Blackjack - build cargo dependencies with Bazel

Generate bazel targets for crates.io dependencies in your Cargo.toml.

Blackjack reads standard Cargo.toml, so the same project can build with both cargo and Bazel. Adding Bazel support to an existing cargo project is as simple as adding a WORKSPACE and BUILD file.

WORKSPACE:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

# Configure a Rust toolchain.
# See https://github.com/bazelbuild/rules_rust for more details.
http_archive(
    name = "rules_rust",
    sha256 = "cf2e82c56c73c3213bc2d94303aee88d7e44634ad1e1fe183befa85b17b5021d",
    strip_prefix = "rules_rust-0e3593fc5d839e4a74523f07e885b761ee19e662",
    urls = [
        # Master branch as of 2021-12-19
        "https://github.com/bazelbuild/rules_rust/archive/0e3593fc5d839e4a74523f07e885b761ee19e662.tar.gz",
    ],
)
load("@rules_rust//rust:repositories.bzl", "rust_repositories")
rust_repositories()

# Set up blackjack
http_archive(
    name = "blackjack",
    url = "https://github.com/wildarch/blackjack/archive/909b66db4782cf963791aa4a24793244dffdba17.zip",
    strip_prefix = "blackjack-909b66db4782cf963791aa4a24793244dffdba17",
)
load("@blackjack//:workspace.bzl", "blackjack_cargo")
blackjack_cargo()

BUILD file in the same directory as Cargo.toml:

load("@blackjack//:blackjack.bzl", "blackjack")

blackjack(name = "blackjack")

Now run bazel run //:blackjack. You'll find a newly created cargo_dependencies.bzl file next to your Cargo.toml.

Note: Blackjack pulls in its own cargo executable, so you do not need to have it installed.

Import the generated dependencies into your WORKSPACE:

load("//:cargo_dependencies.bzl", "cargo_dependencies")
cargo_dependencies()

Dependencies from Cargo.toml are now available as rust_library rules under the label @crates_io_{name}//:{name}.

For an example, see here.

Updates

If you make any changes to the dependency graph, simply re-run bazel run //:blackjack to build an updated cargo_dependencies.bzl file.

Configuration

Many crates will build out of the box. For more exotic crates (linking to C libraries, build scripts etc.), there are a few configuration options. This configuration is embedded in the Cargo.toml in the metadata section.

To pass extra flags to rustc when compiling a particular crate, add the following to the end of Cargo.toml:

[package.metadata.blackjack.proc-macro2]
rustc_flags = ["--cfg=use_proc_macro"]

Note: Blackjack provides default settings for a few common crates such as proc-macro2, so you don't always need to add them yourself. Please send PRs for default settings for other common crates!

If a dependency needs a build script, but does not otherwise do anything complicated, try adding something like:

[package.metadata.blackjack.proc-macro-nested]
build_script = true

If all else fails, you can replace the dependency with a custom rust_library target:

[package.metadata.blackjack.libz-sys]
replace = "@custom_libz_sys"

For a simple example of replacing a dependency and linking to a C library, see here.

If you want to change the default crates_io_ prefix for generated dependency targets, that is possible too:

[package.metadata.blackjack]
prefix = "blackjack_crates_io"

Now instead of @crates_io_serde//:serde, use blackjack_crates_io_serde//:serde.

Things that don't work yet (but would gladly accept a PR for)

• Support for Windows. All that is really needed here is to add it to the list of supported platforms, and fix the platform specific workspace test.