expectBuiltinCall

fn expectBuiltinCall(p: *Parse) !Node.Index {
    const builtin_token = p.assertToken(.builtin);
    _ = p.eatToken(.l_paren) orelse {
        try p.warn(.expected_param_list);
        // Pretend this was an identifier so we can continue parsing.
        return p.addNode(.{
            .tag = .identifier,
            .main_token = builtin_token,
            .data = undefined,
        });
    };
    const scratch_top = p.scratch.items.len;
    defer p.scratch.shrinkRetainingCapacity(scratch_top);
    while (true) {
        if (p.eatToken(.r_paren)) |_| break;
        const param = try p.expectExpr();
        try p.scratch.append(p.gpa, param);
        switch (p.tokenTag(p.tok_i)) {
            .comma => p.tok_i += 1,
            .r_paren => {
                p.tok_i += 1;
                break;
            },
            // Likely just a missing comma; give error but continue parsing.
            else => try p.warn(.expected_comma_after_arg),
        }
    }
    const comma = (p.tokenTag(p.tok_i - 2)) == .comma;
    const params = p.scratch.items[scratch_top..];
    if (params.len <= 2) {
        return p.addNode(.{
            .tag = if (comma) .builtin_call_two_comma else .builtin_call_two,
            .main_token = builtin_token,
            .data = .{ .opt_node_and_opt_node = .{
                if (params.len >= 1) .fromOptional(params[0]) else .none,
                if (params.len >= 2) .fromOptional(params[1]) else .none,
            } },
        });
    } else {
        const span = try p.listToSpan(params);
        return p.addNode(.{
            .tag = if (comma) .builtin_call_comma else .builtin_call,
            .main_token = builtin_token,
            .data = .{ .extra_range = span },
        });
    }
}