zig fmt: canonicalize stray backslashes in identifiers

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);
                     },
                 }

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}