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 [COMMAND]`	Print the JSON feature-combination matrix to stdout. Add `--pretty` for indented JSON. `COMMAND` is an optional cargo subcommand whose per-command configuration the matrix should reflect. See The matrix subcommand and `cargo fc matrix --help`.
`version`	Print version information.

Both are cargo-fc’s own subcommands: they print and exit, so they parse everything after their token themselves instead of forwarding it to Cargo. cargo fc matrix --help prints the matrix help (not the matrix), and an argument neither subcommand can honor is an error rather than a silently dropped token.

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.
`--prune-implied`	Automatic pruning of redundant combinations, on by default; `=false` disables it for the run.
`--maximal-features`	Run only maximal feature sets: combinations that are not a subset of another generated combination. An unconstrained matrix collapses into a single all-features invocation per package-target, while matrix constraints keep one invocation per alternative.
`--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 (`--target`, then `CARGO_BUILD_TARGET`, then host). With an explicit `--target <triple>` this also lifts package-level `targets` constraints, forcing every selected package onto that triple.
`--install-missing-targets`	Install missing Rust target components with `rustup` before running. Explicit opt-in — may mutate the toolchain and use the network.
`--omit-host-target-flag`	Build a configured target that is the host without an injected `--target`, sharing `target/debug` with an ordinary `cargo build`. On by default; `=false` builds it under `target/<triple>/` like every other configured target. See Configured targets.
`--driver <bin>`	Program invoked in place of `cargo` for each build (e.g. `cargo-zigbuild`, `cross`). See Build drivers.
`--env <KEY=VALUE>`	Set a variable in every matrix-cell Cargo process (repeatable; the last value for a key wins). Overrides scoped `env` config.
`--unset-env <KEY>`	Remove a variable from every matrix-cell Cargo process (repeatable). Applied before CLI `--env` additions.

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

Environment variables#

Variable	Effect
`CARGO`	Program used for plain Cargo invocations. Set in child processes to the `+toolchain` cargo when an override is given, unless child `env` config overrides it.
`CARGO_DRIVER`	Set in child processes to the resolved driver unless child `env` config or CLI overrides it.
`RUSTUP_TOOLCHAIN`	Set in child processes to the `+toolchain` override unless child `env` config overrides it.
`CARGO_FC_VERBOSE`	Boolean default for verbose `cargo fc` headers.

Notes#

Cargo-fc boolean flags take an optional inline value: --summary-only means --summary-only=true, and --summary-only=false turns a Cargo.toml default back off for one invocation. no, off, and 0 are accepted alongside false (and yes/on/1 alongside true). There are no --no-<flag> spellings — those tokens belong to cargo (--no-fail-fast for test, --no-dedupe for tree) and cargo-fc forwards them untouched.
Switches with no configurable default — --workspace, --pretty, --help, --version — take no value. --pretty belongs to cargo fc matrix and must follow that subcommand.
--dedupe implies --diagnostics-only. Setting --dedupe together with diagnostics_only = false in config is rejected as contradictory.
Everything after -- is forwarded to the invoked program and never interpreted by cargo fc. cargo fc matrix and cargo fc version invoke nothing, so they reject -- instead of dropping what follows it.
Cargo’s lockfile flags (--frozen, --locked, --offline) are forwarded to every cargo invocation and applied to the initial cargo metadata discovery, so --locked cannot be defeated by the metadata step updating Cargo.lock. cargo fc matrix accepts them for the metadata step alone.
--env requires KEY=VALUE, split at the first =; KEY= sets an empty value.