Returning a std::vector from C++ fails with STL debug mode enabled

[mlaveaux]

It seems that enabling various STL debug features that change the ABI of standard data types might break accessing these types from the Rust side. Perhaps this is to be expected, but it did caught me by surprise. I used gcc to reproduce the issue, using the latest cxx version 1.0.191.

Here is a simple example that introduces a obtain_vector function, which returns a vector from the C++ side to the Rust side (via a UniquePtr<CxxVector<usize>> construction). Furthermore, it prints the values contained the in the vector on both sides. When running this example with the build.define calls in build.rs commented out we see the results I expect:

contains: 1
Obtained [1]

However, when running it with the various _LIBCPP_DEBUG_XXX and _GLIBCXX_DEBUG defines enabled I instead see the following output.

contains: 1
Obtained []

This also does not cause any segfaults or errors reported by AddressSanitizer.

cxx_bug.zip

[dtolnay]

Relevant documentation: https://gcc.gnu.org/onlinedocs/libstdc++/manual/debug_mode_using.html

"Note that this flag changes the sizes and behavior of standard class templates such as std::vector, and therefore you can only link code compiled with debug mode and code compiled without debug mode if no instantiation of a container is passed between the two translation units."

In your case your build consists of more than one C++ translation unit, and they do pass container instantiations.

You would want to set _GLIBCXX_DEBUG globally in a way that applies to all C++ compilation in the build, not only obtain_vector().

[mlaveaux]

Granted, I don't quite understand how cxx builds the ffi bindings internally with cc, but in the minimal example I am only using a single cxx_build::bridges with the relevant defines provided. Is there a way to see which compiler invocations are being used during the build? Setting cargo_debug does not quite work, but It only seems to produce a single object file. So I am not quite sure where (and how) these defines should be added globally.