From 537b5b38cccda20fb60bf54377ffd695bab5d885 Mon Sep 17 00:00:00 2001
From: Jeremy Fitzhardinge <jeremy@goop.org>
Date: Sat, 9 Sep 2023 11:30:23 -0700
Subject: [PATCH] Use serde_json::to_writer for JsonEmitter::emit

Avoids an unnecessary intermediate string.
---
 compiler/rustc_errors/src/json.rs | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/compiler/rustc_errors/src/json.rs b/compiler/rustc_errors/src/json.rs
index 7b92ce95f830..2f7145ceadc4 100644
--- a/compiler/rustc_errors/src/json.rs
+++ b/compiler/rustc_errors/src/json.rs
@@ -141,11 +141,12 @@ impl JsonEmitter {
 
     fn emit(&mut self, val: EmitTyped<'_>) -> io::Result<()> {
         if self.pretty {
-            writeln!(self.dst, "{}", serde_json::to_string_pretty(&val).unwrap())
+            serde_json::to_writer_pretty(&mut *self.dst, &val)?
         } else {
-            writeln!(self.dst, "{}", serde_json::to_string(&val).unwrap())
-        }
-        .and_then(|_| self.dst.flush())
+            serde_json::to_writer(&mut *self.dst, &val)?
+        };
+        self.dst.write_all(b"\n")?;
+        self.dst.flush()
     }
 }