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.

`--dedupe`#

Like --diagnostics-only, but also collapses identical diagnostics that repeat across combinations, so a warning present in twenty combinations is shown once.

cargo fc --dedupe clippy

$ cargo fc --dedupe 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] )
       Note 2 duplicate diagnostics suppressed
     Linting [4/4] loader ( features = [validation] )
       Note 1 duplicate diagnostic suppressed
 
    Finished 4 feature combinations for 1 package in 0.00s
 
        WARN loader ( 0 errors, 1 warnings, 0 suppressed, features = [] )
        FAIL loader ( 1 errors, 0 warnings, 0 suppressed, features = [experimental] )
        FAIL loader ( 1 errors, 0 warnings, 2 suppressed, features = [experimental, validation] )
        WARN loader ( 0 errors, 1 warnings, 1 suppressed, features = [validation] )

--dedupe implies --diagnostics-only.

`--summary-only`#

Hide cargo output entirely and print only the final per-combination result table.

cargo fc --summary-only check

$ cargo fc --summary-only check --workspace
 
     Checking [1/4] loader ( features = [] )
     Checking [2/4] loader ( features = [experimental] )
     Checking [3/4] loader ( features = [experimental, validation] )
     Checking [4/4] loader ( features = [validation] )
 
    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] )

`--fail-fast`#

Stop at the first combination that fails instead of running the whole matrix. Useful locally when you just want the first problem.

cargo fc --fail-fast test

`--pedantic`#

Treat warnings as failures in the summary and under --fail-fast. A combination with warnings but no errors is then reported as failing.

cargo fc --pedantic --fail-fast clippy

`--errors-only`#

Allow all warnings and surface errors only (equivalent to -A warnings). This appends to RUSTFLAGS / CARGO_ENCODED_RUSTFLAGS, and — like any RUSTFLAGS override — it shadows rustflags set in config files.

cargo fc --errors-only check

Pruning: `--show-pruned` and `--no-prune-implied`#

cargo fc prunes redundant feature combinations by default (you never need to enable this). --show-pruned includes them in the summary marked SKIP; --no-prune-implied disables pruning for the run:

cargo fc --show-pruned check
cargo fc --no-prune-implied check

See Automatic pruning for what it does and a worked example.

Combining flags#

Flags compose. A common local loop:

# Fast feedback: first failure only, warnings-as-errors, deduped diagnostics
cargo fc --dedupe --pedantic --fail-fast clippy

For the exhaustive list, see the CLI reference.

Output modes#

--diagnostics-only#

--dedupe#

--summary-only#

--fail-fast#

--pedantic#

--errors-only#

Pruning: --show-pruned and --no-prune-implied#