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71 lines
3.0 KiB
Markdown
71 lines
3.0 KiB
Markdown
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# Exercise: Interoperability with C++
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## Part one
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* In the Rust file you previously created, add a `#[cxx::bridge]` which specifies a single function,
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to be called from C++, called `hello_from_rust`, taking no parameters and returning
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no value.
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* Modify your previous `hello_from_rust` function to remove `extern "C"` and `#[no_mangle]`.
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This is now just a standard Rust function.
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* Modify your `gn` target to build these bindings.
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* In your C++ code, remove the forward-declaration of `hello_from_rust`. Instead, include
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the generated header file.
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* Build and run!
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## Part two
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It's a good idea to play with cxx a little. It helps you think about how flexible
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Rust in Chromium actually is.
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Some things to try:
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* Call back into C++ from Rust. You will need:
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* An additional header file which you can `include!` from your `cxx::bridge`.
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You'll need to declare your C++ function in that new header file.
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* An `unsafe` block to call such a function, or alternatively specify the `unsafe`
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keyword in your `#[cxx::bridge]` [as described here][0].
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* You may also need to `#include "third_party/rust/cxx/v1/crate/include/cxx.h"`
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* Pass a C++ string from C++ into Rust.
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* Pass a reference to a C++ object into Rust.
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* Intentionally get the Rust function signatures mismatched from the `#[cxx::bridge]`,
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and get used to the errors you see.
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* Intentionally get the C++ function signatures mismatched from the `#[cxx::bridge]`,
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and get used to the errors you see.
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* Pass a `std::unique_ptr` of some type from C++ into Rust, so that Rust
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can own some C++ object.
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* Create a Rust object and pass it into C++, so that C++ owns it. (Hint:
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you need a `Box`).
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* Declare some methods on a C++ type. Call them from Rust.
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* Declare some methods on a Rust type. Call them from C++.
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## Part three
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Now you understand the strengths and limitations of cxx interop, think of
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a couple of use-cases for Rust in Chromium where the interface would be
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sufficiently simple. Sketch how you might define that interface.
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## Where to find help
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* The [cxx binding reference][1]
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* The [`rust_static_library` gn template][2]
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<details>
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As students explore Part Two, they're bound to have lots of questions about how
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to achieve these things, and also how cxx works behind the scenes.
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Some of the questions you may encounter:
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* I'm seeing a problem initializing a variable of type X with type Y, where
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X and Y are both function types.
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This is because your C++ function doesn't quite match the declaration in your
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`cxx::bridge`.
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* I seem to be able to freely convert C++ references into Rust references.
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Doesn't that risk UB?
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For cxx's _opaque_ types, no, because they are zero-sized. For cxx trivial types
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yes, it's _possible_ to cause UB, although cxx's design makes it quite
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difficult to craft such an example.
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</details>
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[0]: https://cxx.rs/extern-c++.html#functions-and-member-functions
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[1]: https://cxx.rs/bindings.html
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[2]: https://source.chromium.org/chromium/chromium/src/+/main:build/rust/rust_static_library.gni;l=16
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