Feature Request: enable outputting cxx.cc itself to the target directory

[Racinettee]

Hello

Recently I've integrated cxx into a legacy visual c++ application and had a couple of difficulties around linking. It seems that rust and also cxx use a specific version of the msvc runtime library that will cause many conflicts when you link statically - things along the lines of runtime library conflicts and _ITERATOR_DEBUG_LEVEL macros.

After trying to link to cxxbridge1.lib a couple of times using different settings what I ended up going with making my crate a cdylib and to also compile cxx.cc directly into my c++ project.

My suggestion for this issue is to add an option somewhere that will have cxx or cxx-build output cxx.cc alongside cxx.h in the target/cxxbridge output directory so that users of this project will have greater flexibility in how they integrate cxx their projects. Currently my solution isn't future proof - but if you were to accept this proposal then it would be.

[dtolnay]

I would prefer not to make this change, because what you've described sounds like a build system that does not understand C++ that depends on Rust that depends on C++, not anything specific to how cxx.cc is linked by the cxx crate. For example if one of cxx's dependencies, or a dependency of one of your other Rust dependencies, added a dependency on some piece of C++ code it needs, you'd be back to square 1. I don't believe that papering over this case in the way that you suggest would be beneficial. I know that better approaches are possible because other build systems don't appear to have difficulty with this case.

[sajattack]

@dtolnay The issue boils down to cc-rs defaulting to the MSVC /MD runtime library. Integrating cxx into a /MT static library is therefore impossible in the current state. This is configurable through the following cc-rs function, perhaps you could expose a way to allow the /MT library to be used https://docs.rs/cc/latest/cc/struct.Build.html#method.static_crt

[dtolnay]

That is not something that would need to be exposed in cxx. You need to use RUSTFLAGS='-C target-feature=+crt-static' and cc will pick it up automatically.

[sajattack]

You're right, I tried to pass that flag via .cargo/config.toml, but it failed silently when I passed it incorrectly and I thought it didn't help. Thanks for bringing it back to my attention and helping me get things working.

[ultimaweapon]

@Racinettee how did you get cxx.cc to compile using your C++ build system?

[Racinettee]

@ultimaweapon we didn't do anything special in terms of flags or manual modifications if that's what you mean. we copy cxx.cc directly into a location in our source tree where our build system (visual c++) expects to find it.

if it helps and you're also using visual c++ - we use the c++latest flag and we're on the v143 tool-chain now, but i dont imagine those matter too much, and nothing else stands out to me as particularly important to make it compile

[ultimaweapon]

Thank you for the information.