boost::scoped_ptr demo

[dtolnay]

Example code in response to #251 (comment).

Note this line is using cxx::kind::Opaque:

cxx/scoped_ptr_demo/src/main.rs

Line 16 in bb4fda0

type Kind = cxx::kind::Opaque;

because passing a boost::scoped_ptr by value across the FFI is not possible due to both of the following being true:

1. It uses the "small class" ABI in C++ which Rust has no representation for. Any Rust data structure will always either look like "small POD" or "big classes" as far as the ABI in C++.

2. It exposes no way to exit out of "small class" state (analogous to unique_ptr<T>::release which is what we use for making UniquePtr work by value on the language boundary).

For a different smart pointer for which one or both of these doesn't apply you'd probably use cxx::kind::Trivial because Rust's move semantics are non-problematic for anything pointer-like.

[dtolnay]

#525 shows an approach where item 1 in #524 (comment) does not apply -- it just works around "small class" by adding padding, which isn't great, but is compatible with Rust.