From 2ee6d55c62e9b62a20b5a87a7665e9836b879120 Mon Sep 17 00:00:00 2001
From: Mark Rousskov <mark.simulacrum@gmail.com>
Date: Sat, 19 Feb 2022 14:49:35 -0500
Subject: [PATCH] Preallocate a buffer in FmtPrinter

---
 compiler/rustc_middle/src/ty/print/pretty.rs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index 0e94ac40ff7..80ba7232a4a 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -1583,7 +1583,9 @@ impl<'a, 'tcx> FmtPrinter<'a, 'tcx> {
     pub fn new(tcx: TyCtxt<'tcx>, ns: Namespace) -> Self {
         FmtPrinter(Box::new(FmtPrinterData {
             tcx,
-            fmt: String::new(),
+            // Estimated reasonable capacity to allocate upfront based on a few
+            // benchmarks.
+            fmt: String::with_capacity(64),
             empty_path: false,
             in_value: ns == Namespace::ValueNS,
             print_alloc_ids: false,