Writing tests with CXX code causes linker issues

[Jolyon-S]

Problem

I use CXX to define some bindings against an external C++ library (that I manage via Nix). This builds perfectly well locally, and perfectly well in CI. All good.

Then, I write an integration test in the same crate as the C++ bridge is defined. Builds fine locally, but breaks on a very strange linker error in CI:

error: linking with `cc` failed: exit status: 1

and the main error appearing below of the form:

<...> undefined reference to <...>

where these are calls to the external library from my C++ code.

build.rs for interest:

fn main() {
    let library = pkg_config::Config::new().probe("my-external-library").unwrap();
    cxx_build::bridge("src/foo.rs")
        .file("src/foo.cc")
        .flag("-std=c++14")
        .includes(library.include_paths)
        .compile("my-crate");

    println!("cargo:rerun-if-changed=src/foo.rs");
    println!("cargo:rerun-if-changed=src/foo.cc");
    println!("cargo:rerun-if-changed=include/foo.h");
}

Workaround

Disabling the test again (by commenting it out) makes the linking work again perfectly. Additionally, just moving the test to a different crate in the workspace makes it work perfectly - the test runs fine locally and in CI, and no linker issues appear.

Is this just a known issue, or is there some other thing I need to configure to make this work?