[3.13] gh-154711: Strip comments after escaped quotes in f-string debug text (#158310)

When a replacement field ended with a string that had an escaped quote and
was followed by a comment, the comment leaked into the debug f-string output.

The comment detection loop in set_fstring_expr skips escaped characters, but
the comment stripping loop did not. So an escaped quote left the scanner
thinking it was still inside a string, and the comment was never removed.

This makes the stripping loop skip escaped characters the same way the
detection loop does. 3.14 and later already have this fix from gh-154719.

Author: ArneshBanerjee

Lib/test/test_fstring.py

@@ -1664,6 +1664,11 @@ def __repr__(self):
         self.assertEqual(f'{" # nooo "=}', '" # nooo "=\' # nooo \'')
         self.assertEqual(f'{" \" # nooo \" "=}', '" \\" # nooo \\" "=\' " # nooo " \'')
 
+        # A real comment after a string that ends with an escaped quote must
+        # still be stripped (gh-154711).
+        self.assertEqual(f"{'\'' = # comment
+}", "'\\'' = \n" + repr("'"))
+
         self.assertEqual(f'{ # some comment goes here
   """hello"""=}',  ' \n  """hello"""=\'hello\'')
         self.assertEqual(f'{"""# this is not a comment

Misc/NEWS.d/next/Core_and_Builtins/2026-07-30-14-40-00.gh-issue-154711.nUxYXc.rst

@@ -0,0 +1,3 @@
+Fix a bug where a comment placed after a string that ends with an escaped
+quote could leak into the output of a debug f-string. Now the comment is
+stripped as expected.

Parser/lexer/lexer.c

@@ -162,6 +162,19 @@ set_fstring_expr(struct tok_state* tok, struct token *token, char c) {
         while (i < expression_size) {
             char ch = expression[i];
 
+            // Copy escaped characters as-is. This keeps an escaped quote from
+            // flipping the in_string state, which would otherwise stop a real
+            // comment from being detected (see the detection loop above).
+            if (ch == '\\') {
+                result[j++] = ch;
+                i++;
+                if (i < expression_size) {
+                    result[j++] = expression[i];
+                    i++;
+                }
+                continue;
+            }
+
             // Handle string quotes
             if (ch == '"' || ch == '\'') {
                 // See comment above to understand this part