Function parse 

pub fn parse<'v, T>(
    spec: &Spec<'_>,
    argv: &[&'v OsStr],
    parse_from: impl FnOnce(&[&'v OsStr]) -> Result<T, Error<'static, 'v>>,
) -> Result<T, String>

The parsed struct, or the text a user would have read instead.

outcome without the four-way distinction, for the common test: parse this, and if it does not parse, show me what the user gets. A help or version request is text here too, since neither is a parsed struct.