cargo-crap

[!TIP] For more context on the motivation behind this crate, read: cargo-crap: Finding Untested Complexity in AI-Generated Rust Code or watch Your AI Code Might Be CRAP! (Here's How To Fix It).

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

From the AUR:

paru -S 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

# 6. Only selected workspace members (changed-file CI).
cargo crap -p backend_core -p backend_identity --lcov lcov.info

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

| Flag | Default | Purpose | | ---------------------------------------------------------------- | ------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | --lcov <FILE> | — | LCOV file from cargo llvm-cov or cargo tarpaulin. | | --path <DIR> | . | Root to walk for .rs files (respects .gitignore). | | --threshold <N> | 30 | Score above which a function is flagged. | | --min <SCORE> | — | Hide entries below this score. | | --top <N> | — | Show only the N worst offenders. | | --sort {crap,file} | crap | Final ordering of entries. crap sorts by score descending (best for reading top-down). file sorts by (file, function, line) ascending — stable across score changes, so a committed JSON baseline produces minimal diffs. --top always selects the N highest-CRAP functions first, then --sort reorders them. Applies to every format. | | --missing {pessimistic,optimistic,skip}