Fix an f-string crash in ESP. (#3463)

CHANGES.md

@@ -19,6 +19,8 @@
 - Fix a crash in preview style with assert + parenthesized string (#3415)
 - Fix crashes in preview style with walrus operators used in function return annotations
   and except clauses (#3423)
+- Fix a crash in preview advanced string processing where mixed implicitly concatenated
+  regular and f-strings start with an empty span (#3463)
 - Do not put the closing quotes in a docstring on a separate line, even if the line is
   too long (#3430)
 - Long values in dict literals are now wrapped in parentheses; correspondingly

src/black/trans.py

@@ -1359,9 +1359,14 @@ def more_splits_should_be_made() -> bool:
             # prefix, and the current custom split did NOT originally use a
             # prefix...
             if (
-                next_value != self._normalize_f_string(next_value, prefix)
-                and use_custom_breakpoints
+                use_custom_breakpoints
                 and not csplit.has_prefix
+                and (
+                    # `next_value == prefix + QUOTE` happens when the custom
+                    # split is an empty string.
+                    next_value == prefix + QUOTE
+                    or next_value != self._normalize_f_string(next_value, prefix)
+                )
             ):
                 # Then `csplit.break_idx` will be off by one after removing
                 # the 'f' prefix.

tests/data/preview/long_strings__regression.py

@@ -531,6 +531,18 @@ async def foo(self):
         r"signiferumque, duo ea vocibus consetetur scriptorem. Facer \t",
 }
 
+# Regression test for https://github.com/psf/black/issues/3459.
+xxxx(
+    empty_str_as_first_split=''
+    f'xxxxxxx {xxxxxxxxxx} xxx xxxxxxxxxx xxxxx xxx xxx xx '
+    'xxxxx xxxxxxxxx xxxxxxx, xxx xxxxxxxxxxx xxx xxxxx. '
+    f'xxxxxxxxxxxxx xxxx xx xxxxxxxxxx. xxxxx: {x.xxx}',
+    empty_u_str_as_first_split=u''
+    f'xxxxxxx {xxxxxxxxxx} xxx xxxxxxxxxx xxxxx xxx xxx xx '
+    'xxxxx xxxxxxxxx xxxxxxx, xxx xxxxxxxxxxx xxx xxxxx. '
+    f'xxxxxxxxxxxxx xxxx xx xxxxxxxxxx. xxxxx: {x.xxx}',
+)
+
 
 # output
 
@@ -1193,3 +1205,19 @@ async def foo(self):
         r"signiferumque, duo ea vocibus consetetur scriptorem. Facer \t"
     ),
 }
+
+# Regression test for https://github.com/psf/black/issues/3459.
+xxxx(
+    empty_str_as_first_split=(
+        ""
+        f"xxxxxxx {xxxxxxxxxx} xxx xxxxxxxxxx xxxxx xxx xxx xx "
+        "xxxxx xxxxxxxxx xxxxxxx, xxx xxxxxxxxxxx xxx xxxxx. "
+        f"xxxxxxxxxxxxx xxxx xx xxxxxxxxxx. xxxxx: {x.xxx}"
+    ),
+    empty_u_str_as_first_split=(
+        ""
+        f"xxxxxxx {xxxxxxxxxx} xxx xxxxxxxxxx xxxxx xxx xxx xx "
+        "xxxxx xxxxxxxxx xxxxxxx, xxx xxxxxxxxxxx xxx xxxxx. "
+        f"xxxxxxxxxxxxx xxxx xx xxxxxxxxxx. xxxxx: {x.xxx}"
+    ),
+)