Rust code

Brave uses Rust language code, but there are some caveats.

Since 1.58 brave-core has used the upstream chromium build system for rust code. All code must be in-tree, with 3rd-party crates (libraries) vendored either in brave/third_party/rust or upstream in src/third_party/rust. We largely follow the chromium guidelines for determining acceptable use, although we have some components (SKUs, Speedreader, STAR) implemented in rust. Please contact Clifton and Brian Johnson when planning any new work involving substantial Rust code.

Instead of cargo, our build calls rustc directly. Source files, config settings, and dependencies are extracted from Cargo.toml files and an equivalent BUILD.gn is created and checked into the tree. There is a gnrt tool to automate this, but it is currently very rough and doesn't support our setup of with multiple third_party/rust trees. See the instructions for how to build and run this tool.

Updating a dependency

This is pretty painful right now. Since gnrt doesn't support downstream two-level third_party/rust tree, one approach is to copy src/brave/third_party/rust into src/third_party/rust, update third_party.toml and then run gnrt download and gnrt gen until it's happy. They copy everything back into the brave-core tree, and use git status to work split out which files belong where, and manually fix up the paths for upstream crates.

Alternatively, downloading the published crate directly and writing/updating BUILD.gn by hand can be easier.

Until we have better tooling for this, it can be a real time sink. Please ask one of the @rust-deps reviewers for help if you're not sure how this works. The #rust slack channel is also a good place to ask for help.

Review guidelines

A checklist for reviewing Rust code changes, including dependent library crates.

• Third-party dependency versions should be aligned with what's already in the upstream chromium or brave trees. If a change needs to update one of those crates, make sure there's at least an issue open for all other users to coordinate migration so we don't ship duplicate code.
• Carefully check any thread_local use. Leaf code may be called from different C++ threads in sequence, so thread-local variables won't be consistent.
• build.rs is forbidden. We don't use cargo to build Rust code, so many affordances there don't work. Any custom artefact generation would need to be ported to BUILD.gn.
• Any use of autocfg in dependencies must be patched out and replaced by config settings in BUILD.gn based on variables there. Since we don't use cargo, these checks won't work.