deserialization error on `ChunkSource::Chunks`

[baehyunsol]

serde has a problem deserializing Chunk whose source is ChunkSource::Chunks. To be more specific, when deserializing one, it throws:

JsonSerdeError(Error("u128 is not supported", line: 16, column: 3))

Some interesting things are that

1. It can serialize the struct, but cannot deserialize it.
2. It can deserialize u128 in Uid, but not the ones in ChunkSource::Chunks. I suspect it's because of #[serde(tag = "type")], but I'm not sure yet.

I thought this pr is a fix for the problem, and I tested locally. It gives me another error message:

JsonSerdeError(Error("invalid type: map, expected u128", line: 16, column: 3))

It could either be 1) a bug in the PR or 2) the PR is not for this issue, but I'm not sure.

Below is a minimal code that can reproduce this error:

use serde::{Deserialize, Serialize};

#[derive(Debug, Deserialize, Serialize)]
#[serde(tag = "type")]
pub enum ChunkSource {
    File { source: String, index: usize },
    Chunks { uids: Vec<Uid> },
}

#[derive(Debug, Deserialize, Serialize)]
pub struct Uid {
    high: u128,
    low: u128,
}

fn main() {
    let uid = Uid { high: 290035462369354878339780249989510800979, low: 181325506523271356483045718667112222021 };
    let u = ChunkSource::Chunks { uids: vec![uid] };
    let us = serde_json::to_string_pretty(&u).unwrap();
    println!("{:?}", serde_json::from_str::<ChunkSource>(&us).unwrap());
}

In the local version (the one with the PR), it does not panic if high and low of Uid are less than or equal to u64::MAX. With crates.io version, it dies regardless the value of high and low.

Below is the json it generates:

{
  "type": "Chunks",
  "uids": [
    {
      "high": 290035462369354878339780249989510800979,
      "low": 181325506523271356483045718667112222021
    }
  ]
}

[baehyunsol]

It seems like serde_json's arbitrary_precision feature has a problem. The PR mentioned above behaves differently with and without the feature.

With arbitrary_precision, the error message says

Error("invalid type: map, expected u128", line: 0, column: 0)

Without it, it's

Error("invalid type: floating point 2.9003546236935486e38, expected u128", line: 0, column: 0)

It seems like the feature changes how serde_json deserializes tagged enums.