mycpp tool improvements

[andychu]

Functionality

• It should be able to export headers.
  • frontend/args.py needs a header, exported to the cpp/frontend_arg_def.cc
    • args::_Attributes should be exported, as well as args::Reader
  • asdl/format.py needs a header
  • probably tdop or arith_parse? and more

Other:

• it's way too slow. Can we leverage the caching somehow? I think it serializes to JSON or something?
• I don't like the "inverted" visitor pattern.

Maybe dump the AST to an ASDL schema? It would solve both problems. Compile-time reference counting could motivate us to do this?

• Weird bug: why do we get asdl.tdop and oil.asdl.tdop after looking at MyPy's result.graph ?

[andychu]

• ASDL enhancements #443 -- related

[andychu]

• Essential bug detection: I think you have to detect and disallow allocation inside arg lists, because C++ evaluation order isn't defined?

f(Alloc<Foo>(), Alloc<Bar>());   // bad!

myobj->Method(Alloc<Foo>());  // bad because myobj will be moved!!!

[andychu]

Discussion with author of mjs about the above evaluation order bug:

https://old.reddit.com/r/Compilers/comments/v291jy/a_garbagecollected_heap_in_c_shaped_like_typed/iat53lw/

Discussion with Erik Corry about it:

https://old.reddit.com/r/cpp/comments/v1vkce/a_garbagecollected_heap_in_c_shaped_like_typed/icf70rq/

[andychu]

• It should detect the f(g()) missing stack root problem.
  • related: generated ASDL code should support garbage collection #1176

[andychu]

• try/finally hack TODO around this weird "caught" hack catch (std::exception) { }\n
• remove hard-coded lists
  • cppgen_pass.py: list of functions with default arguments
  • cppgen_pass.py: list of modules vs. types to disambiguate the . operator. module.Func() vs. sum_type.Variant()
  • cppgen_pass.py: _GetCastKind() has some hard-coded names
  • mycpp_main.py: ModulesToCompile() is a weird hack

[andychu]

Subsumed by #1231