How to access byte count of arbitrary number using num_traits?

October 19, 2023

I want to implement an extension trait to std::io::Read in order to read arbitrary numbers. However I cannot find out, how to get the byte count of an arbitary number using the num_traits crate (or what other crate might provide this information):

use num_traits::FromBytes;
use std::io::{Read, Result};

pub trait ReadNums: Read {
    fn read_num_be<N>(&mut self) -> Result<N>
    where
        N: FromBytes,
    {
        let mut buffer = [0; N::WHAT_HERE];
        self.read_exact(&mut buffer)?;
        Ok(N::from_be_bytes(&buffer))
    }
}

impl<T> ReadNums for T where T: Read {}

[package]
name = "numtrait_test"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
num-traits = "0.2.17"

I don’t know what tu put at N::WHAT_HERE. I need the respective size of N in bytes here as a const.
Using nightly features is out of the question for my use case.

>Solution :

FromBytes provides the Bytes associated type, but since FromBytes is also meant to work with arbitrary-precision types like BigInt, the Bytes type is not necessarily fixed width.

You can, however, force the association to be an array, with a const generic for the size:

use num_traits::FromBytes;
use std::io::{Read, Result};

pub trait ReadNums: Read {
    fn read_num_be<V, const N: usize>(&mut self) -> Result<V>
    where
        V: FromBytes<Bytes=[u8; N]>,
    {
        let mut buffer = [0; N];
        self.read_exact(&mut buffer)?;
        Ok(V::from_be_bytes(&buffer))
    }
}

impl<T> ReadNums for T where T: Read {}

This will work with all of the primitive integer types, since their FromBytes implementation take in fixed-sized arrays.