Fix sus returns in strings.py (#4546)

src/black/linegen.py

@@ -67,7 +67,7 @@
 )
 from black.numerics import normalize_numeric_literal
 from black.strings import (
-    fix_docstring,
+    fix_multiline_docstring,
     get_string_prefix,
     normalize_string_prefix,
     normalize_string_quotes,
@@ -444,7 +444,7 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
             indent = " " * 4 * self.current_line.depth
 
             if is_multiline_string(leaf):
-                docstring = fix_docstring(docstring, indent)
+                docstring = fix_multiline_docstring(docstring, indent)
             else:
                 docstring = docstring.strip()
 

src/black/strings.py

@@ -63,10 +63,9 @@ def lines_with_leading_tabs_expanded(s: str) -> list[str]:
     return lines
 
 
-def fix_docstring(docstring: str, prefix: str) -> str:
+def fix_multiline_docstring(docstring: str, prefix: str) -> str:
     # https://www.python.org/dev/peps/pep-0257/#handling-docstring-indentation
-    if not docstring:
-        return ""
+    assert docstring, "INTERNAL ERROR: Multiline docstrings cannot be empty"
     lines = lines_with_leading_tabs_expanded(docstring)
     # Determine minimum indentation (first line doesn't count):
     indent = sys.maxsize
@@ -186,8 +185,7 @@ def normalize_string_quotes(s: str) -> str:
         orig_quote = "'"
         new_quote = '"'
     first_quote_pos = s.find(orig_quote)
-    if first_quote_pos == -1:
-        return s  # There's an internal error
+    assert first_quote_pos != -1, f"INTERNAL ERROR: Malformed string {s!r}"
 
     prefix = s[:first_quote_pos]
     unescaped_new_quote = _cached_compile(rf"(([^\\]|^)(\\\\)*){new_quote}")