Handle escaped braces in f-strings

src/flake8/processor.py

@@ -203,7 +203,13 @@ def build_logical_line_tokens(self) -> _Logical:  # noqa: C901
             if token_type == tokenize.STRING:
                 text = mutate_string(text)
             elif token_type == FSTRING_MIDDLE:  # pragma: >=3.12 cover
-                text = "x" * len(text)
+                # A curly brace in an FSTRING_MIDDLE token must be an escaped
+                # curly brace. Both 'text' and 'end' will account for the
+                # escaped version of the token (i.e. a single brace) rather
+                # than the raw double brace version, so we must counteract this
+                brace_offset = text.count("{") + text.count("}")
+                text = "x" * (len(text) + brace_offset)
+                end = (end[0], end[1] + brace_offset)
             if previous_row:
                 (start_row, start_column) = start
                 if previous_row != start_row:

tests/integration/test_plugins.py

@@ -1,6 +1,8 @@
 """Integration tests for plugin loading."""
 from __future__ import annotations
 
+import sys
+
 import pytest
 
 from flake8.main.cli import main
@@ -261,3 +263,36 @@ def test_logical_line_plugin(tmpdir, capsys):
 """
     out, err = capsys.readouterr()
     assert out == expected
+
+
+def test_escaping_of_fstrings_in_string_redacter(tmpdir, capsys):
+    cfg_s = f"""\
+[flake8]
+extend-ignore = F
+[flake8:local-plugins]
+extension =
+    T = {yields_logical_line.__module__}:{yields_logical_line.__name__}
+"""
+
+    cfg = tmpdir.join("tox.ini")
+    cfg.write(cfg_s)
+
+    src = """\
+f'{{"{hello}": "{world}"}}'
+"""
+    t_py = tmpdir.join("t.py")
+    t_py.write_binary(src.encode())
+
+    with tmpdir.as_cwd():
+        assert main(("t.py", "--config", str(cfg))) == 1
+
+    if sys.version_info >= (3, 12):  # pragma: >=3.12 cover
+        expected = """\
+t.py:1:1: T001 "f'xxx{hello}xxxx{world}xxx'"
+"""
+    else:  # pragma: <3.12 cover
+        expected = """\
+t.py:1:1: T001 "f'xxxxxxxxxxxxxxxxxxxxxxxx'"
+"""
+    out, err = capsys.readouterr()
+    assert out == expected