Bug in cxxbridge-cmd? Generated incorrect namespace

[XiangpengHao]

Hi, I encountered this after upgrading the cxxbridge-cmd to 0.5.6:

// lib.rs
#[cxx::bridge(namespace = rust_part)]
mod ffi {
    extern "C++" {
        type Person;
        fn get_name(person: &Person) -> &CxxString;
    }
}

Run cxxbridge:

cxxbridge lib.rs --header
cxxbridge lib.rs

And get the header:

#pragma once
#include <string>

namespace rust_part {
  using Person = ::rust_part::Person;
}

and source:

#include <string>

namespace rust_part {
  using Person = ::rust_part::Person;
}

namespace rust_part {
extern "C" {
const ::std::string *rust_part$cxxbridge05$get_name(const ::rust_part::Person &person) noexcept {
  const ::std::string &(*get_name$)(const ::rust_part::Person &) = ::rust_part::get_name;
  return &get_name$(person);
}
} // extern "C"
} // namespace rust_part

I suppose the Person should not be in rust_part namespace, in fact (at least) in 0.5.2 it yields the following code and compiles for me.

#pragma once
#include <string>

namespace rust_part {

using Person = Person;

} // namespace rust_part

Am I missing something? This also relates to the problem discussed #434

[dtolnay]

This looks like #416. Sorry, this was not intentional. :(

For now you can work around it by writing Person without a namespace attribute, like this:

#[cxx::bridge]
mod ffi {
    extern "C++" {
        type Person;
    }

    #[namespace = "rust_part"]
    extern "C++" {
        fn get_name(person: &Person) -> &CxxString;
    }
}

or like this:

#[cxx::bridge]
mod ffi {
    extern "C++" {
        type Person;

        #[namespace = "rust_part"]
        fn get_name(person: &Person) -> &CxxString;
    }
}

[XiangpengHao]

Thanks for the reply!

But this doesn't work:

#[cxx::bridge]
mod ffi {
    #[namespace = "rust_part"]
    extern "Rust"{
        fn hello();
    }
}

fn hello() {
    println!("hello");
}

-->

#pragma once

void hello() noexcept;

I can only specify namespace on the function/type not the entire extern block, is that intended?

[dtolnay]

Ah, I forgot that this was called out as not implemented yet in #370. I put up #444 with the implementation for namespace attributes on extern blocks and will publish 0.5.8 shortly.

[XiangpengHao]

Great! Thanks for the support!

[dtolnay]

Published 0.5.8:

$ cxxbridge --version
cxxbridge 0.5.8

$ echo '#[cxx::bridge] mod ffi { #[namespace = "rust_part"] extern "Rust" { fn hello(); } }' | cxxbridge --header -
#pragma once

namespace rust_part {
void hello() noexcept;
} // namespace rust_part

[XiangpengHao]

How do you consider this case:

#[cxx::bridge]
mod ffi {
    #[namespace = "shared"]
    struct Color {
        r: u8,
        g: u8,
        b: u8
    }

    #[namespace = "rust_part"]
    extern "Rust"{
        fn is_white(self: &Color) -> bool;
    }
}

Note that the Color is in shared namespace while its member function is_white is in rust_part namespace

Probably it's better to just reject the code, but I haven't got a chance to look at the implementation.

[dtolnay]

Looks like we currently generated something like the following, which does not compile. Good catch.

namespace shared {
struct Color final {
  uint8_t r;
  uint8_t g;
  uint8_t b;

  bool is_white() const noexcept;
};
} // namespace shared

namespace rust_part {
bool Color::is_white() const noexcept {
  ...
}
} // namespace rust_part

cxxbridge/sources/demo/src/main.rs.cc:25:6: error: ‘Color’ has not been declared
   25 | bool Color::is_white() const noexcept {
      |      ^~~~~

I wouldn't be opposed to accepting that code and just making it put the member function in the struct's namespace all the time, regardless of namespace attribute on the member function.

But if someone feels strongly this should be an error (with a better message), that would be reasonable too.

[dtolnay]

I filed #460 to track the bug described in #441 (comment) of method with a different explicit namespace than the receiver type.