Commands on cargo-feature-combinations

Running commands

Mon, 01 Jan 0001 00:00:00 +0000

Running commands#

Use cargo fc as if it were cargo. The command and its arguments are forwarded to each combination’s invocation:

cargo fc check
cargo fc clippy
cargo fc build
cargo fc test
cargo fc doc

Forwarded arguments#

Every cargo argument is passed through except the three that would conflict with the matrix cargo fc builds for you:

--features
--all-features
--no-default-features

Everything else is forwarded verbatim, so this works as expected:

cargo fc check -p my-crate --all-targets
cargo fc test --release -- --nocapture

Arguments after -- are passed to the invoked program (for example the test binary), never interpreted by cargo fc.

Output modes

Mon, 01 Jan 0001 00:00:00 +0000

Output modes#

A full matrix run can print a lot. These flags control how much you see. They are CLI flags, but most can also be set as defaults in Cargo.toml.

`--diagnostics-only`#

Show only warnings and errors per combination, suppressing build progress and “Finished” chatter.

cargo fc --diagnostics-only clippy

$ cargo fc --diagnostics-only clippy --workspace
 
 Linting [1/4] loader ( features = [] )
warning: function `checksum` is never used
 --> crates/loader/src/lib.rs:12:4
 |
12 | fn checksum(bytes: &[u8]) -> usize {
 | ^^^^^^^^
 |
 = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
 
 Linting [2/4] loader ( features = [experimental] )
error[E0425]: cannot find function `missing_helper` in this scope
 --> crates/loader/src/lib.rs:32:5
 |
32 | missing_helper()
 | ^^^^^^^^^^^^^^ not found in this scope
 
For more information about this error, try `rustc --explain E0425`.
 Linting [3/4] loader ( features = [experimental, validation] )
error[E0425]: cannot find function `missing_helper` in this scope
 --> crates/loader/src/lib.rs:32:5
 |
32 | missing_helper()
 | ^^^^^^^^^^^^^^ not found in this scope
 
For more information about this error, try `rustc --explain E0425`.
 Linting [4/4] loader ( features = [validation] )
warning: function `checksum` is never used
 --> crates/loader/src/lib.rs:12:4
 |
12 | fn checksum(bytes: &[u8]) -> usize {
 | ^^^^^^^^
 |
 = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
 
 
 Finished 4 feature combinations for 1 package in 0.00s
 
 WARN loader ( 0 errors, 1 warnings, features = [] )
 FAIL loader ( 1 errors, 0 warnings, features = [experimental] )
 FAIL loader ( 1 errors, 0 warnings, features = [experimental, validation] )
 WARN loader ( 0 errors, 1 warnings, features = [validation] )

The command must accept --message-format and emit rustc JSON diagnostics. That is true for build, check, clippy, and doc — and any alias or wrapper that does the same. It is not safe for test or run, which is why broad diagnostics defaults don’t apply to them automatically.

The matrix subcommand

Mon, 01 Jan 0001 00:00:00 +0000

The `matrix` subcommand#

cargo fc matrix prints the feature matrix as JSON instead of running a command. It’s the bridge to CI: emit the matrix in one job, then fan out a build/test job per row.

cargo fc matrix
cargo fc matrix --pretty

Output shape#

The matrix is a JSON array with one object per combination. Each object carries the package name, the comma-joined features string, the effective target, and any configured metadata:

CLI reference

Mon, 01 Jan 0001 00:00:00 +0000

CLI reference#

The authoritative reference is cargo fc --help. This page mirrors it.

Usage#

cargo fc [+toolchain] [SUBCOMMAND] [SUBCOMMAND_OPTIONS]
cargo fc [+toolchain] [OPTIONS] [CARGO_OPTIONS] [CARGO_SUBCOMMAND]

Subcommands#

Subcommand	Description
`matrix`	Print the JSON feature-combination matrix to stdout. Add `--pretty` for indented JSON.
`version`	Print version information.

Options#

Flag	Description
`-h`, `--help`	Print help information.
`-V`, `--version`	Print version information.
`--manifest-path <path>`	Path to the `Cargo.toml` to inspect.
`-p`, `--package <name>`	Include only this workspace package (repeatable).
`--exclude <name>`	Exclude a workspace package (repeatable). Pairs with `--workspace` for Cargo-compatible selection.
`--diagnostics-only`	Show only diagnostics (warnings/errors) per combination. Requires a command that emits rustc JSON diagnostics (`build`, `check`, `clippy`, `doc`, or an equivalent alias/wrapper).
`--dedupe`	Like `--diagnostics-only`, but also deduplicate identical diagnostics across combinations.
`--summary-only`	Hide cargo output; show only the final summary.
`--fail-fast`	Stop on the first failing combination.
`--errors-only`	Allow all warnings, show errors only (`-A warnings`). Appends to `RUSTFLAGS`.
`--pedantic`	Treat warnings like errors in the summary and under `--fail-fast`.
`--show-pruned`	Show pruned (redundant) combinations in the summary.
`--no-prune-implied`	Disable automatic pruning of redundant combinations.
`--packages-only`	In `matrix` mode, emit one row per package-target instead of per combination.
`--only-packages-with-lib-target`	Only consider packages that have a library target.
`--aggregate-targets`	Batch a combination’s configured targets into a single Cargo invocation (one `--target` each). Faster on many cores; falls back to serial for `run` and pruned summaries.
`--no-targets`	Ignore configured target lists for this run; use Cargo’s default single target.
`--install-missing-targets`	Install missing Rust target components with `rustup` before running. Explicit opt-in — may mutate the toolchain and use the network.
`--driver <bin>`	Program invoked in place of `cargo` for each build (e.g. `cargo-zigbuild`, `cross`). See Build drivers.

Most boolean flags can also be set as defaults in Cargo.toml; CLI flags always win for a single invocation.

Commands on cargo-feature-combinations

Running commands

Running commands#

Forwarded arguments#

Output modes

Output modes#

--diagnostics-only#

The matrix subcommand

The matrix subcommand#

Output shape#

CLI reference

CLI reference#

Usage#

Subcommands#

Options#

`--diagnostics-only`#

The `matrix` subcommand#