fix: avoid formatting backslash strings inside f-strings (#4401)

Author: tusharsadhwani

CHANGES.md

@@ -34,6 +34,9 @@
 - Fix regression where Black failed to parse a multiline f-string containing another
   multiline string (#4339)
 
+- Fix regression where Black failed to parse an escaped single quote inside an f-string
+  (#4401)
+
 - Fix bug with Black incorrectly parsing empty lines with a backslash (#4343)
 
 ### Performance

src/black/linegen.py

@@ -510,6 +510,15 @@ def visit_fstring(self, node: Node) -> Iterator[Line]:
         # currently we don't want to format and split f-strings at all.
         string_leaf = fstring_to_string(node)
         node.replace(string_leaf)
+        if "\\" in string_leaf.value and any(
+            "\\" in str(child)
+            for child in node.children
+            if child.type == syms.fstring_replacement_field
+        ):
+            # string normalization doesn't account for nested quotes,
+            # causing breakages. skip normalization when nested quotes exist
+            yield from self.visit_default(string_leaf)
+            return
         yield from self.visit_STRING(string_leaf)
 
         # TODO: Uncomment Implementation to format f-string children

tests/data/cases/pep_701.py

@@ -128,6 +128,9 @@
 f"""{'''
 '''}"""
 
+f"{'\''}"
+f"{f'\''}"
+
 # output
 
 x = f"foo"
@@ -258,3 +261,6 @@
 
 f"""{'''
 '''}"""
+
+f"{'\''}"
+f"{f'\''}"