FAQ & troubleshooting#

How is this different from --all-features?#

--all-features enables every feature at once — a single configuration. cargo fc runs many configurations: the combinations of your features. That’s what catches a conflict between two features that each work alone but not together, or a crate that breaks with --no-default-features.

How is this different from cargo-all-features?#

Both explore feature combinations. cargo fc adds a single readable summary, diagnostics de-duplication, automatic pruning of redundant combinations, a JSON matrix for CI, per-target and per-command configuration, and driver-based cross-compilation. The skip_optional_dependencies behavior is intentionally compatible.

My matrix is huge / I hit max_combinations.#

The powerset grows as 2ⁿ. Rather than only raising max_combinations, shape the matrix:

• skip_optional_dependencies if optional deps are inflating it.
• isolated_feature_sets for independent feature groups.
• only_features or allow_feature_sets to restrict what’s varied.

My target.'cfg(...)' addition replaced the base instead of extending it.#

An array is always an override. Use a patch object:

# replaces the inherited value
exclude_features = ["cuda"]
# extends the inherited value
exclude_features = { add = ["cuda"] }

See the override model.

--diagnostics-only shows nothing for cargo fc test.#

Diagnostics-only needs a command that emits rustc JSON diagnostics (build, check, clippy, doc). test and run aren’t diagnostics-safe, so broad diagnostics defaults are ignored for them. You can opt a specific command in explicitly — see Flags in config.

cargo fc test fails for a non-host target.#

test executes the compiled binary, which can’t run a foreign target. Keep test/run host-only: narrow with --target, --no-targets, or expand_targets = false. See Configured targets.

A configured target isn’t installed.#

By default cargo fc won’t touch the toolchain. Preinstall the target with rustup target add, or pass --install-missing-targets.

The driver cargo-zigbuild is missing.#

When a non-host target is planned, cargo fc defaults to cargo-zigbuild so native-C deps cross-compile. Install zig and cargo-zigbuild, or force plain cargo with --driver cargo (or driver = "cargo" in config). See Build drivers.

CARGO_BUILD_TARGET is being ignored.#

Configured target lists intentionally take precedence over CARGO_BUILD_TARGET, so repository config isn’t silently collapsed by your environment. Use --target <triple> to force a single target for one run, or --no-targets to fall back to Cargo’s default. See Configured targets.

Can I rely on the Rust API?#

No. The CLI is the supported interface. The Rust API exists for the tool’s own binaries and integration tests and has no stability guarantees.

Where’s the canonical flag reference?#

cargo fc --help, mirrored in the CLI reference.