From 8def2fc122a9f698914606c51a9e8c18aed168df Mon Sep 17 00:00:00 2001
From: Vadim Petrochenkov <vadim.petrochenkov@gmail.com>
Date: Wed, 4 Nov 2020 16:57:51 +0300
Subject: [PATCH] rustc_ast: Never clone empty token streams in mutable visitor

---
 compiler/rustc_ast/src/mut_visit.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index af8b8ad34c8f2..517717eebd9d0 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -656,7 +656,7 @@ pub fn visit_tt<T: MutVisitor>(tt: &mut TokenTree, vis: &mut T) {
 
 // No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
 pub fn visit_tts<T: MutVisitor>(TokenStream(tts): &mut TokenStream, vis: &mut T) {
-    if vis.token_visiting_enabled() {
+    if vis.token_visiting_enabled() && !tts.is_empty() {
         let tts = Lrc::make_mut(tts);
         visit_vec(tts, |(tree, _is_joint)| visit_tt(tree, vis));
     }