cargo-feature-combinations

Plugin for cargo to run commands against selected combinations of features.

Installation

brew install --cask romnn/tap/cargo-fc

# or install from source
cargo install --locked cargo-feature-combinations

Usage

In most cases, just use the command as if it was cargo:

cargo fc check
cargo fc test
cargo fc build

In addition, there are a few optional flags and the matrix subcommand. To get an idea, consider these examples:

# run tests and fail on the first failing combination of features
cargo fc --fail-fast test

# silence output and only show final summary
cargo fc --silent build

# print all combinations of features in JSON (useful for usage in github actions)
cargo fc matrix --pretty

For details, please refer to --help:

$ cargo fc --help

USAGE:
    cargo [+toolchain] [SUBCOMMAND] [SUBCOMMAND_OPTIONS]
    cargo [+toolchain] [OPTIONS] [CARGO_OPTIONS] [CARGO_SUBCOMMAND]

SUBCOMMAND:
    matrix                  Print JSON feature combination matrix to stdout
        --pretty            Print pretty JSON

OPTIONS:
    --help                  Print help information
    --silent                Hide cargo output and only show summary
    --fail-fast             Fail fast on the first bad feature combination
    --exclude-package       Exclude a package from feature combinations 
    --only-packages-with-lib-target
                            Only consider packages with a library target
    --errors-only           Allow all warnings, show errors only (-Awarnings)
    --pedantic              Treat warnings like errors in summary and
                            when using --fail-fast

Configuration

In your Cargo.toml, you can configure the feature combination matrix:

[package.metadata.cargo-feature-combinations]
# When at least one isolated feature set is configured, stop taking all project 
# features as a whole, and instead take them in these isolated sets. Build a 
# sub-matrix for each isolated set, then merge sub-matrices into the overall 
# feature matrix. If any two isolated sets produce an identical feature 
# combination, such combination will be included in the overall matrix only once.
#
# This feature is intended for projects with large number of features, sub-sets 
# of which are completely independent, and thus don’t need cross-play.
#
# Non-existent features are ignored. Other configuration options are still 
# respected.
isolated_feature_sets = [
    ["foo-a", "foo-b", "foo-c"],
    ["bar-a", "bar-b"],
    ["other-a", "other-b", "other-c"],
]

# Exclude groupings of features that are incompatible or do not make sense
exclude_feature_sets = [ ["foo", "bar"], ] # formerly "skip_feature_sets"

# Exclude features from the feature combination matrix
exclude_features = ["default", "full"] # formerly "denylist"

# Skip implicit features that correspond to optional dependencies from the
# matrix.
#
# When enabled, the implicit features that Cargo generates for optional
# dependencies (of the form `foo = ["dep:foo"]` in the feature graph) are
# removed from the combinatorial matrix. This mirrors the behaviour of the
# `skip_optional_dependencies` flag in the `cargo-all-features` crate.
skip_optional_dependencies = true

# Include features in the feature combination matrix
include_features = ["feature-that-must-always-be-set"]

# In the end, always add these exact combinations to the overall feature matrix, 
# unless one is already present there.
#
# Non-existent features are ignored. Other configuration options are ignored.
include_feature_sets = [
    ["foo-a", "bar-a", "other-a"],
] # formerly "exact_combinations"

Example: skipping optional dependency features

[features]
default = []
core = []
cli = ["core"]

[dependencies]
tokio = { version = "1", optional = true }
serde = { version = "1", optional = true }

[package.metadata.cargo-feature-combinations]
exclude_features = ["default"]
skip_optional_dependencies = true

With this configuration, the feature matrix will only vary the core and cli features. The implicit tokio and serde features that correspond to optional dependencies are excluded from the matrix, avoiding a combinatorial explosion over integration features. If you still want to test specific combinations that include tokio or serde, you can list them explicitly in include_feature_sets.

When using a cargo workspace, you can also exclude packages in your workspace Cargo.toml:

[workspace.metadata.cargo-feature-combinations]
# Exclude packages in the workspace metadata, or the metadata of the *root* package.
exclude_packages = ["package-a", "package-b"]

Usage with github-actions

The github-actions matrix feature can be used together with cargo fc to more efficiently test combinations of features in CI.

First, add a workflow feature-matrix.yaml that computes the feature matrix for your project. We will re-use this workflow in our build.yaml workflow.

# .github/workflows/feature-matrix.yaml
name: feature-matrix
on:
  workflow_call:
    outputs:
      matrix:
        description: "feature matrix"
        value: ${{ jobs.matrix.outputs.matrix }}
jobs:
  matrix:
    name: Generate feature matrix
    runs-on: ubuntu-24.04
    outputs:
      matrix: ${{ steps.compute-matrix.outputs.matrix }}
    steps:
      - uses: actions/checkout@v4
      - uses: romnn/cargo-feature-combinations@main
      - name: Compute feature matrix
        id: compute-matrix
        run: |-
          MATRIX="$(cargo fc matrix)"
          echo "${MATRIX}"
          echo "matrix=${MATRIX}" >> "$GITHUB_OUTPUT"

Now, we can use the feature-matrix.yaml workflow result to dynamically create jobs that build each combination of features in parallel.

# .github/workflows/build.yaml
name: build
on:
  push: {}
  pull_request: {}
jobs:
  feature-matrix:
    uses: ./.github/workflows/feature-matrix.yaml

  build:
    name: build ${{ matrix.package.name }} (${{ matrix.os }}, features ${{ matrix.package.features }})
    runs-on: ${{ matrix.os }}
    needs: [feature-matrix]
    strategy:
      fail-fast: false
      matrix:
        os: [macos-latest, ubuntu-24.04]
        package: ${{ fromJson(needs.feature-matrix.outputs.matrix) }}

    steps:
      - uses: actions/checkout@v4
      - uses: dtolnay/rust-toolchain@stable
      - name: Build
        # prettier-ignore
        run: >-
          cargo build
          --package "${{ matrix.package.name }}"
          --features "${{ matrix.package.features }}"
          --all-targets

Of course you can also apply the same approach for your test.yaml or lint.yaml workflows! Per job, up to 256 feature sets can be processed in parallel.

Local development

For local development and testing, you can point cargo fc to another project using the --manifest-path flag.

cargo run -- cargo check --manifest-path ../path/to/Cargo.toml
cargo run -- cargo matrix --manifest-path ../path/to/Cargo.toml --pretty

Acknowledgements

The cargo-all-features crate is similar yet offers more complex configuration and is lacking a summary.