When a C++ library enables Crubit, that library can be used directly from Rust. This page documents roughly what that entails, and additional subpages (available in the left-hand navigation) document specific aspects of the generated bindings.
Tip: The code examples below are pulled straight from https://github.com/google/crubit/tree/main/examples/cpp/function/. The other examples in https://github.com/google/crubit/tree/main/examples/cpp/ are also useful. If you prefer just copy-pasting something, start there.
Crubit allows you to call some C++ interfaces from Rust. It supports functions, classes and structs, and enums. Crubit does not support advanced features like templates or virtual inheritance.
The rest of this document goes over how to create a C++ library that can be called from Rust, and how to actually call it from Rust. The quick summary is:
Define a rust_api_from_cpp target in the same BUILD file as your cc_library.
Add the generated .hint target (e.g. :<name_of_rust_target>.hint) to the aspect_hints of your cc_library.
Depend on the rust_api_from_cpp target in the deps of your Rust target.
The bindings can be previewed using the following command:
```sh $ bazel build --config=crubit-genfiles //path/to:target ```
cc_library target {#cc_library}The first part of creating a library that can be used by Crubit is to write a cc_library target. For example:
{{ #include ../../examples/cpp/function/example.h }}
If you write a BUILD target as normal, it will not actually get Crubit bindings, but we'll start from there:
{{ #include ../../examples/cpp/function/BUILD }}
To enable Crubit on a C++ target, you must define a rust_api_from_cpp target associated with it, and link them using aspect_hints.
Behind the scenes, Rust APIs are generated via Bazel aspects which run on the cc_library target. When examining a cc_library, Rust API generation looks for the aspect_hints so that it can find the corresponding rust_api_from_cpp target.
Define a rust_api_from_cpp target in the same BUILD file as your cc_library, and add its .hint target to the aspect_hints of the cc_library:
{{ #include ../../examples/cpp/function/BUILD }}
{{ #include ../../examples/cpp/function/BUILD }}
The .hint target is automatically created by the rust_api_from_cpp macro (named <name>.hint) and is used to avoid circular dependencies between the C++ library and the generated Rust API.
Note that having Rust callers does constrain library evolution. Certain changes cannot be made in C++ without breaking Rust callers, unless care is taken. crubit.rs/cpp/cookbook#compatibility
To examine the generated C++ bindings for the target, you can run the following command:
$ bazel build --config=crubit-genfiles //examples/cpp/function:example_lib_broken
This is the best way to preview the generated bindings for a given C++ target right now. You might end up using this a lot, so keep it in your shell history.
If you run the above command, you should see some output like the following:
Aspect //rs_bindings_from_cc/bazel_support:rust_bindings_from_cc_aspect.bzl%rust_bindings_from_cc_aspect of //examples/cpp/function:example_lib up-to-date: bazel-bin/examples/cpp/function/example_lib_rust_api_impl.cc bazel-bin/examples/cpp/function/example_lib_rust_api.rs bazel-bin/examples/cpp/function/example_lib_namespaces.json
These files are the generated bindings which are used under the hood when depending on a C++ target from Rust. They consist of:
.cc file.).rs file.).json file.)You don't need to check them in, as they are regenerated automatically whenever you build a Rust build target which depends on C++.
The .rs file is the interesting one for end users. It should contain an actually useful API for the target:
{{ #include ../../examples/cpp/function/example_generated.rs }}
To depend on a C++ library from Rust, add the corresponding rust_api_from_cpp target to your Rust target's deps:
{{ #include ../../examples/cpp/function/BUILD }}
At that point, the bindings are directly usable from Rust. The interface is identical to the .rs file previewed earlier, but can be used directly:
{{ #include ../../examples/cpp/function/main.rs }}
See crubit.rs/errors
Some features are either unsupported, or else only supported with experimental feature flags . In order to get bindings for a C++ interface, that interface must only use the subset of features currently supported.
The way to work around this kind of problem, in all cases, is to wrap or hide the problematic interface behind an interface Crubit can handle:
std::set<T> is not supported, but a struct which wraps a set::set<T> is. crubit.rs/errors/unsupported_type describes the process in more detail.inline_cpp! to allow embedding arbitrary C++ code within a Rust target using the rust_library_with_embedded_cpp Bazel rule.[^aspects]: Crubit is an aspect: an automatically generated entity that exists on every build target. It is disabled by default, so that Rust callers don‘t accidentally impose on C++ libraries that weren’t expecting them.
Aspects allow Crubit to fully understand the dependency graph: the bindings for X are in the Crubit aspect of X. This allows Crubit to generate bindings which themselves rely on bindings: if a function in target `A` returns a struct from target `B`, we know that the bindings for `A` will depend on the bindings for `B`. Because Crubit is an aspect, it already knows the name of the bindings for `B`: it's simply the Crubit aspect on `B`! Without aspects, or something like aspects, you would need to write down, for every library, the location of its Rust bindings. There is no need for that kind of boilerplate when aspects are involved, and that is why most things shaped like Crubit use aspects. For example, protocol buffers use aspects for their generated implementations in multiple languages. (They *also* use named rules, but the rules simply re-export the aspect, and the underlying aspect is what is used within the rule for referring to transitive dependencies.) Thanks to aspects, the `proto_library` doesn't need to re-specify "ah, and the Go proto is named `'x'`". Be not afraid! Aspects are what make transitive dependencies work seamlessly, without boilerplate. So when you see aspect this, or aspect that, remember: this is a Good Thing.