Working with rust

Our rust toolchain is very different from cargo, and fits into bazel's norms.

Creating a new 1st-party crate

For now, writing crates in pure bazel doesn‘t play nice with cargo’s IDE support, so we make the crate support both rust and cargo. To do so, we:

Create a new directory bazel/path/to/my_crate/src/ containing either main.rs (for a binary crate) or lib.rs (for a library crate) Similar to any other language, create a .rs file, and then create a rust_library/binary target which lists that file as it's srcs.
Add the following to bazel/path/to/my_crate/BUILD.bazel:

# Copyright 2024 The ChromiumOS Authors
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

load("@rules_rust//rust:defs.bzl", "rust_library", "rust_test")
load("//bazel/build_defs:generate_cargo_toml.bzl", "generate_cargo_toml")

rust_library(
    name = "my_crate",
    srcs = glob(["src/**/*.rs"]),
    deps = [],
)

rust_test(
    name = "my_crate_test",
    size = "small",
    crate = ":my_crate",
    deps = [],
)

generate_cargo_toml(
    name = "cargo_toml",
    crate = ":my_crate",
    tests = [":my_crate_test"],
)

Add //bazel/path/to/my_crate:cargo_toml to //bazel:cargo_workspace's deps.
You will now have failing tests. They'll give you instructions on how to fix them, but the TLDR is:

bazel run //bazel/path/to/my_crate:generate_cargo_toml
bazel run //bazel:generate_cargo_files

Adding dependencies on crates

Note that when adding dependencies, you will probably break some diff tests. However, you don't need to worry too much, since SKIP_PORTAGE_TESTS=1 bazel/portage/tools/run_tests.sh should pick up on these

1st-party crates

Add the corresponding target to your deps.

3rd-party crates

Add a dep on @crates//:<crate name> (for chromeos) or @alchemy-crates//:<crate name> (for infra).

If the crate doesn't exist, or if the version needs to be updated, then:

@alchemy-crates

Modify the //bazel/rust/alchemy_crates/Cargo.toml, then cd to the directory that contains it and run cargo update --workspace to update the lockfile.

After you‘ve added a crate that uses it, you’ll need to run bazel run //bazel:generate_cargo_files (but don't worry, if you forget this step, a test will fail and tell you you need to run this command).

@crates

Modify //third_party/rust_crates/projects/.../Cargo.toml, then run third_party/rust_crates/vendor.py.