How to check that Rust type size is equal to C++ type size.

[noy1993]

I have a question: If the size of types in C++ and Rust are different, the code might still compile, but it would be very unsafe. Is there a way to make the compiler throw an error when the sizes of C++ and Rust types do not match?

For example, with the following code, a Rust Foo struct could be directly transmuted into a C++ Foo.

rust code:

struct Foo{
    v:f64
    v2:f64
}

unsafe impl ExternType for Foo {
    type Id = cxx::type_id!("Foo");
    type Kind = cxx::kind::Trivial;
}

c++ code:

class Foo{
public:
    double v;
};

[dtolnay]

Yes, the best way would be to run bindgen (https://github.com/rust-lang/rust-bindgen) on the C++ header at build time to get a correctly sized Rust representation of the C++ class. Then either use that generated Rust struct as the type that you implement cxx::ExternType on:

mod bindgen {
    include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
}

unsafe impl ExternType for bindgen::Foo {
    type Id = cxx::type_id!("Foo");
    type Kind = cxx::kind::Trivial;
}

or keep your own type and add a const assertion that checks the layout against the bindgen-generated one.

mod bindgen {
    include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
}

#[repr(C)]
pub struct Foo {
    v: f64,
    v2: f64,
}

unsafe impl ExternType for Foo {
    type Id = cxx::type_id!("Foo");
    type Kind = cxx::kind::Trivial;
}

const _: () = const {
    assert!(std::mem::size_of::<Foo>() == std::mem::size_of::<bindgen::Foo>());
    assert!(std::mem::align_of::<Foo>() == std::mem::align_of::<bindgen::Foo>());
};