Panic 'no entry found for key' when translating mruby 2.0.0 about c2rust

[holmesmr]

Ah, actually, is this covered by this limitation notice in the wiki?

[take-cheeze]

There is too many features used in mruby that is in the known limitation:

• longjmp/setjmp
• bitfields
• variadic function definitions
• C macros

[holmesmr]

Apologies for not reading thoroughly before opening this issue. Will close.

[rinon]

Wanted to see what was going on here so I dug a bit deeper. This failure is actually not one of the limitations mentioned above, it is due to a non-C99 gcc extension, Labels as Values. Ruby can use a threaded interpreter dispatch style that stores goto labels in an array and dispatches directly to one of these labels after each opcode is handled. Rust does not have an analogous feature, so we have no plans to support this extension.

However, I was able to disable the feature in mruby with the following build_config.rb:

MRuby::Build.new do |conf|
  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
    toolchain :visualcpp
  else
    toolchain :gcc
  end

  conf.cc do |cc|
    cc.defines = %w(MRB_DISABLE_DIRECT_THREADING)
  end
end

Note that while this allowed the project to be fully translated, varargs functions are not (yet) supported. mrb_get_args, mrb_format, mrb_raisef, etc. will need to either be compiled as C code and statically linked into the rust code or re-implemented manually. There are also some warnings about potential flexible array member struct fields. At first glance these appear to be false positive warnings, but I may have missed a real problem in there.