Avoid allocation in JsonEmitter::emit_footer

Write directly to the output instead of creating an intermediate `String`.

src/emitter/json.rs

@@ -1,7 +1,7 @@
 use super::*;
 use crate::rustfmt_diff::{DiffLine, Mismatch, make_diff};
 use serde::Serialize;
-use serde_json::to_string as to_json_string;
+use serde_json::to_writer as to_json_writer;
 
 #[derive(Debug, Default)]
 pub(crate) struct JsonEmitter {
@@ -26,7 +26,8 @@ struct MismatchedFile {
 
 impl Emitter for JsonEmitter {
  fn emit_footer(&self, output: &mut dyn Write) -> Result<(), io::Error> {
- writeln!(output, "{}", &to_json_string(&self.mismatched_files)?)
+ to_json_writer(&mut *output, &self.mismatched_files)?;
+ writeln!(output)
  }
 
  fn emit_formatted_file(
@@ -252,7 +253,7 @@ mod tests {
  )
  .unwrap();
  let _ = emitter.emit_footer(&mut writer);
- let exp_json = to_json_string(&vec![MismatchedFile {
+ let exp_json = serde_json::to_string(&vec![MismatchedFile {
  name: String::from(file_name),
  mismatches: vec![
  MismatchedBlock {
@@ -338,7 +339,7 @@ mod tests {
  }],
  };
 
- let exp_json = to_json_string(&vec![exp_bin, exp_lib]).unwrap();
+ let exp_json = serde_json::to_string(&vec![exp_bin, exp_lib]).unwrap();
  assert_eq!(&writer[..], format!("{exp_json}\n").as_bytes());
  }
 }