Is there currently a way to do conditional compilation?

[laptou]

I want to bind to some C++ methods which only exist on Android. However, if I try to do something like this:

        #[cfg(target_os = "android")]
        type CallInvokerHolder;

        #[cfg(target_os = "android")]
        pub fn getCallInvoker(self: Pin<&mut CallInvokerHolder>) -> SharedPtr<CallInvoker>;

I get the following error:

  error[cxxbridge]: unsupported attribute
      ┌─ src/ffi.rs:326:5
      │
  326 │     #[cfg(target_os = "android")]
      │     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unsupported attribute

Is there a workaround?

[dtolnay]

This is definitely something we can support, but it's not implemented yet.

Basically in Cargo-based builds cxx-build would need to examine CARGO_CFG_TARGET_OS in the build script's environment to decide whether or not to write those types/functions into the generated C++ code, with no other action required on the user's part other than writing cfg(...) inside their cxx::bridge module; and Bazel/Buck/CMake/etc would need the build system to pass something like --cfg=target_os=android / --cfg=target_os=linux as part of their cxxbridge invocation.

For now the easiest workaround would be to move the android-specific signatures to a different cxx::bridge module in a different file, and gate that whole file with #[cfg(target_os = "android")].

Something like:

// lib.rs

#[cfg(target_os = "android")]
mod android;

// android.rs

#[cxx::bridge]
mod ffi {
    extern "C++" {
        type CallInvokerHolder;
        fn getCallInvoker(self: Pin<&mut CallInvokerHolder>) -> SharedPtr<CallInvoker>;
        ...
    }
}

// build.rs

let mut bridge_modules = vec!["src/lib.rs"];
if std::env::var_os("CARGO_CFG_TARGET_OS").unwrap() == "android" {
    bridge_modules.push("src/android.rs");
}
cxx_build::bridges(bridge_modules)
    ...
    .compile(...);

[laptou]

Thanks for the tip! This is exactly what I ended up doing.

[dtolnay]

I published an implementation of #[cfg(...)] conditional compilation in cxx 1.0.63. In Cargo-based builds it should just work. In non-Cargo builds you'll need your build system to pass --cfg flags to the cxxbridge CLI just like you'd pass to rustc.

[lukaz-vaultree-com]

Just bumped my cxx version (build and non build versions) to 1.0.63. Then doing:

#[allow(dead_code)]
#[cxx::bridge]
pub(crate) mod ffi {
       #[cfg(debug_assertions)]
       unsafe extern "C++" {
            unsafe fn do_something(
                context: SharedPtr<Context>,
                buffer: *const u8,
                size: usize,
        ) -> bool;
       }

gives undefined reference error on

e::ffi::do_something':
          /workspaces/something/src/bridge.rs:108: undefined reference to `cxxbridge1$do_something'
          collect2: error: ld returned 1 exit status

help: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
  = note: use the `-l` flag to specify native libraries to link

Edit: simply taking out #[cfg(debug_assertions)] makes it work

[dtolnay]

There are a few special cfgs that Cargo conceals from build scripts. I added a workaround for that one in cxx-build 1.0.64.