cargo-crap

Compute the CRAP (Change Risk Anti-Patterns) metric for Rust projects.

CRAP combines cyclomatic complexity and test coverage into a single number that is high when code is both hard to understand and poorly tested — i.e. where bugs love to hide. The metric was introduced by Savoia & Evans in 2007 and was originally implemented for Java (Crap4j) and .NET (NDepend). cargo-crap brings it to the Rust ecosystem.

CRAP(m) = comp(m)² × (1 − cov(m)/100)³ + comp(m)

A few properties worth internalizing before you use the output:

• A trivial function (CC=1, 100% covered) scores exactly 1.0. That's the lower bound.
• At 100% coverage the quadratic term collapses and CRAP equals CC. When you see matching values in those two columns, that function is fully covered — tests are capping the damage, but the complexity itself remains. It's a good sign, not a bug.
• Above CC ≈ 30 no amount of coverage keeps you under the default threshold of 30. That's not a bug in the formula — it's the formula saying "this function is too big to certify as clean, regardless of tests."

Install

Via cargo binstall (downloads the right pre-built binary automatically):

cargo binstall cargo-crap

From source (requires Rust stable ≥ 1.88):

cargo install cargo-crap

Pre-built binary (manual download):

# macOS (Apple Silicon)
curl --proto '=https' --tlsv1.2 -LsSf https://github.com/minikin/cargo-crap/releases/latest/download/cargo-crap-aarch64-apple-darwin.tar.gz | tar xz -C ~/.cargo/bin

# macOS (Intel)
curl --proto '=https' --tlsv1.2 -LsSf https://github.com/minikin/cargo-crap/releases/latest/download/cargo-crap-x86_64-apple-darwin.tar.gz | tar xz -C ~/.cargo/bin

# Linux (x86_64)
curl --proto '=https' --tlsv1.2 -LsSf https://github.com/minikin/cargo-crap/releases/latest/download/cargo-crap-x86_64-unknown-linux-gnu.tar.gz | tar xz -C ~/.cargo/bin

# Linux (aarch64)
curl --proto '=https' --tlsv1.2 -LsSf https://github.com/minikin/cargo-crap/releases/latest/download/cargo-crap-aarch64-unknown-linux-gnu.tar.gz | tar xz -C ~/.cargo/bin

Windows: download cargo-crap-x86_64-pc-windows-msvc.zip from the latest release and extract cargo-crap.exe into a directory on your PATH.

Quick start

# 1. Generate an LCOV coverage report.
cargo llvm-cov --lcov --output-path lcov.info

# 2. Score every function.
cargo crap --lcov lcov.info

# 3. Gate CI on the threshold.
cargo crap --lcov lcov.info --fail-above

# 4. Whole-workspace analysis (monorepos).
cargo llvm-cov --workspace --lcov --output-path lcov.info
cargo crap --workspace --lcov lcov.info

# 5. Quick aggregate summary (no table).
cargo crap --workspace --lcov lcov.info --summary

Example output:

┌───┬───────┬────┬───────────────────┬──────────┬───────────────┐
│   │  CRAP │ CC │ Coverage          │ Function │ Location      │
╞═══╪═══════╪════╪═══════════════════╪══════════╪═══════════════╡
│ ✗ │ 156.0 │ 12 │ ░░░░░░░░░░   0.0% │ crappy   │ src/lib.rs:24 │
│ ▲ │   6.7 │  4 │ ████░░░░░░  44.4% │ moderate │ src/lib.rs:12 │
│ ✓ │   1.0 │  1 │ ██████████ 100.0% │ trivial  │ src/lib.rs:8  │
└───┴───────┴────┴───────────────────┴──────────┴───────────────┘
✗ 1/3 function(s) exceed CRAP threshold 30.

Flags

JSON output schema

--format json produces a versioned envelope with a $schema URL pointing at the published JSON Schema. Consumers can validate output offline or generate types directly from the schema.

// cargo crap --format json
{
  "$schema": "https://raw.githubusercontent.com/minikin/cargo-crap/main/schemas/report-v1.json",
  "version": "0.0.2",
  "entries": [
    {
      "file": "src/lib.rs",
      "function": "do_thing",
      "line": 12,
      "cyclomatic": 4.0,
      "coverage": 75.0,        // null when no coverage data was found
      "crap": 5.5625,
      "crate": "my-crate"      // present only with --workspace
    }
  ]
}

// cargo crap --format json --baseline baseline.json
{
  "$schema": "https://raw.githubusercontent.com/minikin/cargo-crap/main/schemas/delta-v2.json",
  "version": "0.0.2",
  "entries": [ /* DeltaEntry — current + baseline_crap + delta + status (+ optional previous_file when moved) */ ],
  "removed": [ /* RemovedEntry — function, file, baseline_crap */ ]
}

--baseline only reads files in this envelope shape; bare-array baselines from older runs must be regenerated.

Configuration file

Any flag can be set persistently in .cargo-crap.toml at the project root (or any parent directory — the tool walks up until it finds one). CLI flags always take precedence.

# .cargo-crap.toml
threshold = 30.0
fail-above = true
missing = "pessimistic"   # pessimistic | optimistic | skip
exclude = ["tests/**", "benches/**"]
allow   = ["generated::*"]
epsilon = 0.01            # regression-detector tolerance
jobs    = 4               # cap parallel analysis at 4 threads

All keys are optional. Unknown keys are rejected to catch typos.

Design

The tool has six orthogonal modules. Each is testable in isolation; the join between them has its own integration test.

  cargo llvm-cov                  syn
  (LCOV file)                 (Rust AST)
        │                         │
        ▼                         ▼
  ┌───────────┐            ┌────────────┐
  │ coverage  │            │ complexity │
  │  module   │            │   module   │
  └─────┬─────┘            └──────┬─────┘
        │                         │
        └──────────┬──────────────┘
                   ▼
             ┌──────────┐
             │  merge   │  ← path normalization lives here
             └─────┬────┘
                   ▼
             ┌──────────┐     ┌───────┐
             │  score   │ ──▶ │ delta │  ← baseline comparison (optional)
             └─────┬────┘     └───────┘
                   ▼
             ┌──────────┐
             │  report  │  ← human / JSON / GitHub / Markdown
             └──────────┘

The path-matching problem

This is where silent failures happen. Complexity analysis produces absolute paths (whatever was passed to the walker). LCOV files contain whatever the coverage tool decided to write:

1. Absolute paths — /home/alice/project/src/foo.rs
2. Workspace-relative paths — src/foo.rs
3. Crate-relative paths in a workspace — crates/core/src/foo.rs
4. Paths with ./ or ../ components

A naïve HashMap<PathBuf, _> lookup silently returns None for 100% of files when the two don't agree, and every function reports as 0% covered. cargo-crap handles this with a two-level index:

• Absolute coverage paths → direct canonical-path hash lookup.
• Relative coverage paths → suffix match on path components (not bytes — /foo/bar.rs must not match oofoo/bar.rs).

Relative paths are never canonicalized against the process's CWD, which would otherwise silently bind them to whatever file happened to exist under the tool's working directory. The regression test relative_coverage_paths_are_not_resolved_against_cwd in src/merge.rs pins this.

The --missing policy

Some functions have complexity data but no coverage data — the coverage tool didn't instrument them, or they were excluded via #[cfg(test)], or the coverage run was scoped to a subset of the workspace. Three policies:

• pessimistic (default): treat as 0% covered. Surfaces unmapped code as a red flag. Correct for CI gates.
• optimistic: treat as 100% covered. Useful during local development when you're iterating on a specific module.
• skip: drop the row entirely.

Integrating with CI

Absolute threshold gate

- run: cargo llvm-cov --lcov --output-path lcov.info
- run: cargo crap --lcov lcov.info --fail-above --threshold 30

Regression gate (recommended for teams)

Save a baseline on main, then fail on any PR that makes a score go up. This works regardless of the absolute threshold and catches regressions as they are introduced, not weeks later.

# On main branch — upload baseline as a CI artifact
- run: cargo llvm-cov --lcov --output-path lcov.info
- run: cargo crap --lcov lcov.info --format json --output baseline.json
- uses: actions/upload-artifact@v4
  with:
    name: crap-baseline
    path: baseline.json

# On pull requests — download baseline and compare
# NOTE: actions/download-artifact@v4 extracts to a subfolder named after the
# artifact by default — pin `path:` so the file lands somewhere predictable.
- uses: actions/download-artifact@v4
  with:
    name: crap-baseline
    path: baseline
- run: cargo llvm-cov --lcov --output-path lcov.info
- run: cargo crap --lcov lcov.info --baseline baseline/baseline.json --fail-regression

PR comment bot

--format pr-comment produces a sticky comment that surfaces regressions and new functions in the primary table and tucks improvements / removed functions / above-threshold hot-spots into collapsed <details> blocks. A hidden marker (<!-- cargo-crap-report -->) lets the script update an existing comment instead of posting duplicates. The job needs pull-requests: write.

self_score:
  permissions:
    pull-requests: write
  steps:
    # ...generate lcov.info and download the baseline as above...

    - name: Generate PR comment
      if: github.event_name == 'pull_request'
      run: |
        cargo crap \
          --lcov lcov.info \
          --baseline baseline.json \
          --format pr-comment \
          --output crap-comment.md

    - name: Post or update PR comment
      if: github.event_name == 'pull_request'
      uses: actions/github-script@v7
      with:
        script: |
          const fs = require('fs');
          const body = fs.readFileSync('crap-comment.md', 'utf8');
          const marker = '<!-- cargo-crap-report -->';
          const { data: comments } = await github.rest.issues.listComments({
            owner: context.repo.owner,
            repo: context.repo.repo,
            issue_number: context.issue.number,
          });
          const existing = comments.find(c => c.body.startsWith(marker));
          const args = {
            owner: context.repo.owner,
            repo: context.repo.repo,
            body,
          };
          if (existing) {
            await github.rest.issues.updateComment({ ...args, comment_id: existing.id });
          } else {
            await github.rest.issues.createComment({ ...args, issue_number: context.issue.number });
          }

Prior art and references

• Savoia, A. & Evans, B. (2007). The CRAP Metric.
• Crap4j — the original Java implementation.

License

This project is licensed under the MIT License - see the LICENSE file for details.