Crate cargo_hakari

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cargo hakari is a command-line application to manage workspace-hack crates. Use it to speed up local cargo build and cargo check commands by 15-95%, and cumulatively by 20-25% or more.

For an explanation of what workspace-hack packages are and how they make your builds faster, see the about module.


The cargo-guppy repository uses a workspace-hack crate managed by cargo hakari. See the generated Cargo.toml.

§Platform support

  • Unix platforms: Hakari works and is supported.
  • Windows: Hakari works and outputs file paths with forward slashes for consistency with Unix. CRLF line endings are not supported in the workspace-hack’s Cargo.toml. Within Git repositories, cargo hakari init automatically does this for you. Here’s how to do it manually. (Pull requests to improve this are welcome.)


§Release binaries

Release binaries are available on GitHub Releases, via cargo binstall:

cargo binstall cargo-hakari

In GitHub Actions CI, use taiki-e/install-action, which uses cargo binstall under the hood:

- name: Install cargo-hakari
  uses: taiki-e/install-action@v2
     tool: cargo-hakari

§Installing from source

To install or update cargo-hakari, run:

cargo install cargo-hakari --locked

If $HOME/.cargo/bin is in your PATH, the cargo hakari command will be available.


§Getting started

There are four steps you must take for cargo hakari to work properly.

§1. Check in your Cargo.lock

For hakari to work correctly, you must add your Cargo.lock to version control, even if you don’t have any binary crates. This is because patch version bumps in dependencies can add or remove features or even entire transitive dependencies.

§2. Initialize the workspace-hack

Initialize a workspace-hack crate for a workspace at path my-workspace-hack:

cargo hakari init my-workspace-hack

§3. Generate the Cargo.toml

Generate or update the contents of a workspace-hack crate:

cargo hakari generate

§4. Add dependencies to the workspace-hack

Add the workspace-hack crate as a dependency to all other workspace crates:

cargo hakari manage-deps

§Making hakari work well

These are things that are not absolutely necessary to do, but will make cargo hakari work better.

§1. Update the hakari config

Open up .config/hakari.toml, then:

  • uncomment or add commonly-used developer platforms
  • read the note about the resolver, and strongly consider setting resolver = "2" in your workspace’s Cargo.toml.

Remember to run cargo hakari generate after changing the config.

§2. Keep the workspace-hack up-to-date in CI

Run the following commands in CI:

cargo hakari generate --diff  # workspace-hack Cargo.toml is up-to-date
cargo hakari manage-deps --dry-run  # all workspace crates depend on workspace-hack

If either of these commands exits with a non-zero status, you can choose to fail CI or produce a warning message.

For an example, see this GitHub action used by cargo-guppy.

All cargo hakari commands take a --quiet option to suppress output, though showing diff output in CI is often useful.

§3. Consider a [patch] directive

If your workspace is depended on as a Git or path dependency, it is strongly recommended that you follow the instructions in the [patch] directive section.

§Information about the workspace-hack

The commands in this section provide information about components in the workspace-hack.

§Why is a dependency in the workspace-hack?

Print out information about why a dependency is present in the workspace-hack:

cargo hakari explain <dependency-name>

§Does the workspace-hack ensure that each dependency is built with exactly one feature set?

cargo hakari verify

If some dependencies are built with more than one feature set, this command will print out details about them. This is always a bug—if you encounter it, a bug report with more information would be greatly appreciated!


§Publishing a crate

If you publish crates to or other registries, see the publishing module.

§Disabling and uninstalling

Disable the workspace-hack crate temporarily by removing generated lines from Cargo.toml. (Re-enable by running cargo hakari generate.)

cargo hakari disable

Remove the workspace-hack crate as a dependency from all other workspace crates:

cargo hakari remove-deps


cargo hakari is configured through .config/hakari.toml at the root of the workspace. Running cargo hakari init causes a new file to be created at this location.

Example configuration:

## The name of the package used for workspace-hack unification.
hakari-package = "workspace-hack"
## Cargo resolver version in use -- version 2 is highly recommended.
resolver = "2"

## Format for `workspace-hack = ...` lines in other Cargo.tomls. Version 2 requires cargo-hakari
## 0.9.8 or above.
dep-format-version = "2"

## Add triples corresponding to platforms commonly used by developers here.
platforms = [
    ## "x86_64-unknown-linux-gnu",
    ## "x86_64-apple-darwin",
    ## "x86_64-pc-windows-msvc",

## Write out exact versions rather than specifications. Set this to true if version numbers in
## `Cargo.toml` and `Cargo.lock` files are kept in sync, e.g. in some configurations of
## exact-versions = false

For more options, including how to exclude crates from the output, see the config module.

§Stability guarantees

cargo-hakari follows semantic versioning, where the public API is the command-line interface.

Within a given series, the command-line interface will be treated as append-only.

The generated Cargo.toml will also be kept the same unless:

  • a new config option is added, in which case the different output will be gated on the new option, or
  • there is a bugfix involved.


  • About workspace-hack crates, how cargo hakari manages them, and how much faster they make builds.
  • Configuration for cargo hakari.
  • Using a [patch] directive.
  • Publishing a package to or other registries.