include! can't read header when cxx is missing from dependencies

[alexxbb]

Hi! I'm having issues with including a c++ header file and I'd appreciate your help :)

#[cxx::bridge]
mod ffi {
    extern "C" {
        include!("cpp/wrap/wrap.h");
        type Spref;
        fn new_spref(name: &str) -> UniquePtr<Spref>;
    }
}

Note the "src/cpp/wrap/wrap.h"

error: couldn't read src/cpp/wrap/wrap.h: No such file or directory (os error 2)
 --> src/lib.rs:4:9
  |
4 |         include!("cpp/wrap/wrap.h");
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |

Here's my builld.rs

fn main() {
    cxx_build::bridge("src/lib.rs")  // returns a cc::Build
        .file("cpp/wrap/wrap.cpp")
        .include("cpp/lib")
        .flag_if_supported("-std=c++14")
        .compile("cpp-wrap");

    println!("cargo:rerun-if-changed=src/lib.rs");
    println!("cargo:rerun-if-changed=cpp/wrap/wrap.h");
    println!("cargo:rerun-if-changed=cpp/wrap/wrap.cpp");
}

If I change the path to something like:

include!("../cpp/wrap/wrap.h")

I'll get a different error:

running: "c++" "-O0" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-m64" "-shared" "-I" "/home/alex/Sandbox/rust/cxx_test/target/cxxbridge" "-I" "/home/alex/Sandbox/rust/cxx_test" "-I" "cpp/lib" "-Wall" "-Wextra" "-std=c++14" "-o" "/home/alex/Sandbox/rust/cxx_test/target/debug/build/cxx_test-38d75340743cc24c/out/src/lib.rs.o" "-c" "/home/alex/Sandbox/rust/cxx_test/target/debug/build/cxx_test-38d75340743cc24c/out/src/lib.rs.cc"
cargo:warning=/home/alex/Sandbox/rust/cxx_test/target/debug/build/cxx_test-38d75340743cc24c/out/src/lib.rs.cc:1:10: fatal error: ../cpp/wrap/wrap.h: No such file or directory
cargo:warning= #include "../cpp/wrap/wrap.h"
cargo:warning=          ^~~~~~~~~~~~~~~~~~~~
cargo:warning=compilation terminated.
exit code: 1

Thx,
Alex

[dtolnay]

I think this means cxx::bridge is not being invoked. Does your Cargo.toml have cxx under [dependencies] and does it have edition = "2018" under [package]?

[alexxbb]

Oh you're right, I only added cxx-build but totally forgot to add cxx itself. It's now compiled. Thanks @dtolnay

[alexxbb]

I stumbled upon another related issue:

I created a new cargo bin project alongside the cxx_test (which is a lib crate)
Cargo.toml:

[dependencies]
cxx_test = {path = ".."}

Building this gives me an error:

running: "c++" "-O0" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-m64" "-shared" "-I" "/home/alex/Sandbox/rust/cxx_test/consume/target/cxxbridge" "-I" "/home/alex/Sandbox/rust/cxx_test/consume" "-I" "cpp/lib" "-Wall" "-Wextra" "-std=c++14" "-lcxxlib" "-o" "/home/alex/Sandbox/rust/cxx_test/consume/target/debug/build/cxx_test-b3a77f1cdae677d0/out/src/lib.rs.o" "-c" "/home/alex/Sandbox/rust/cxx_test/consume/target/debug/build/cxx_test-b3a77f1cdae677d0/out/src/lib.rs.cc"
cargo:warning=/home/alex/Sandbox/rust/cxx_test/consume/target/debug/build/cxx_test-b3a77f1cdae677d0/out/src/lib.rs.cc:1:10: fatal error: cpp/wrap/wrap.h: No such file or directory
cargo:warning= #include "cpp/wrap/wrap.h"
cargo:warning=          ^~~~~~~~~~~~~~~~~
cargo:warning=compilation terminated.
exit code: 1

I guess this has something to do with include! uses a relative path of the cxx_test crate. Is there a way to fix it?

[alexxbb]

Adding .include(".") to the build script fixes the problem.
I wish there was more examples and explanation of how things work. In particular it' be very helpful to see an example of how to write a simple binding to a c++ shared library.

[dtolnay]

Glad you got it working! As you noticed, the Cargo workflow is not well trodden yet. So far I have mostly been developing using Bazel, and using Buck at work. We are looking at making the Cargo workflow more robust in #214 and of course documenting it better in #179.