Rust code

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

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.