Rust third-party code

This directory contains third-party Rust libraries from https://crates.io (and sometimes thin wrappers around them for Chromium interop).

gnrt: Cargo.toml => vendor + BUILD.gn + README.chromium

Third-party libraries from crates.io, depend on Cargo as a build system. Chromium uses gnrt tool to automate

• Vendoring crates sources into Chromium repository
• Generating BUILD.gn and README.chromium files based on crate's Cargo.toml.

The tool can be found in //tools/crates/gnrt and is typically invoked using //tools/crates/run_gnrt.py.

Directory structure for third-party crates

The directory structure for a crate “foo” version 3.4.2 is:

//third_party/
    rust/
        foo/
            wrapper/             (optional FFI glue or Chromium integration)
            v3/                  (version 3.4.2 maps to the v3 epoch)
                BUILD.gn         (generated by gnrt gen)
                README.chromium  (generated by gnrt vendor)
        chromium_crates_io/
            vendor/
                foo-v3/  (sources downloaded from crates.io by gnrt vendor)
            patches/
                foo-v3/  (patches automatically applied during gnrt vendor)
                    0001-Some-changes.diff
                    0002-Other-changes.diff
            Cargo.toml        (version and enabled features of "foo")
            Cargo.lock
            gnrt_config.toml  (additional configuration of "foo")

TODO(https://crbug.com/417250983): Patch directory should be epoch-specific.

Most of the files above are managed automatically by gnrt. The only “foo”-specific manually-authored files and directories are:

• //third_party/rust/Cargo.toml
• //third_party/rust/gnrt_config.toml
• //third_party/rust/chromium_crates_io/patches/ (see also //third_party/rust/chromium_crates_io/patches/README.md)
• `//third_party/rust/foo/wrapper

There are also other, non-crate-specific files such as:

• //third_party/rust:
  • .md - this doc and other docs
  • OWNERS, PRESUBMIT.py
• //third_party/rust/chromium_crates_io:
  • BUILD.gn.hbs - template for BUILD.gn files
  • README.chromium.hbs - template for README.chromium files
  • PRESUBMIT.py
  • .py helper scripts used by PRESUBMIT.py and by //tools/crates.
  • Somewhat obsolete cargo vet-support items - we keep supply-chain/audits.toml to preserve Chromium audits that are imported into https://github.com/google/rust-crate-audits/blob/main/sources.list)
  • Various infrastructure pieces to disable auto-formatting of the vendored files (e.g. .gitattributes, .rustfmt.toml, .style.yapf, etc.)

Importing new third-party crates

See //third_party/rust/README-importing-new-crates.md for instructions on how to import a crate from https://crates.io into Chromium.

Updating existing third-party crates

Third-party crates will get updated semi-automatically through the process described in ../tools/crates/create_update_cl.md. If you nevertheless need to manually update a crate to its latest minor or major version, then follow the steps below. To facilitate easier review, we recommend uploading separate patchsets for 1) manual changes, and 2) tool-driven, automated changes.

1. Change directory to the root src/ dir of Chromium.
2. Update the versions in //third_party/rust/chromium_crates_io/Cargo.toml.
   • vpython3 ./tools/crates/run_gnrt.py update <crate name>.
   • Under the hood this invokes cargo update and accepts the same command line parameters. In particular, you may need to specify --breaking when working on major version updates.
3. Download any updated crate's files:
   • ./tools/crates/run_gnrt.py vendor
4. Add the downloaded files to git:
   • git add -f third_party/rust/chromium_crates_io/vendor
   • The -f is important, as files may be skipped otherwise from a .gitignore inside the crate.
5. Generate the BUILD.gn files
   • vpython3 ./tools/crates/run_gnrt.py gen
   • Or, directly through (nightly) cargo: cargo run --release --manifest-path tools/crates/gnrt/Cargo.toml --target-dir out/gnrt gen
6. Add the generated files to git:
   • git add third_party/rust

Writing a wrapper for binding generation

Most Rust libraries will need a more C++-friendly API written on top of them in order to generate C++ bindings to them. The wrapper library can be placed in //third_party/rust/<cratename>/wrapper or at another single place that all C++ goes through to access the library. The CXX is used to generate bindings between C++ and Rust.

See //third_party/rust/serde_json_lenient/v0_1/wrapper/ and //components/qr_code_generator for examples.

See //docs/rust-ffi.md for information on C++/Rust FFI.