Read>(reader: &mut R) -> Result<u64>;

that read some bytes and perform some decoding (e.g. zigzag). They are semantically equal apart from how we call reader.read_exact([u8; 1])? vs reader.read_exact([u8; 1]).await?.

E.g. the sync version reads:

fn decode_variable<R: Read>(reader: &mut R) -> Result<u64> {
    let mut i = 0u64;
    let mut buf = [0u8; 1];

    let mut j = 0;
    loop {
        if j > 9 {
            // if j * 7 > 64
            panic!() // todo move to an error
        }
        reader.read_exact(&mut buf[..])?;
        i |= (u64::from(buf[0] & 0x7F)) << (j * 7);
        if (buf[0] >> 7) == 0 {
            break;
        } else {
            j += 1;
        }
    }

    Ok(i)
}

Is there a macro that I can use to encapsulate the same semantics above without copying the whole algorithm? E.g. something like

async fn decode_variable_async<R: AsyncRead + Unpin + Send>(reader: &mut R) -> Result<u64> {
    decode!(.await?)
}

async fn decode_variable<R: Read>(reader: &mut R) -> Result<u64> {
    decode!(?)
}

I also tried

decode!(AsyncReadExt::read_exact(reader, &buf[..]).async?)
decode!(Read::read_exact(reader, &buf)?)

but I am being unable to achieve this.

2

The only problem with this is that you’ll need to pass the variable name reader, otherwise the code inside the macro can’t reference that variable due to hygiene rules.

macro_rules! decode {
    ($reader:ident $($_await:tt)*) => {
        {
            let mut i = 0u64;
            let mut buf = [0u8; 1];
        
            let mut j = 0;
            loop {
                if j > 9 {
                    // if j * 7 > 64
                    panic!() // todo move to an error
                }
                $reader.read_exact(&mut buf[..])$($_await)*?;
                i |= (u64::from(buf[0] & 0x7F)) << (j * 7);
                if (buf[0] >> 7) == 0 {
                    break;
                } else {
                    j += 1;
                }
            }
        
            Ok(i)
        }
    }
}

async fn decode_variable_async<R: AsyncRead + Unpin + Send>(reader: &mut R) -> Result<u64> {
    decode!(reader .await)
}

async fn decode_variable<R: Read>(reader: &mut R) -> Result<u64> {
    decode!(reader)
}
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