Extensible "builtin" types for adding custom type interfaces

[rinon]

As far as I can tell, the only types allowed to cross the ffi boundary by value are the builtin types or new shared types? This makes perfect sense from a safety perspective, but limits the usability of existing custom C++ ADTs across the boundary.

Would it be possible to make an extensible interface to define how new, custom C++ types should be handled? For example, some codebases have their own implementation of ref-counted pointers, which I don't currently see a good way to expose through cxx (although I may just be missing something). It would be nice to allow users to implement their own "builtin" types, perhaps by implementing a trait defining the new type, then providing C++ and Rust interfaces to it. I'm not familiar enough with the cxx codebase (yet) to know what the interface for extensions might have to look like, but I wanted to check and see if this was worth looking into further.

If a sufficiently generic type extension interface existed, it might be useful to rewrite the current builtin types to fit this interface and thus extract their implementation from the core of the code generators?

Sorry if this has been discussed elsewhere, I looked and didn't see anything.

[dtolnay]

Extensibility has come up before in #16 (comment) but I think we're not quite in a position to tackle it yet. I agree that we want to though.

For now, two pieces of feature work that might help partially are:

• Allow raw pointers in extern fn signature #164, which would work for pointer-sized custom smart pointers.
• Multiple FFI blocks referencing types from one another #99, which I am currently working on; the approach in Multiple FFI blocks referencing types from one another #99 (comment) gives a way to unsafely claim that a local Rust type (whether handwritten or bindgen-generated) matches the ABI of some particular C++ type and will then be able to be passed by value.

[rinon]

Makes sense, thanks. I might be able to take a pass at #164, as I'd like to get more familiar with the codebase, but I can't promise how soon. I'm pretty slammed with other stuff for the moment.