autocxx-reduce may need to try to build the resulting code

[adetaylor]

Sometimes problems show up only when building the resulting C++ code (e.g. #489, #490, #491). It would be good to be able to minimize the test cases based on C++ compilation results. We should probably make it possible to build the Rust code too.

[adetaylor]

This turns out to be annoyingly hard, because the reduction process wants to work on a single preprocessed file, yet the .cxx files generated by autocxx try to #include a bunch of headers (including transitively via cxx.h). They obviously conflict.

[adetaylor]

Here are the things which need to be done here:

• Clean up test_gen a bit (Use generate_all! in s2 example. #497). Unrelated in theory, but may help with subsequent changes.
• Add a reduction test (in Attempt to build .rs and .cxx code in reduction. #495 but doesn't work yet).
• Stick include guards around the preprocessed header, before passing it into creduce. Surround with include guards #516 . This is because when we compile gen0.cc as produced by cxx_gen, it includes the preprocessed header via two routes:
  • Via a direct include! directive which we put into the cxx::bridge
  • Via the include! directive for autocxxgen.h which in turn #includes the preprocessed header.
    (The code in Attempt to build .rs and .cxx code in reduction. #495 does this using an --omit-includes autocxx_gen flag, but include guards would probably be better).
• When generating the preprocessed header in engine/src/lib.rs, before preprocessing, append #includes for all system headers which cxx ever includes in its generated C++ code. There's no current API to get that list out of cxx_gen so we will either need to add it, or hard-code it. The idea is that the preprocessed header is fully hermetic and we need never refer to local headers during the reduction process. Include all in preprocess #508.
• When reducing, make fake blank headers for each of those headers.
• Add an optional precompilation step to the interestingness test, triggered by command-line argument to autocxx_reduce.
  • Allow override of the clang used, using the CLANG_PATH code which already exists
• Add an optional postcompilation step to the interestingness test, triggered by command-line argument to autocxx_reduce.
• Possibly: In autocxxgen.h, as generated by engine/src/conversion/codegen_cpp/mod.rs, #include the preprocessed header before any additional headers required by the generated C++ (for example string, memory, cxx.h). Specifically, we need to include the preprocessed header before including cxx.h so that the preprocessed header continues to define symbols like CXXBRIDGE1_RUST_STRING. That way, when the compiler reads cxx.h it won't come across duplicate definitions of the cxx types. I'm not sure this is actually necessary any more.