parseParamDeclList

fn parseParamDeclList(p: *Parse) !SmallSpan {
    _ = try p.expectToken(.l_paren);
    const scratch_top = p.scratch.items.len;
    defer p.scratch.shrinkRetainingCapacity(scratch_top);
    var varargs: enum { none, seen, err } = .none;
    while (true) {
        if (p.eatToken(.r_paren)) |_| break;
        if (varargs == .seen) {
            try p.warnMsg(.{ .tag = .varargs_nonfinal, .token = p.tok_i });
            varargs = .err;
        }
        const opt_param = try p.expectParamDecl();
        if (opt_param) |param| {
            try p.scratch.append(p.gpa, param);
        } else if (p.tokenTag(p.tok_i - 1) == .ellipsis3) {
            if (varargs == .none) varargs = .seen;
        }
        switch (p.tokenTag(p.tok_i)) {
            .comma => p.tok_i += 1,
            .r_paren => {
                p.tok_i += 1;
                break;
            },
            .colon, .r_brace, .r_bracket => return p.failExpected(.r_paren),
            // Likely just a missing comma; give error but continue parsing.
            else => try p.warn(.expected_comma_after_param),
        }
    }
    const params = p.scratch.items[scratch_top..];
    return switch (params.len) {
        0 => .{ .zero_or_one = .none },
        1 => .{ .zero_or_one = params[0].toOptional() },
        else => .{ .multi = try p.listToSpan(params) },
    };
}