Boost.Spirit X3 编译时间因递归规则而爆炸

下面的程序需要10秒才能编译。当我更改下面的parenProcess规则以'(' >> process >> ')'编译器花费 CPU 但似乎没有完成时。（我尝试制作一个较小的可重现程序 - 通过删除process和parenProcess之间的规则，但随后编译时间不再爆炸）。

嵌入process时如何修复编译（时间）？

（其他小问题：有没有更好的方法来制定规则"x"和"xActual"？

#include <boost/config/warning_disable.hpp>
#include <boost/spirit/home/x3.hpp>
#include <boost/spirit/home/x3/support/ast/variant.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <iostream>
#include <string>
#include <vector>
namespace wccs_parser {
namespace x3 = boost::spirit::x3;
namespace ascii = x3::ascii;
using x3::long_;
using x3::ulong_;
using x3::lexeme;
//--- Ast structures
struct AstChannel {
    std::string label;
    bool complement;
};
struct AstAction {
    AstChannel channel;
    uint32_t weight;
};
struct AstRenaming {
    std::string from;
    std::string to;
};
struct AstNullProcess;
struct AstActionPrefixProcess;
struct AstChoiceProcess;
struct AstCompositionProcess;
struct AstRestrictionProcess;
struct AstRenamingProcess;
struct AstConstantProcess;
using AstAnyProcess = x3::variant<
    x3::forward_ast<AstNullProcess>,
    x3::forward_ast<AstActionPrefixProcess>,
    x3::forward_ast<AstChoiceProcess>,
    x3::forward_ast<AstCompositionProcess>,
    x3::forward_ast<AstRestrictionProcess>,
    x3::forward_ast<AstRenamingProcess>,
    x3::forward_ast<AstConstantProcess>
>;
struct AstNullProcess {};
struct AstActionPrefixProcess {
    AstAction action;
    AstAnyProcess subProcess;
};
struct AstChoiceProcess {
    std::vector<AstAnyProcess> subProcesses;
};
struct AstCompositionProcess {
    std::vector<AstAnyProcess> subProcesses;
};
struct AstRestrictionProcess {
    AstAnyProcess subProcess;
    std::vector<std::string> labels;
};
struct AstRenamingProcess {
    AstAnyProcess subProcess;
    std::vector<AstRenaming> renamings;
};
struct AstConstantProcess {
    std::string processName;
};
} // End namespace
BOOST_FUSION_ADAPT_STRUCT(wccs_parser::AstChannel, label, complement)
BOOST_FUSION_ADAPT_STRUCT(wccs_parser::AstAction, channel, weight)
BOOST_FUSION_ADAPT_STRUCT(wccs_parser::AstRenaming, from, to)
BOOST_FUSION_ADAPT_STRUCT(wccs_parser::AstActionPrefixProcess, action, subProcess)
BOOST_FUSION_ADAPT_STRUCT(wccs_parser::AstChoiceProcess, subProcesses)
BOOST_FUSION_ADAPT_STRUCT(wccs_parser::AstCompositionProcess, subProcesses)
BOOST_FUSION_ADAPT_STRUCT(wccs_parser::AstRestrictionProcess, subProcess, labels)
BOOST_FUSION_ADAPT_STRUCT(wccs_parser::AstRenamingProcess, subProcess, renamings)
BOOST_FUSION_ADAPT_STRUCT(wccs_parser::AstConstantProcess, processName)
namespace wccs_parser {
//--- Rules
auto const constantName = x3::rule<struct constantRule, std::string> {"constantName"} =
    x3::lexeme[ascii::upper >> *(ascii::alnum)];
auto const label = x3::rule<struct labelRule, std::string> {"label"} =
    x3::lexeme[ascii::lower >> *(ascii::alnum)];
auto const channel = x3::rule<struct channelRule, AstChannel> {"channel"} = 
    label >> x3::matches['!'];
auto const action = x3::rule<struct actionRule, AstAction> {"action"} = 
    '<' >> channel >> ',' >> ulong_ >> '>'; 
auto renamingPair = x3::rule<struct renamingPairRule, AstRenaming> {"renamingPair"} =
    label > "=>" > label;
x3::rule<struct processRule, AstAnyProcess> process{"process"};
auto const nullProcess = x3::rule<struct nullProcessRule, AstNullProcess> {"nullProcess"} = '0' >> x3::attr(AstNullProcess());
auto const constant = x3::rule<struct constantRule, AstConstantProcess> {"constant"} = constantName;
///   HERE:
auto const parenProcess = '(' > nullProcess > ')';
auto const primitive = x3::rule<struct primitiveRule, AstAnyProcess> {"primitive"} =
      parenProcess
    | nullProcess
    | constant; 
auto const restrictionActual = x3::rule<struct restrictionActual, AstRestrictionProcess> {"restrictionActual"} =
    primitive >> '' >> '{' >> label % ',' >> '}';
auto const restriction = x3::rule<struct restrictionRule, AstAnyProcess> {"restriction"} =
      primitive >> !x3::lit('')
    | restrictionActual;
auto const renamingActual = x3::rule<struct renamingActualRule, AstRenamingProcess> {"renamingActual"} =
    restriction >> '[' >> renamingPair % ',' >> ']';
auto const renaming = x3::rule<struct renamingRule, AstAnyProcess> {"renaming"} =
      restriction >> !x3::lit('[')
    | renamingActual;
x3::rule<struct actionPrefixingRule, AstAnyProcess> actionPrefix{"actionPrefix"};
auto const actionPrefixActual = x3::rule<struct actionPrefixActualRule, AstActionPrefixProcess> {"actionPrefixActual"} =
    action > ('.' > actionPrefix);
auto const actionPrefix_def =
      actionPrefixActual
    | renaming;
BOOST_SPIRIT_DEFINE(actionPrefix)
auto const compositionActual = x3::rule<struct choiceActualrule, AstCompositionProcess> {"compositionActual"} =
    actionPrefix % '|'; 
auto const composition = x3::rule<struct compositionRule, AstAnyProcess> {"composition"} =
      actionPrefix >> !x3::lit('|')
    | compositionActual;
auto const choiceActual = x3::rule<struct choiceActualrule, AstChoiceProcess> {"choiceActual"} =
    composition % '+'; 
auto const choice = x3::rule<struct choiceRule, AstAnyProcess> {"choice"} =
      composition >> !x3::lit('+')
    | choiceActual;
auto const process_def = choice;
BOOST_SPIRIT_DEFINE(process)
auto const entry = x3::skip(ascii::space) [process];
} //End namespace
int main() {
    std::string str("0 + (0)");
    wccs_parser::AstAnyProcess root;
    auto iter = str.begin();
    auto end = str.end();
    bool r = parse(iter, end, wccs_parser::entry, root);
    if (r) {
        std::cout << str << std::endl << std::endl << " Parses OK: " << std::endl;
    }
    else {
        std::cout << "Parsing failedn";
    }
    if (iter != end) std::cout << "Partial match" << std::endl;
    return 0;
}