How to serialise structs into io::Write in a loop - json

Question

I need to do a simple read/process/write in Rust like this:
#[derive(serde::Deserialize)]
struct Incoming {
first: String,
last: String,
}
#[derive(serde::Serialize)]
struct Outgoing {
name: String,
}
// Keep the read/write traits as generic as possible
fn stream_things<R: std::io::Read, W: std::io::Write>(reader: R, writer: W) {
let incoming: Vec<Incoming> = serde_json::from_reader(reader).unwrap();
for a in incoming {
let b = Outgoing {
name: format!("{} {}", a.first, a.last),
};
serde_json::to_writer(writer, &b).unwrap();
}
}
fn main() {
stream_things(std::io::stdin(), std::io::stdout());
}
This does not compile because:
error[E0382]: use of moved value: `writer`
--> src/main.rs:20:31
|
13 | fn stream_things<R: std::io::Read, W: std::io::Write>(reader: R, writer: W) {
| -- ------ move occurs because `writer` has type `W`, which does not implement the `Copy` trait
| |
| help: consider further restricting this bound: `W: Copy +`
...
20 | serde_json::to_writer(writer, &b).unwrap();
| ^^^^^^ value moved here, in previous iteration of loop
What is the correct way to write to std::io::Write in a loop?
Also how to correctly do it with serde's to_writer?
See the plaground.

Answer 1

Given W is io::Write, then &mut W is also io::Write:
impl<'_, W: Write + ?Sized> Write for &'_ mut W
so the following compiles:
fn stream_things<R: std::io::Read, W: std::io::Write>(reader: R, mut writer: W) {
let incoming: Vec<Incoming> = serde_json::from_reader(reader).unwrap();
for a in incoming {
let b = Outgoing {
name: format!("{} {}", a.first, a.last),
};
serde_json::to_writer(&mut writer, &b).unwrap();
}
}