Use shared type from another bridge module

[mikhailzagurskiy]

I tried to use a Shared type from one module inside another module, but have a quite strange error message:

error: `extern` block uses type `std::string::String`, which is not FFI-safe
  --> linkage-test/src/two.rs:12:31
   |
12 |     fn IClass2_create(params: &CommonParams) -> SharedPtr<IClass2>;
   |                               ^^^^^^^^^^^^^ not FFI-safe
   |
note: the lint level is defined here
  --> linkage-test/src/two.rs:1:1
   |
1  | #[cxx::bridge]
   | ^^^^^^^^^^^^^^
   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
   = note: this struct has unspecified layout
   = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

Here an MVP for this bug. CXX version 1.0.34.

The main.rs

pub mod one;
pub mod two;

#[cxx::bridge]
mod ffi {}

fn main() {
    let a = one::ffi::CommonParams {
        count: 123,
        name: String::from("somename"),
    };

    let b = one::ffi::IClass1_create(&a);
    println!("B is not empty: {}", b.get_name());

    let c = two::ffi::IClass2_create(&a);
    println!("C is not empty: {}", c.get_name());
}

The one.rs

#[cxx::bridge]
pub mod ffi {

  #[derive(Debug, Default)]
  pub struct CommonParams {
    pub count: u32,
    pub name: String,
  }

  unsafe extern "C++" {
    include!("linkage-test/wrap/wrapper.hpp");
    type IClass1;
    fn IClass1_create(params: &CommonParams) -> SharedPtr<IClass1>;
    fn get_name(&self) -> String;
  }
}

The two.rs

#[cxx::bridge]
pub mod ffi {
  unsafe extern "C++" {
    include!("linkage-test/wrap/one.rs.hpp");
    type CommonParams = crate::one::ffi::CommonParams;
  }

  unsafe extern "C++" {
    include!("linkage-test/wrap/wrapper.hpp");

    type IClass2;
    fn IClass2_create(params: &CommonParams) -> SharedPtr<IClass2>;
    fn get_name(self: &IClass2) -> String;
  }
}

The wrapper.hpp

#pragma once

#include <rust/cxx.h>

#include <memory>

struct IClass1 {
  int64_t count;
  std::string name;

  rust::String get_name() const;
};

struct IClass2 {
  int64_t count;
  std::string name;

  rust::String get_name() const;
};

struct CommonParams;

std::shared_ptr<IClass1> IClass1_create(const CommonParams &params);

std::shared_ptr<IClass2> IClass2_create(const CommonParams &params);

I omit wrapper.cpp to simplify the example. Nothing interesting there.

The build.rs

fn main() {
  cxx_build::bridges(vec!["src/main.rs", "src/one.rs", "src/two.rs"])
    .file("wrap/wrapper.cpp")
    .flag_if_supported("-std=c++17")
    .compile("test-link");

  println!("cargo:rustc-link-lib=lprsdk");
  println!("cargo:rerun-if-changed=src/main.rs");
  println!("cargo:rerun-if-changed=wrap/wrapper.cpp");
  println!("cargo:rerun-if-changed=wrap/wrapper.hpp");
}

You can see, that I can use the IClass1_create function properly, but can't use IClass2_create with the same argument.
Also, as you know, I can't use #[repr(...)] for shared types inside the #[cxx::bridge].

What should I do to prevent this error? Is it correct behavior? How can I fix or, maybe, workaround this?

P. S.

For now, I'm tried to defined the same Shared type CommonParam in the both modules, but it's cause compile failure, because he thinks, that there are two different types, and he can't handle the type as the argument of the function.

[mikhailzagurskiy]

You notified here how to define the type from other module, but it doesn't work

[dtolnay]

Thanks, I was able to reproduce this from your minimal repro. I've published a fix in cxx 1.0.37.