Exercise: Interoperability with C++

Part one

In the Rust file you previously created, add a #[cxx::bridge] which specifies a single function, to be called from C++, called hello_from_rust, taking no parameters and returning no value.
Modify your previous hello_from_rust function to remove extern "C" and #[unsafe(no_mangle)]. This is now just a standard Rust function.
Modify your gn target to build these bindings.
In your C++ code, remove the forward-declaration of hello_from_rust. Instead, include the generated header file.
Build and run!

Part two

It's a good idea to play with CXX a little. It helps you think about how flexible Rust in Chromium actually is.

Some things to try:

Call back into C++ from Rust. You will need:
- An additional header file which you can include! from your cxx::bridge. You'll need to declare your C++ function in that new header file.
- An unsafe block to call such a function, or alternatively specify the unsafe keyword in your #[cxx::bridge] as described here.
- You may also need to #include "third_party/rust/cxx/v1/crate/include/cxx.h"
Pass a C++ string from C++ into Rust.
Pass a reference to a C++ object into Rust.
Intentionally get the Rust function signatures mismatched from the #[cxx::bridge], and get used to the errors you see.
Intentionally get the C++ function signatures mismatched from the #[cxx::bridge], and get used to the errors you see.
Pass a std::unique_ptr of some type from C++ into Rust, so that Rust can own some C++ object.
Create a Rust object and pass it into C++, so that C++ owns it. (Hint: you need a Box).
Declare some methods on a C++ type. Call them from Rust.
Declare some methods on a Rust type. Call them from C++.

Part three

Now you understand the strengths and limitations of CXX interop, think of a couple of use-cases for Rust in Chromium where the interface would be sufficiently simple. Sketch how you might define that interface.

Where to find help

The cxx binding reference
The rust_static_library gn template

<details> As students explore Part Two, they're bound to have lots of questions about how to achieve these things, and also how CXX works behind the scenes.

Some of the questions you may encounter:

I'm seeing a problem initializing a variable of type X with type Y, where X and Y are both function types. This is because your C++ function doesn't quite match the declaration in your cxx::bridge.
I seem to be able to freely convert C++ references into Rust references. Doesn't that risk UB? For CXX's opaque types, no, because they are zero-sized. For CXX trivial types yes, it's possible to cause UB, although CXX's design makes it quite difficult to craft such an example.

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