Add note about Maps to readme

[c-git]

Hi,

Thanks you very much really great crate. I understand why maps cannot be supported and it makes sense but it's not really mentioned in the readme or anywhere that I checked (only checked the readme and I the first page on docs.rs). Might be good to mention it so beginners don't get tripped up. Let me know if that's ok and I can do the PR or feel free to say it in your own way.

[VictorKoenders]

Which maps are not supported? If it's in the standard library I think we should be able to support it

[c-git]

I'll try to produce an example. IIRC it was a BTreeMap from std. But I will double check and get back to you on this.

[c-git]

Works now with suggestion from #697 (comment)

Old Text of comment

I tried to use the example in the readme to make the example of the map but I can't quite get it to compile. I'm not sure what I'm doing wrong. I converted it into a single file script to show the dependencies that I assumed. I'm not sure if the API has changed or if I'm doing something wrong. Will try to create the example without using the sample.

Fixed code

#!/usr/bin/env -S cargo +nightly -Zscript
```cargo
package.edition = "2021"
# dependencies.bincode = "1.3.3" # doesn't work with this version.
dependencies.bincode = "2.0.0-rc"
```

use bincode::{config, Decode, Encode};

#[derive(Encode, Decode, PartialEq, Debug)]
struct Entity {
    x: f32,
    y: f32,
}

#[derive(Encode, Decode, PartialEq, Debug)]
struct World(Vec<Entity>);

fn main() {
    let config = config::standard();

    let world = World(vec![Entity { x: 0.0, y: 4.0 }, Entity { x: 10.0, y: 20.5 }]);

    let encoded: Vec<u8> = bincode::encode_to_vec(&world, config).unwrap();

    // The length of the vector is encoded as a varint u64, which in this case gets collapsed to a single byte
    // See the documentation on varint for more info for that.
    // The 4 floats are encoded in 4 bytes each.
    assert_eq!(encoded.len(), 1 + 4 * 4);

    let (decoded, len): (World, usize) = bincode::decode_from_slice(&encoded[..], config).unwrap();

    assert_eq!(world, decoded);
    assert_eq!(len, encoded.len()); // read all bytes
}

[c-git]

I created a test case using a map and found no problem so I must have had an incorrect root cause analysis when I came to this conclusion. My apologies for creating this issue without creating a MRE to show the problem. I don't recall what the issue was but I recall being short on time and creating this to help the next person but seems I may have just mislead people instead of helping. Again my apologies. My example with a map that worked for the next person looking at this can be found below.

#!/usr/bin/env -S cargo +nightly -Zscript
```cargo
package.edition = "2021"

[dependencies]
bincode = "1.3.3"
serde = { version = "1.0.197", features = ["derive"] }

```

use std::collections::BTreeMap;

#[derive(serde::Serialize, serde::Deserialize, PartialEq, Debug)]
struct Entity {
    x: f32,
    y: f32,
    other: BTreeMap<i32, i32>,
}

#[derive(serde::Serialize, serde::Deserialize, PartialEq, Debug)]
struct World(Vec<Entity>);

fn main() {
    let mut other = BTreeMap::new();
    other.insert(3, 4);
    other.insert(5, 6);

    // The object that we will serialize.
    let target: Option<World> = Some(World(vec![Entity {
        x: 1.,
        y: 2.,
        other,
    }]));

    let encoded: Vec<u8> = bincode::serialize(&target).unwrap();
    let decoded: Option<World> = bincode::deserialize(&encoded[..]).unwrap();
    assert_eq!(target, decoded);
}

[c-git]

I'm not sure if the issue with not being able to compile the example #697 (comment) from the readme is valid and if I should create a separate issue for it as this one should be closed as it wasn't valid.

[VictorKoenders]

Oh I just realized why the example is not working. The readme is an example for the upcoming 2.0 release, you're adding a 1.3.3 dependency.

Try adding bincode = "2.0.0-rc" as a dependency and the example should compile

[c-git]

Thanks that does fix it.