def _validate(self, line: Line, string_idx: int) -> TResult[None]:
        """
        Checks that @line meets all of the requirements listed in this classes'
        docstring. Refer to `help(BaseStringSplitter)` for a detailed
        description of those requirements.

        Returns:
            * Ok(None), if ALL of the requirements are met.
              OR
            * Err(CannotTransform), if ANY of the requirements are NOT met.
        """
        LL = line.leaves

        string_leaf = LL[string_idx]

        max_string_length = self._get_max_string_length(line, string_idx)
        if len(string_leaf.value) <= max_string_length:
            return TErr(
                "The string itself is not what is causing this line to be too long."
            )

        if not string_leaf.parent or [L.type for L in string_leaf.parent.children] == [
            token.STRING,
            token.NEWLINE,
        ]:
            return TErr(
                f"This string ({string_leaf.value}) appears to be pointless (i.e. has"
                " no parent)."
            )

        if id(line.leaves[string_idx]) in line.comments and contains_pragma_comment(
            line.comments[id(line.leaves[string_idx])]
        ):
            return TErr(
                "Line appears to end with an inline pragma comment. Splitting the line"
                " could modify the pragma's behavior."
            )

        if has_triple_quotes(string_leaf.value):
            return TErr("We cannot split multiline strings.")

        return Ok(None)

        Checks that @line meets all of the requirements listed in this classes'
        docstring. Refer to `help(BaseStringSplitter)` for a detailed
        description of those requirements.

        Returns:
            * Ok(None), if ALL of the requirements are met.
              OR
            * Err(CannotTransform), if ANY of the requirements are NOT met.
        
src/black/trans.py