element

pub fn element(
    self: *Decoder,
    expected: ExpectedTag,
) (error{ EndOfStream, UnexpectedElement } || Element.DecodeError)!Element {
    if (self.index >= self.bytes.len) return error.EndOfStream;

    const res = try Element.decode(self.bytes, self.index);
    var e = expected;
    if (self.field_tag) |ft| {
        e.number = @enumFromInt(ft.number);
        e.class = ft.class;
    }
    if (!e.match(res.tag)) {
        return error.UnexpectedElement;
    }

    self.index = if (res.tag.constructed) res.slice.start else res.slice.end;
    return res;
}