def _iter_fexpr_slices(self, string: str) -> Iterator[tuple[Index, Index]]:
        """
        Yields:
            All ranges of @string which, if @string were to be split there,
            would result in the splitting of an f-expression (which is NOT
            allowed).
        """
        if "f" not in get_string_prefix(string).lower():
            return
        yield from iter_fexpr_spans(string)

        Yields:
            All ranges of @string which, if @string were to be split there,
            would result in the splitting of an f-expression (which is NOT
            allowed).
        
src/black/trans.py