Cannot recover from gap while searching for lookahead

[bbannier]

At least in some situations when a parser encounters a gap while searching for a lookahead token one cannot recovery with &synchronize/confirm, e.g.,

# sync.spicy
module foo;

type X = unit {
	a: /A/ &synchronize;

	on %synced { print "sync"; confirm; }
};

public type Xs = unit {
	xs: (X &synchronize)[] { confirm; }
};

// foo.cc
#include "sync.cc"
#include "sync_linker.cc"

int main() {
    hilti::rt::init();
    spicy::rt::init();

    const spicy::rt::Parser* parser = nullptr;
    for ( auto* p : spicy::rt::parsers() ) {
        parser = p;
        break;
    }

    assert(parser);

    auto xs = hilti::rt::reference::make_value<__hlt::foo::Xs>();

    hilti::rt::ValueReference<hilti::rt::Stream> stream;
    stream->append("A");
    stream->append(nullptr, 1);
    stream->append("A");

     __hlt::foo::Xs::parse2(xs, stream, {}, {});
    std::cerr << xs << '\n';
}

$ spicyc -dco sync.cc sync.spicy
$ spicyc -dlo sync_linker.cc sync.spicy
$ $(spicy-config --cxx-launcher --cxx --cxxflags --debug --ldflags) -o foo foo.cc
$ ./foo
llibc++abi.dylib: terminating with uncaught exception of type hilti::rt::MissingData: data is missing (sync.spicy:10:2)

Looking at the backtrace, this error originates in stage 1 parsing which has no recovery code,

frame #5: 0x00000001000b1895 foo`__hlt::foo::Xs::__parse_xs_stage1(this=0x0000000100706d88, __data=0x00007ffeefbfed28, __cur=View @ 0x00007ffeefbfdad8, __trim=(_value = false), __lah=(m_int = 0), __lahe=SafeConstIterator @ 0x00007ffeefbfdab0, __error= Has Value=false , __dst=0x0000000100706d98) at sync.cc:495:49
   492                      for ( ::hilti::rt::regexp::MatchState ms = __hlt::foo::__re_xs_l1.tokenMatcher(); ::hilti::rt::Bool(true);  ) {
   493                          ::hilti::rt::integer::safe<int32_t> rc;
   494                            __location__("sync.spicy:10:2");
-> 495                          std::tie(rc, ncur) = ms.advance(ncur);
   496                            __location__("sync.spicy:10:2");
   497                          if ( ::hilti::rt::integer::safe<int32_t> __x = rc; __x == ::hilti::rt::integer::safe<std::int32_t>{-1} ) {
   498                              if ( ::spicy::rt::detail::waitForInputOrEod(__data, __cur, (*__self).__filters) ) {
(lldb)
frame #6: 0x00000001000b0693 foo`__hlt::foo::Xs::__parse_foo_Xs_stage2(this=0x0000000100706d88, __data=0x00007ffeefbfed28, __cur=View @ 0x00007ffeefbfe128, __trim=(_value = false), __lah=(m_int = 0), __lahe=SafeConstIterator @ 0x00007ffeefbfe100, __error= Has Value=false ) at sync.cc:324:57
   321      spicy::rt::detail::printParserState(std::string("foo::Xs"), __data, __cur, __lah, __lahe, std::string("default"), __trim, __error);
   322      HILTI_RT_DEBUG(std::string("spicy-verbose"), std::string("- parsing production: While: xs  -> while(<look-ahead-found>): foo_X_2"));
   323      ::hilti::rt::debug::indent(std::string("spicy-verbose"));
-> 324      std::tie(__cur, __lah, __lahe, __error) = (*__self).__parse_xs_stage1(__data, __cur, __trim, __lah, __lahe, __error, ::hilti::rt::optional::valueOrInit((*__self).xs));
   325      ::hilti::rt::debug::dedent(std::string("spicy-verbose"));
   326      // End parsing production: While: xs  -> while(<look-ahead-found>): foo_X_2;
   327