Comparing Chromium and Cargo Ecosystems

The Rust community typically uses cargo and libraries from crates.io. Chromium is built using gn and ninja and a curated set of dependencies.

When writing code in Rust, your choices are:

• Use gn and ninja with the help of the templates from //build/rust/*.gni (e.g. rust_static_library that we’ll meet later). This uses Chromium’s audited toolchain and crates.
• Use cargo, but restrict yourself to Chromium’s audited toolchain and crates
• Use cargo, trusting a toolchain and/or crates downloaded from the internet

From here on we’ll be focusing on gn and ninja, because this is how Rust code can be built into the Chromium browser. At the same time, Cargo is an important part of the Rust ecosystem and you should keep it in your toolbox.

Mini exercise

Split into small groups and:

• Brainstorm scenarios where cargo may offer an advantage and assess the risk profile of these scenarios.
• Discuss which tools, libraries, and groups of people need to be trusted when using gn and ninja, offline cargo, etc.

Ask students to avoid peeking at the speaker notes before completing the exercise. Assuming folks taking the course are physically together, ask them to discuss in small groups of 3-4 people.

Notes/hints related to the first part of the exercise (“scenarios where Cargo may offer an advantage”):

• It’s fantastic that when writing a tool, or prototyping a part of Chromium, one has access to the rich ecosystem of crates.io libraries. There is a crate for almost anything and they are usually quite pleasant to use. (clap for command-line parsing, serde for serializing/deserializing to/from various formats, itertools for working with iterators, etc.).

  • cargo makes it easy to try a library (just add a single line to Cargo.toml and start writing code)
  • It may be worth comparing how CPAN helped make perl a popular choice. Or comparing with python + pip.

• Development experience is made really nice not only by core Rust tools (e.g. using rustup to switch to a different rustc version when testing a crate that needs to work on nightly, current stable, and older stable) but also by an ecosystem of third-party tools (e.g. Mozilla provides cargo vet for streamlining and sharing security audits; criterion crate gives a streamlined way to run benchmarks).

  • cargo makes it easy to add a tool via cargo install --locked cargo-vet.
  • It may be worth comparing with Chrome Extensions or VScode extensions.

• Broad, generic examples of projects where cargo may be the right choice:

  • Perhaps surprisingly, Rust is becoming increasingly popular in the industry for writing command line tools. The breadth and ergonomics of libraries is comparable to Python, while being more robust (thanks to the rich typesystem) and running faster (as a compiled, rather than interpreted language).
  • Participating in the Rust ecosystem requires using standard Rust tools like Cargo. Libraries that want to get external contributions, and want to be used outside of Chromium (e.g. in Bazel or Android/Soong build environments) should probably use Cargo.

• Examples of Chromium-related projects that are cargo-based:

  • serde_json_lenient (experimented with in other parts of Google which resulted in PRs with performance improvements)
  • Fontations libraries like font-types
  • gnrt tool (we will meet it later in the course) which depends on clap for command-line parsing and on toml for configuration files.
    • Disclaimer: a unique reason for using cargo was unavailability of gn when building and bootstrapping Rust standard library when building Rust toolchain.
    • run_gnrt.py uses Chromium’s copy of cargo and rustc. gnrt depends on third-party libraries downloaded from the internet, but run_gnrt.py asks cargo that only --locked content is allowed via Cargo.lock.)

Students may identify the following items as being implicitly or explicitly trusted:

• rustc (the Rust compiler) which in turn depends on the LLVM libraries, the Clang compiler, the rustc sources (fetched from GitHub, reviewed by Rust compiler team), binary Rust compiler downloaded for bootstrapping
• rustup (it may be worth pointing out that rustup is developed under the umbrella of the https://github.com/rust-lang/ organization - same as rustc)
• cargo, rustfmt, etc.
• Various internal infrastructure (bots that build rustc, system for distributing the prebuilt toolchain to Chromium engineers, etc.)
• Cargo tools like cargo audit, cargo vet, etc.
• Rust libraries vendored into //third_party/rust (audited by security@chromium.org)
• Other Rust libraries (some niche, some quite popular and commonly used)