How can bincode distinguish between tuples and (fixed size) arrays?

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1

Continuing the discussion from Is it better to use bincode or postcard?:

// uses `bincode` version "2.0.0-rc.2"

fn main() {
    let tuple: (u16, u16) = (44, 55);
    let array: [u16; 2] = [44, 55];
    let stdcfg = bincode::config::standard();
    assert_eq!(bincode::encode_to_vec(&tuple, stdcfg).unwrap(), &[44, 55]);
    assert_eq!(bincode::encode_to_vec(&array, stdcfg).unwrap(), &[2, 44, 55]);
}

How can bincode distinguish between tuples and (fixed size) arrays? Aren't both going through the serde type tuple?

2

There's no 2.0 version of bincode. With the latest 1.3 version, the two data types serialize to the exact same representation.

3

Seems like the bincode function in the code example doesn’t use serde, and there’s another one that does, treating arrays and tuples equally: Rust Explorer

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4

:scream:

This means that the following code:

// uses `bincode` version "2.0.0-rc.2"

fn main() {
    let array: [u8; 3] = [11, 12, 13];
    let stdcfg = bincode::config::standard();
    assert_eq!(
        bincode::encode_to_vec(&array, stdcfg).unwrap(),
        bincode::serde::encode_to_vec(&array, stdcfg).unwrap(),
    );
}

Fails with:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[3, 11, 12, 13]`,
 right: `[11, 12, 13]`', src/main.rs:6:5

That's not good. This means bincode::encode_to_vec isn't compatible with bincode::serde::encode_to_vec. I don't think this is intended, is it?

Note I'm referring to version 2.0.0-rc.2, which is a published (non-yanked) release candidate.

5

I don’t know, I’m not familiar with that crate at all. If you feel like this might be not intended, open an issue on their repo :slight_smile:

Even if it is intended, it might be insufficiently documented.

6

I opened the following issue:

7

Right… it’s almost as if they got their point of which one of skip_fixed_array_length and write_fixed_array_length is supported by bincode::serde::… the wrong way around.

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