From 712fde5a6fce0e7bee3615ab5c7545ce442bf034 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Sun, 24 Dec 2023 20:19:37 -0800
Subject: [PATCH] Fix ToTokens for PatTuple to insert trailing comma

---
 src/pat.rs        | 9 +++++++++
 tests/test_pat.rs | 9 ++++++---
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/pat.rs b/src/pat.rs
index df7da5bbe4..249960459a 100644
--- a/src/pat.rs
+++ b/src/pat.rs
@@ -856,6 +856,15 @@ mod printing {
             tokens.append_all(self.attrs.outer());
             self.paren_token.surround(tokens, |tokens| {
                 self.elems.to_tokens(tokens);
+                // If there is only one element, a trailing comma is needed to
+                // distinguish PatTuple from PatParen, unless this is `(..)`
+                // which is a tuple pattern even without comma.
+                if self.elems.len() == 1
+                    && !self.elems.trailing_punct()
+                    && !matches!(self.elems[0], Pat::Rest { .. })
+                {
+                    <Token![,]>::default().to_tokens(tokens);
+                }
             });
         }
     }
diff --git a/tests/test_pat.rs b/tests/test_pat.rs
index 89033083c3..7b5f8b0257 100644
--- a/tests/test_pat.rs
+++ b/tests/test_pat.rs
@@ -107,10 +107,13 @@ fn test_tuple_comma() {
     snapshot!(expr.to_token_stream() as Pat, @"Pat::Tuple");
 
     expr.elems.push_value(parse_quote!(_));
-    // FIXME: must parse to Pat::Tuple, not Pat::Paren
+    // Must not parse to Pat::Paren
     snapshot!(expr.to_token_stream() as Pat, @r###"
-    Pat::Paren {
-        pat: Pat::Wild,
+    Pat::Tuple {
+        elems: [
+            Pat::Wild,
+            Token![,],
+        ],
     }
     "###);