Lifetimes on an opaque C++ type

[dtolnay]

For example suppose we have a C++ type which stores references:

class Resource;

class Holder {
  Holder(const Resource &resource) : resource(resource) {}
  const Resource &resource;
};

std::shared_ptr<Holder> makeHolder(const Resource &resource) {
  return std::make_shared<Holder>(resource);
}

It would be most appropriate to describe this in Rust with a lifetime as:

#[cxx::bridge]
mod ffi {
    unsafe extern "C++" {
        type Resource;
        type Holder<'a>;

        fn makeHolder<'a>(resource: &'a Resource) -> SharedPtr<Holder<'a>>;

        // or with lifetime elision
        fn makeHolder(resource: &Resource) -> SharedPtr<Holder>;
    }
}

Currently CXX doesn't handle the lifetimes on an extern type:

  error[cxxbridge]: expected `=` or `:`
    ┌─ src/main.rs:5:20
    │
  5 │         type Holder<'a>;
    │                    ^ expected `=` or `:`

[dtolnay]

Hit an ICE while working on this: rust-lang/rust#80468. I think it happens only in otherwise erroneous code so it shouldn't be a blocker, but it's possible it would come up often when using this feature.