Using CXX in Chromium

In Chromium, we define an independent #[cxx::bridge] mod for each leaf-node where we want to use Rust. You’d typically have one for each rust_static_library. Just add

cxx_bindings = [ "my_rust_file.rs" ]
   # list of files containing #[cxx::bridge], not all source files
allow_unsafe = true

to your existing rust_static_library target alongside crate_root and sources.

C++ headers will be generated at a sensible location, so you can just

#include "ui/base/my_rust_file.rs.h"

You will find some utility functions in //base to convert to/from Chromium C++ types to CXX Rust types — for example SpanToRustSlice.

Students may ask — why do we still need allow_unsafe = true?

The broad answer is that no C/C++ code is “safe” by the normal Rust standards. Calling back and forth to C/C++ from Rust may do arbitrary things to memory, and compromise the safety of Rust’s own data layouts. Presence of too many unsafe keywords in C/C++ interop can harm the signal-to-noise ratio of such a keyword, and is controversial, but strictly, bringing any foreign code into a Rust binary can cause unexpected behavior from Rust’s perspective.

The narrow answer lies in the diagram at the top of this page — behind the scenes, CXX generates Rust unsafe and extern "C" functions just like we did manually in the previous section.