From 409541bc77cb1d6a447dbcdf74f9fcbc2a8b0397 Mon Sep 17 00:00:00 2001
From: Kendall Condon <goon.pri.low@gmail.com>
Date: Mon, 28 Jul 2025 14:10:18 -0400
Subject: [PATCH] zig fmt: canonicalize stray backslashes in identifiers

---
 lib/std/zig/Ast/Render.zig  |  3 +++
 lib/std/zig/parser_test.zig | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/lib/std/zig/Ast/Render.zig b/lib/std/zig/Ast/Render.zig
index e9caab17a3..a501b21d10 100644
--- a/lib/std/zig/Ast/Render.zig
+++ b/lib/std/zig/Ast/Render.zig
@@ -3257,6 +3257,9 @@ fn renderIdentifierContents(ais: *AutoIndentingStream, bytes: []const u8) !void
                         }
                     },
                     .failure => {
+                        // Escape the stray backslash
+                        // This also avoids cases like "\x3\x39" becoming "\x39"
+                        try ais.writeByte('\\');
                         try ais.writeAll(escape_sequence);
                     },
                 }
diff --git a/lib/std/zig/parser_test.zig b/lib/std/zig/parser_test.zig
index ab0181ede8..800caf0e94 100644
--- a/lib/std/zig/parser_test.zig
+++ b/lib/std/zig/parser_test.zig
@@ -6666,6 +6666,24 @@ test "zig fmt: doc comments on fn parameters" {
     );
 }
 
+test "zig fmt: canonicalize stray backslashes in identifiers" {
+    try testTransform(
+        \\const @"\x" = undefined;
+        \\const @"\x3" = undefined;
+        \\const @"\x3\x39" = undefined;
+        \\const @"\u{" = undefined;
+        \\const @"\?" = undefined;
+        \\
+    ,
+        \\const @"\\x" = undefined;
+        \\const @"\\x3" = undefined;
+        \\const @"\\x39" = undefined;
+        \\const @"\\u{" = undefined;
+        \\const @"\\?" = undefined;
+        \\
+    );
+}
+
 test "recovery: top level" {
     try testError(
         \\test "" {inline}