cargo-raze: Bazel BUILD generation for Rust Crates

An experimental support Cargo plugin for distilling a workspace-level Cargo.toml into BUILD targets that code using rules_rust can depend on directly.

Disclaimer

This is not an official Google product (experimental or otherwise), it is just code that happens to be owned by Google.

Overview

This project synthesizes the dependency resolution logic and some of the functionality of Cargo such as features and build scripts into executable rules that Bazel can run to compile Rust crates. Though the standard rules_rust rules can be used to compile Rust code from scratch, the fine granularity of the dependency ecosystem makes transforming dependency trees based on that ecosystem onerous, even for code with few dependencies.

Usage

cargo-raze can generate buildable targets in one of two modes: Vendoring, or Non-Vendoring. In the vendoring mode, developers use the common cargo-vendor tool to retrieve the dependencies indicated by their workspace Cargo.toml into directories that cargo-raze then populates with BUILD files. In the non-vendoring mode, cargo-raze generates a flat list of BUILD files, and a workspace-level macro that can be invoked in the WORKSPACE file to pull down the dependencies automatically in similar fashion to Cargo itself.

In both cases, the first step is to decide where to situate the Cargo dependencies in the workspace. This library was designed with monorepos in mind, where an organization decides upon a set of dependencies that everyone points at. It is intended that stakeholders in the dependencies collaborate to upgrade dependencies atomically, and fix breakages across their codebase simultaneously. In the event that this isn't feasible, it is still possible to use cargo-raze in a decentralized scenario, but its unlikely that such decoupled repositories would interact well together with the current implementation.

Regardless of the approach chosen, the rust_rules should be brought in to the WORKSPACE. Here is an example:

git_repository(
    name = "io_bazel_rules_rust",
    commit = "5bc46ddca8817072cdae1961b3f9830a2bc3afa7",
    remote = "https://github.com/acmcarther/rules_rust.git",
)
load("@io_bazel_rules_rust//rust:repositories.bzl", "rust_repositories")

rust_repositories()

Vendoring Mode

In Vendoring mode, a root directly is selected that will house the vendored dependencies and become the gateway to those build rules. "//cargo" is conventional, but "//third_party/cargo" may be desirable to satisfy organizational needs. Vendoring directly into root isn't well supported due to implementation-specific idiosyncracies, but it may be supported in the future. From here forward, "//cargo" will be the assumed directory.

Generate a Cargo.toml

First, generate a standard Cargo.toml with the dependencies of interest. Take care to include a [lib] directive so that Cargo does not complain about missing source files for this mock crate. Here is an example:

[package]
name = "compile_with_bazel"
version = "0.0.0"

# Mandatory (or Cargo tooling is unhappy)
[lib]
path = "fake_lib.rs"

[dependencies]
log = "0.3.6"

[raze]
# The WORKSPACE relative path to the Cargo.toml working directory.
workspace_path = "//cargo"

# The target to generate BUILD rules for.
target = "x86_64-unknown-linux-gnu"

Generate buildable targets

First, install the required tools for vendoring and generating BUILDable targets.

$ cargo install cargo-vendor
$ cargo install cargo-raze

Then, generate a lock file for your dependencies

$ cargo generate-lockfile

Following that, vendor your dependencies from within the cargo/ directory (mind the -x, it guarantees the version is included in the file path).

$ cargo vendor -x

Finally, generate your BUILD files, again from within the cargo/ directory

$ cargo raze

You can now depend on any explicit dependencies in any Rust rule by depending on //cargo:your_dependency_name.

Remote Dependency Mode

In Remote mode a directory similiar to the vendoring mode is selected. In this case though it contains only BUILD files, a vendoring instruction for the WORKSPACE, and aliases to the explicit dependencies. Slightly different plumbing is required.

Generate a Cargo.toml

Generate a Cargo.toml, similar to Vendoring mode but add a new directive in the [raze] section

[raze]
genmode = "Remote"

This tells Raze not to expect the dependencies to be vendored and to generate different files.

Generate buildable targets

First, install cargo-raze.

$ cargo install cargo-raze

Next, execute cargo raze from within the cargo directory

$ cargo raze

Finally, invoke the remote library fetching function within your WORKSPACE:

load("//cargo:crates.bzl", "raze_fetch_remote_crates")

raze_fetch_remote_crates()

This tells Bazel where to get the dependencies from, and how to build them: using the files generated into //cargo.

You can depend on any explicit dependencies in any Rust rule by depending on //cargo:your_dependency_name.

Handling Unconventional Crates

Some crates execute a "build script", which, while technically unrestricted in what it can do, usually does one of a few common things.

All options noted below are enumerated in the src/settings.rs file.

Crates that generate files using locally known information

In some cases, a crate uses only basic information in order to generate a Rust source file. These build-scripts rules can actually be executed and used within Bazel by including a directive in your Cargo.toml prior to generation:

[raze.crates.clang-sys.'0.21.1']
gen_buildrs = true

This setting tells cargo-raze to generate a rust_binary target for the build script and to direct its generated (OUT_DIR-style) outputs to the parent crate.

Crates that depend on certain flags being determined by a build script

Some build scripts conditionally emit directives to stdout that Cargo knows how to propagate. Unfortunately, its not so simple to manage build-time generated dependency information, so if the flags are statically known (perhaps, since the compilation target is statically known), they can be provided from within the Cargo.toml, in the following manner

[raze.crates.unicase.'2.1.0']
additional_flags = [
  # Rustc is 1.15, enable all optional settings
  "--cfg=__unicase__iter_cmp",
  "--cfg=__unicase__defauler_hasher",
]

Flags provided in this manner are directly handed to rustc. It may be helpful to refer to the build-script section of the documentation to interpret build scripts and stdout directives that are encountered, available here: https://doc.rust-lang.org/cargo/reference/build-scripts.html

Crates that need system libraries

There are two ways to provide system libraries that a crate needs for compilation. The first is to vendor the system library directly, craft a BUILD rule for it, and add the dependency to the corresponding -sys crate. For openssl, this may in part look like:

[raze.crates.openssl-sys.'0.9.24']
additional_flags = [
  # Vendored openssl is 1.0.2m
  "--cfg=ossl102",
  "--cfg=version=102",
]
additional_deps = [
  "@//third_party/openssl:crypto",
  "@//third_party/openssl:ssl",
]

[raze.crates.openssl.'0.10.2']
additional_flags = [
  # Vendored openssl is 1.0.2m
  "--cfg=ossl102",
  "--cfg=version=102",
  "--cfg=ossl10x",
]

In some cases, directly wiring up a local system dependency may be preferable. To do this, refer to the new_local_repository section of the Bazel documentation. For a precompiled version of llvm in a WORKSPACE, this may look something like:

new_local_repository(
    name = "llvm",
    build_file = "llvm.BUILD",
    path = "/usr/lib/llvm-3.9",
)

In a few cases, the sys crate may need to be overridden entirely. This can be facilitated by removing and supplementing dependencies in the Cargo.toml, pre-generation:

[raze.crates.sdl2.'0.31.0']
skipped_deps = [
  "sdl2-sys-0.31.0"
]
additional_deps = [
  "@//cargo/overrides/sdl2-sys:sdl2_sys"
]

Crates that supply useful binaries

Some crates provide useful binaries that themselves can be used as part of a compilation process: Bindgen is a great example. Bindgen produces Rust source files by processing C or C++ files. A directive can be added to the Cargo.toml to tell Bazel to expose such binaries for you:

[raze.crates.bindgen.'0.32.2']
gen_buildrs = true # needed to build bindgen
extra_aliased_targets = [
  "cargo_bin_bindgen"
]

Cargo-raze prefixes binary targets with "cargo_bin_", as although Cargo permits binaries and libraries to share the same target name, Bazel disallows this.

FAQ

Why choose Bazel to build a Rust project?

Bazel ("fast", "correct", choose two) is a battle tested build system used by Google to compile incredibly large, multilingual projects without duplicating effort, and without compromising on correctness. It accomplishes this in part by limiting what mechanisms a given compilation object can use to discover dependencies and by forcing buildable units to express the complete set of their dependencies. It expects two identical sets of build target inputs to produce a byte-for-byte equivalent final result.

In exchange, users are rewarded with a customizable and extensible build system that compiles any kind of compilable target and allows expressing "unconventional dependencies", such as Protobuf objects, precompiled graphics shaders, or generated code, while remaining fast and correct.

Its also probable (though not yet demonstrated with benchmarks) that large applications built with Bazel's strengths in mind: highly granular build units, will compile significantly faster as they are able to cache more aggressively and avoid recompilation of as much code while iterating.

Why try to integrate Cargo's dependencies into this build tool?

For better or worse, the Rust ecosystem heavily depends on Cargo crates in order to provide functionality that is often present in standard libraries. This is actually a fantastic thing for the evolution of the language, as it describes a structured process to stabilization (experimental crate -> 1.0 crate -> RFC -> inclusion in stdlib), but it means that people who lack access to this ecosystem must reinvent many wheels.

Putting that aside there are also fantastic crates that help Rust developers interact with industry standard systems and libraries which can greatly accelerate development in the language.

Why not build directly with Cargo / Why generate rustc invocations?

Though the burden of emulating Cargo's functionality (where possible at all!) is high, it appears to be the only way to maintain the guarantees (correctness, reproducibility) that Bazel depends on to stay performant. It is possible and likely with inflight RFCs that Cargo will become sufficiently flexible to allow it to be used directly for compilation but at this point in time it appears that maintaining a semblance of feature parity is actually easier than avoiding all of the sharp edges introduced by treating Cargo like the Rust compiler.

What is buildable right now with Bazel, and what is not?

With a little bit of elbow grease it is possible to build nearly everything, including projects that depend on openssl-sys. Many sys crates will require identifying the system library that they wrap, and either vendoring it into the project, or telling Bazel where it lives on your system. Some may require minor source tweaks, such as eliminating hardcoded cargo environment variable requirements. Fixes can be non-trivial in a few cases, but a good number of the most popular crates have been built in an example repo, available at https://github.com/acmcarther/cargo-raze-crater

Example Repos

See these examples of providing crate configuration:

• basic-example
• complicated-example
• complicated-example-remote
• openssl-example

The [raze] section is derived from a struct declared in src/settings.rs.