Legitimate uses for UniquePtrs even if not currently expanded

[adetaylor]

#228 (comment) gives code like this (I've changed the type names but nothing else):

use cxx::UniquePtr;

#[cxx::bridge(namespace = base::test)]
pub mod ffi {
    extern "C" {
        include!("base/test/scoped_feature_list.h");
        
        type ScopedFeatureList;
        
        fn Init(self: &mut ScopedFeatureList);
    }
}

pub struct ScopedFeatureList {
    instance: UniquePtr<ffi::ScopedFeatureList>,
}

impl ScopedFeatureList {
    pub fn init(&mut self) {
        self.instance.Init();
    }
}

This doesn't compile because:

error[E0277]: the trait bound `features::ffi::ScopedFeatureList: cxx::private::UniquePtrTarget` is not satisfied
  --> ../../base/rs/src/features.rs:15:5
   |
15 |     instance: UniquePtr<ffi::ScopedFeatureList>,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `cxx::private::UniquePtrTarget` is not implemented for `features::ffi::ScopedFeatureList`
   | 
  ::: /Users/adetaylor/dev/chromium/src/out/Debug/../../third_party/rust/crates/cxx/src/unique_ptr.rs:14:8
   |
14 |     T: UniquePtrTarget,
   |        --------------- required by this bound in `cxx::UniquePtr`

This is because expand_unique_ptr never gets called, because no methods in the ffi block take a UniquePtr<ScopedFeatureList> as a parameter.

[adetaylor]

(that last sentence may be wrong, but that's what I reckon is happening, based on hunting for a workaround!)

Maybe the fix is to assume any type can be a UniquePtrTarget. If that's your preferred fix, let me know and I can work on a PR to that effect. Then again, the same class of problem might occur for CxxVector and maybe other things.

[dtolnay]

This is because expand_unique_ptr never gets called, because no methods in the ffi block take a UniquePtr<ScopedFeatureList> as a parameter.

You guessed right -- this is exactly why.

The easiest workaround currently would be add an unused struct inside of the mod ffi which has any extra generic types you require as fields:

    struct ExtraInstantiations {
        _0: UniquePtr<ScopedFeatureList>,
    }

I had been hoping not to generate impls for every possible use of every type. Maybe doing it for just UniquePtr wouldn't be so bad. But once we start having maps, we'll need some better solution anyway: it wouldn't be good to instantiate std::map<K, V> for every possible combination of K, V.

As far as hacks go, an unused struct like that isn't the worst thing, but I am definitely open to supporting this better (with an attribute, or something else that makes the intent clearer; I really don't know).