docs/rust.md

# Rust in Chromium

[TOC]

# Why?

Handling untrustworthy data in non-trivial ways is a major source of security
bugs, and it's therefore against Chromium's security policies
[to do it in the Browser or Gpu process](../docs/security/rule-of-2.md) unless
you are working in a memory-safe language.

Rust provides a cross-platform memory-safe language so that all platforms can
handle untrustworthy data directly from a privileged process, without the
performance overheads and complexity of a utility process.

# Status

The Rust toolchain is enabled for and supports all platforms and development
environments that are supported by the Chromium project. The first milestone
to include full production-ready support was M119.

Rust is approved by Chrome ATLs for production use in
[certain third-party scenarios](../docs/adding_to_third_party.md#Rust).

For questions or help, reach out to `rust-dev@chromium.org` or `#rust` on the
[Chromium Slack](https://www.chromium.org/developers/slack/).

If you use VSCode, we have [additional advice below](#using-vscode).

# Adding a third-party Rust library

Third-party libraries are pulled from [crates.io](https://crates.io), but
Chromium does not use Cargo as a build system.

## Third-party review

All third-party crates need to go through third-party review. See
[//docs/adding_to_third_party.md](adding_to_third_party.md) for instructions on
how to have a library reviewed.

## Importing a crate from crates.io

The `//third_party/rust/chromium_crates_io/Cargo.toml` file defines the set of crates
depended on from first-party code. Any transitive dependencies will be found
from those listed there. The file is a [standard `Cargo.toml` file](
https://doc.rust-lang.org/cargo/reference/manifest.html), though the crate
itself is never built, it is only used to collect dependencies through the
`[dependencies]` section.

To use a third-party crate "bar" version 3 from first party code:
1. Change directory to the root `src/` dir of Chromium.
1. Add the crate to `//third_party/rust/chromium_crates_io/Cargo.toml`:
   * `vpython3 ./tools/crates/run_gnrt.py add foo` to add the latest version of `foo`.
   * `vpython3 ./tools/crates/run_gnrt.py add foo@1.2.3` to add a specific version of `foo`.
   * Or, directly through (nightly) cargo:
     `cargo run --release --manifest-path tools/crates/gnrt/Cargo.toml --target-dir out/gnrt add foo`
   * Or, edit the Cargo.toml by hand, finding the version you want from [crates.io](https://crates.io).
1. Download the crate's files:
   * `./tools/crates/run_gnrt.py vendor` to download the new crate.
   * Or, directly through (nightly) cargo:
     `cargo run --release --manifest-path tools/crates/gnrt/Cargo.toml --target-dir out/gnrt vendor`
   * This will also apply any patches in `//third_party/rust/chromium_crates_io/patches`
     for the crates. If a patch can not apply, the crate's download will be cancelled and
     an error will be printed. See [patching errors](#patching-errors) below for how to resolve
     this.
1. Add the new 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.
1. If a crate in `//third_party/rust/chromium_crates_io/patches` was updated
   as part of vendoring, then reapply patches to it:
   * Go to the `//third_party/rust/chromium_crates_io` directory.
   * `./apply_patches.sh` (this currently requires linux).
1. (optional) If the crate is only to be used by tests and tooling, then
   specify the `"test"` group in `//third_party/rust/chromium_crates_io/gnrt_config.toml`:
   ```
   [crate.foo]
   group = "test"
   ```
1. Generate the `BUILD.gn` file for the new crate:
   * `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`
1. Upload the CL, mark any `unsafe` usage with `TODO` code review comments,
   and include a link to it in the request for third-party and security review.

### Cargo features

To enable a feature "spaceships" in the crate, change the entry in
`//third_party/rust/chromium_crates_io/Cargo.toml` to include the feature:
```toml
[dependencies]
bar = { version = "3", features = [ "spaceships" ] }
```

### Patching third-party crates

You may patch a crate in tree, but save any changes made into a diff file in
a `//third_party/rust/chromium_crates_io/patches/` directory for the crate.
The diff file should be generated by `git-format-patch` each new patch numbered
consecutively so that they can be applied in order. For example, these files
might exist if the "foo" crate was patched with a couple of changes:

```
//third_party/rust/chromium_crates_io/patches/foo/patches/0001-Edit-the-Cargo-toml.diff
//third_party/rust/chromium_crates_io/patches/foo/patches/0002-Other-changes.diff
```

The recommended procedure to create such patches is:

1. Commit the plain new version of the crate to your local git branch
2. Modify the crate as necessary
3. Commit that modified version
4. Use `git format-patch <unpatched version>` to generate the patch files
5. Add the patch files in a new, third, commit
6. Squash them, or rely on `git cl upload` doing so

#### Patching errors

If `gnrt vendor` fails to apply a patch for a crate, it will cancel the download of that
crate rather than leave it in a broken state. To recreate patches, first get a pristine
copy of the crate by using the `--no-patches` argument:

1. Download the crate without applying patches:
   * `vpython3 ./tools/crates/run_gnrt.py vendor --no-patches=<CRATE_NAME>`
2. Then recreate the patches as described in [Patching third-party crates](
   #patching-third_party-crates).

To verify the patches work, remove the vendored crate directory in
`//third_party/rust/chromium_crates_io/vendor/`, named after the crate name
and version. Then run the `vendor` action without `--no-patches` which will
download the crate and apply the patches:
   * `vpython3 ./tools/crates/run_gnrt.py vendor`

## Security

If a shipping library needs security review (has any `unsafe`), and the review
finds it's not satisfying the [rule of 2](../docs/security/rule-of-2.md), then
move it to the `"sandbox"` group in `//third_party/rust/chromium_crates_io/gnrt_config.toml`
to make it clear it can't be used in a privileged process:
```
[crate.foo]
group = "sandbox"
```

If a transitive dependency moves from `"safe"` to `"sandbox"` and causes
a dependency chain across the groups, it will break the `gnrt vendor` step.
You will need to fix the new crate so that it's deemed safe in unsafe review,
or move the other dependent crates out of `"safe"` as well by setting their
group in `gnrt_config.toml`.

# Updating existing third-party crates

To update crates to their latest minor versions:
1. Change directory to the root `src/` dir of Chromium.
1. Update the versions in `//third_party/rust/chromium_crates_io/Cargo.lock`.
   * `vpython3 ./tools/crates/run_gnrt.py update`
   * Or, directly through (nightly) cargo:
     `cargo run --release --manifest-path tools/crates/gnrt/Cargo.toml --target-dir out/gnrt update`
1. Download any updated crate's files:
   * `./tools/crates/run_gnrt.py vendor`
   * Or, directly through (nightly) cargo:
     `cargo run --release --manifest-path tools/crates/gnrt/Cargo.toml --target-dir out/gnrt vendor`
1. 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.
1. If a crate in `//third_party/rust/chromium_crates_io/patches` was updated
   as part of vendoring, then reapply patches to it:
   * Go to the `//third_party/rust/chromium_crates_io` directory.
   * `./apply_patches.sh` (this currently requires linux).
1. 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`
1. Add the generated files to git:
   * `git add -f third_party/rust`

### Directory structure for third-party crates

The directory structure for a crate "foo" version 3.4.2 is:
```
//third_party/
    rust/
        foo/  (for the "foo" crate)
            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-3.4.2  (crate sources downloaded from crates.io)
            patches/
                foo/  (patches for the "foo" crate)
                    0001-Edit-the-Cargo-toml.diff
                    0002-Other-changes.diff
            Cargo.toml
            Cargo.lock
            gnrt_config.toml
```

## 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>/<epoch>/wrapper` or at another single place
that all C++ goes through to access the library. The [CXX](https://cxx.rs) is
used to generate bindings between C++ and Rust.

See
[`//third_party/rust/serde_json_lenient/v0_1/wrapper/`](
https://source.chromium.org/chromium/chromium/src/+/main:third_party/rust/serde_json_lenient/v0_1/wrapper/)
and
[`//components/qr_code_generator`](
https://source.chromium.org/chromium/chromium/src/+/main:components/qr_code_generator/;l=1;drc=b185db5d502d4995627e09d62c6934590031a5f2)
for examples.

Rust libraries should use the
[`rust_static_library`](
https://source.chromium.org/chromium/chromium/src/+/main:build/rust/rust_static_library.gni)
GN template (not the built-in `rust_library`) to integrate properly into the
mixed-language Chromium build and get the correct compiler options applied to
them.

The [CXX](https://cxx.rs) tool is used for generating C++ bindings to Rust
code. Since it requires explicit declarations in Rust, an wrapper shim around a
pure Rust library is needed. Add these Rust shims that contain the CXX
`bridge` macro to the `cxx_bindings` GN variable in the `rust_static_library`
to have CXX generate a C++ header for that file. To include the C++ header
file, rooted in the `gen` output directory, use
```
#include "the/path/to/the/rust/file.rs.h"
```

# Using VSCode

1. Ensure you're using the `rust-analyzer` extension for VSCode, rather than
   earlier forms of Rust support.
2. Run `gn` with the `--export-rust-project` flag, such as:
   `gn gen out/Release --export-rust-project`.
3. `ln -s out/Release/rust-project.json rust-project.json`
4. When you run VSCode, or any other IDE that uses
   [rust-analyzer](https://rust-analyzer.github.io/) it should detect the
   `rust-project.json` and use this to give you rich browsing, autocompletion,
   type annotations etc. for all the Rust within the Chromium codebase.
5. Point rust-analyzer to the rust toolchain in Chromium. Otherwise you will
   need to install Rustc in your system, and Chromium uses the nightly
   compiler, so you would need that to match. Add the following to
   `.vscode/settings.json` in the Chromium checkout:
   ```
   {
      // The rest of the settings...

      "rust-analyzer.cargo.extraEnv": {
        "PATH": "../../third_party/rust-toolchain/bin:$PATH",
      }
   }
   ```
   This assumes you are working with an output directory like `out/Debug` which
   has two levels; adjust the number of `..` in the path according to your own
   setup.

# Using cargo

If you are building a throwaway or experimental tool, you might like to use pure
`cargo` tooling rather than `gn` and `ninja`. Even then, you may choose
to restrict yourself to the toolchain and crates that are already approved for
use in Chromium.

Here's how.

```
export PATH_TO_CHROMIUM_SRC=~/chromium/src
mkdir my-rust-tool
cd my-rust-tool
mkdir .cargo
cat <<END > .cargo/config.toml
[source.crates-io]
replace-with = "vendored-sources"

[source.vendored-sources]
directory = "$PATH_TO_CHROMIUM_SRC/third_party/rust/chromium_crates_io/vendor"
END
$PATH_TO_CHROMIUM_SRC/third_party/rust-toolchain/bin/cargo init --offline
$PATH_TO_CHROMIUM_SRC/third_party/rust-toolchain/bin/cargo run --offline
```

Most `cargo` tooling works well with this setup; one exception is `cargo add`,
but you can still add dependencies manually to your `Cargo.toml`:

```
[dependencies]
log = "0.4"
```