Crate hakari

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hakari is the library underlying cargo hakari, a tool to manage workspace-hack packages.


use guppy::MetadataCommand;
use hakari::{HakariBuilder, HakariOutputOptions};

// Use this workspace's PackageGraph for these tests.
let package_graph = MetadataCommand::new()
    .expect("obtained cargo-guppy's PackageGraph");

// The second argument to HakariBuilder::new specifies a Hakari (workspace-hack) package.
// In this repository, the package is called "guppy-workspace-hack".
let hakari_package = package_graph.workspace().member_by_name("guppy-workspace-hack").unwrap().id();
let hakari_builder = HakariBuilder::new(&package_graph, Some(hakari_package))
    .expect("HakariBuilder was constructed");

// HakariBuilder has a number of config options. For this example, use the defaults.
let hakari = hakari_builder.compute();

// hakari can be used to build a TOML representation that forms part of a Cargo.toml file.
// Existing Cargo.toml files can be managed using Hakari::read_toml.
let toml = hakari.to_toml_string(&HakariOutputOptions::default()).expect("TOML output was constructed");

// toml contains the Cargo.toml [dependencies] that would go in the Hakari package. It can be
// written out through `HakariCargoToml` (returned by Hakari::read_toml) or manually.
println!("Cargo.toml contents:\n{}", toml);

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

The cargo-guppy repository also has a number of fixtures that demonstrate Hakari’s output. Here is an example.

§How hakari works

Hakari follows a three-step process.

§1. Configuration

A HakariBuilder provides options to configure how a Hakari computation is done. Options supported include:

With the optional cli-support feature, HakariBuilder options can be read from or written to a file as TOML or some other format.

§2. Computation

Once a HakariBuilder is configured, its compute method can be called to create a Hakari instance. The algorithm runs in three steps:

  1. Use guppy to simulate a Cargo build for every workspace package and every given platform, with no features, default features and all features. Collect the results into a map indexed by every dependency and the different sets of features it was built with.
  2. Scan through the map to figure out which dependencies are built with two or more different feature sets, collecting them into an output map.
  3. If one assumes that the output map will be written out to the workspace-hack package through step 3 below, it is possible that it causes some extra packages to be built with a second feature set. Look for such packages, add them to the output map, and iterate until a fixpoint is reached and no new packages are built more than one way.

This computation is done in a parallel fashion, using the Rayon library.

The result of this computation is a Hakari instance.

§3. Serialization

The last step is to serialize the contents of the output map into the workspace-hack package’s Cargo.toml file.

  1. Hakari::read_toml reads an existing Cargo.toml file on disk. This file is partially generated:

    name = "workspace-hack"
    version = "0.1.0"
    # more options...

    The contents outside the BEGIN HAKARI SECTION and END HAKARI SECTION lines may be edited by hand. The contents within this section are automatically generated.

    On success, a HakariCargoToml is returned.

  2. Hakari::to_toml_string returns the new contents of the automatically generated section.

  3. HakariCargoToml::write_to_file writes out the contents to disk.

HakariCargoToml also supports serializing contents to memory and producing diffs.

§Future work

hakari is still missing a few features:

  • Simulating cross-compilations
  • Platform-specific excludes
  • Only including a subset of packages in the final result (e.g. unifying core packages like syn but not any others)

These features will be added as time permits.



  • Command-line operations for hakari.
  • Information about why a dependency is in the workspace-hack.
  • Access to internal Hakari data structures.
  • Manage configuration and generate summaries for hakari.
  • Code related to ensuring that hakari works properly.