Preserve escaped test section markers in update-data

mypy/test/meta/test_update_data.py

@@ -72,6 +72,14 @@ def test_update_data(self) -> None:
             s1: str = 42  # E: bar
             [file b.py]
             s2: str = 43  # E: baz
+
+            [case testEscapedBracketInFile]
+            # flags: --config-file tmp/mypy.ini
+            s: str = 42  # E: foo
+            [file mypy.ini]
+            \\[mypy]
+            warn_unused_ignores = True
+
             [builtins fixtures/list.pyi]
             """)
 
@@ -126,6 +134,14 @@ def test_update_data(self) -> None:
         s1: str = 42  # E: Incompatible types in assignment (expression has type "int", variable has type "str")
         [file b.py]
         s2: str = 43  # E: Incompatible types in assignment (expression has type "int", variable has type "str")
+
+        [case testEscapedBracketInFile]
+        # flags: --config-file tmp/mypy.ini
+        s: str = 42  # E: Incompatible types in assignment (expression has type "int", variable has type "str")
+        [file mypy.ini]
+        \\[mypy]
+        warn_unused_ignores = True
+
         [builtins fixtures/list.pyi]
         """)
         assert result.input_updated == expected

mypy/test/update_data.py

@@ -67,6 +67,7 @@ def _iter_fixes(
             comment_match = re.search(r"(?P<indent>\s+)(?P<comment># [EWN]: .+)$", source_line)
             if comment_match:
                 source_line = source_line[: comment_match.start("indent")]  # strip old comment
+            source_line = _escape_test_data_line(source_line)
             if reports:
                 indent = comment_match.group("indent") if comment_match else "  "
                 # multiline comments are on the first line and then on subsequent lines empty lines
@@ -85,3 +86,11 @@ def _iter_fixes(
             end_lineno=testcase.line + test_item.end_line - 1,
             lines=fix_lines + [""] * test_item.trimmed_newlines,
         )
+
+
+def _escape_test_data_line(line: str) -> str:
+    if line.startswith("["):
+        return "\\" + line
+    if line.startswith("--"):
+        return "--" + line
+    return line