TeX uses -- (double hyphen) for producing an en dash and --- (triple hyphen) for an em dash, and that's pretty darn longstanding and well-established. And FWIW, the English writing conventions that I learned use em dashes as punctuation without an adjoining space—like this—but allow en dashes surrounded by spaces as an alternative – like this – so I frequently see spaced double hyphen used as the ASCII equivalent of dash punctuation and interpret it as the latter. I've never personally heard of double hyphen for em specifically, only either as en specifically or as a sort of ambiguous whatever-dash.