crap4rust

crap4rust computes CRAP scores for Rust functions by combining complexity and test coverage.

Definition

What is a CRAP score? CRAP (Change Risk Anti-Patterns) combines cognitive complexity and test coverage: CRAP(m) = comp(m)² × (1 − cov(m))³ + comp(m). Functions above a score of 30 are flagged as crappy — they are complex enough that their lack of test coverage makes them a maintenance risk.

It is published as the Cargo subcommand package cargo-crap4rust, so the command is cargo crap4rust.

Current status and release notes:

• IMPLEMENTED-FEATURES.md documents what 0.6.0 supports today
• ROADMAP.md tracks planned capabilities beyond the first release
• CHANGELOG.md records released versions

Install

cargo install cargo-crap4rust

License

Licensed under:

• LICENSE

What It Does

• Computes a CRAP score for each discovered Rust function
• Generates coverage automatically with cargo llvm-cov when --coverage is omitted
• Prints a single report to the console
• Supports multiple --package flags for one aggregated report
• Defaults to analysing all workspace members when --package is omitted in a multi-package workspace
• Does not count try-operator propagation with ? as cognitive complexity
• Supports --output-format json for structured CI-friendly output
• Supports --warn-threshold to set the warning level independently from the crappy threshold
• Cognitive complexity scoring lives in its own dedicated module

Examples

Analyse the default scope for a manifest:

• single-package manifest: analyses the root package
• multi-package workspace: analyses all workspace members unless --package is provided

cargo crap4rust --manifest-path C:\Projects\my-workspace\Cargo.toml

Analyse one specific package in a workspace and override the default all-members selection:

cargo crap4rust --manifest-path C:\Projects\my-workspace\Cargo.toml --package app-core

Analyse multiple packages and produce one combined console report:

cargo crap4rust --manifest-path C:\Projects\my-workspace\Cargo.toml --package app-core --package app-validation

Use a precomputed coverage export instead of generating coverage automatically:

cargo crap4rust --manifest-path C:\Projects\my-workspace\Cargo.toml --package app-core --coverage C:\Projects\my-workspace\target\coverage.json

Use stricter project thresholds:

cargo crap4rust --manifest-path C:\Projects\my-workspace\Cargo.toml --threshold 25 --project-threshold 3.0 --strict

Pass Cargo feature flags to the coverage build:

cargo crap4rust --manifest-path C:\Projects\my-workspace\Cargo.toml --package app-core --features host-tests

Disable default features and enable specific ones:

cargo crap4rust --manifest-path C:\Projects\my-workspace\Cargo.toml --package app-core --no-default-features --features host-analysis

Include test targets in the analysis:

cargo crap4rust --manifest-path C:\Projects\my-workspace\Cargo.toml --package app-validation --include-test-targets

Exclude specific source paths from analysis:

cargo crap4rust --manifest-path C:\Projects\my-workspace\Cargo.toml --package app-core --exclude-path src/scenarios

Output as JSON for CI pipelines:

cargo crap4rust --manifest-path C:\Projects\my-workspace\Cargo.toml --output-format json

Set a custom warning threshold:

cargo crap4rust --manifest-path C:\Projects\my-workspace\Cargo.toml --warn-threshold 15

Real Workspace Example

Example run against the Etheram workspace:

cargo crap4rust --manifest-path C:\Projects\etheram\Cargo.toml --package etheram-node --package etheram-validation

Report excerpt:

crap4rust report for etheram-node, etheram-validation

Package	Function	File	Line	Complexity	Coverage	CRAP	Verdict
etheram-node	ConsensusWal::from_bytes	consensus_wal.rs	115	72	41.7%	1099.3	crappy
etheram-node	execute_bytecode	tiny_evm_engine.rs	572	39	43.3%	316.1	crappy
etheram-node	RecoveryImportValidator::validate_response	recovery_import_validator.rs	13	21	47.9%	83.3	crappy
etheram-node	IbftProtocol::handle_client_message	ibft_protocol_dispatch.rs	82	19	47.1%	72.6	crappy
etheram-node	exec_sha3	tiny_evm_engine.rs	345	12	30.3%	60.7	crappy

Summary: total_functions=388, crappy_functions=12, crappy_percent=3.1%, threshold=30.0, project_threshold=5.0%, verdict=warn. Production functions only — test code and generated code excluded by default.

The report above is abbreviated to the highest-scoring rows, with function names and file paths shortened for readability. When coverage is generated automatically, cargo llvm-cov also emits normal build and test output before the final crap4rust report.

Try-operator propagation with ? is treated as error forwarding rather than decision-making complexity, so CRAP scoring reflects branching and control-flow structure instead of penalising straightforward Result propagation.

Current Scope

The current implementation focuses on:

• console and JSON reporting
• automatic cargo llvm-cov integration
• internal cognitive-complexity scoring in a dedicated module
• workspace package selection and aggregation
• all-workspace-member default selection when --package is omitted in multi-package workspaces
• try-operator propagation excluded from cognitive-complexity scoring
• configurable warning threshold independent from the crappy threshold

See IMPLEMENTED-FEATURES.md for the shipped feature set and ROADMAP.md for the broader plan.