Add Formatter::write_byte_array

[dtolnay]

Example that writes bytes as base64 JSON strings:

// [dependencies]
// base64 = "0.21"
// serde = { version = "1", features = ["derive"] }
// serde_bytes = "0.11"
// serde_json = "1"

use serde::Serialize;
use std::io::{self, Write};

#[derive(Serialize)]
struct Struct {
    #[serde(with = "serde_bytes")]
    buf: Vec<u8>,
}

struct Base64Formatter;

impl serde_json::ser::Formatter for Base64Formatter {
    fn write_byte_array<W>(&mut self, writer: &mut W, value: &[u8]) -> io::Result<()>
    where
        W: ?Sized + io::Write,
    {
        writer.write_all(b"\"")?;
        let engine = &base64::engine::general_purpose::STANDARD_NO_PAD;
        let mut encoder = base64::write::EncoderWriter::new(writer, engine);
        encoder.write_all(value)?;
        let writer = encoder.finish()?;
        writer.write_all(b"\"")
    }
}

fn main() -> serde_json::Result<()> {
    let thing = Struct {
        buf: b".....\xed..\x1e\n.".to_vec(),
    };

    let out = io::stdout();
    let mut ser = serde_json::Serializer::with_formatter(out, Base64Formatter);
    thing.serialize(&mut ser)?;
    Ok(())
}

{"buf":"Li4uLi7tLi4eCi4"}