cargo-machete - Unused Dependency Detection

Detect and remove unused dependencies in Rust projects using cargo-machete.

When to Use This Skill

Use this skill when... Use X instead when...

Auditing for unused dependencies Checking for outdated deps -- use cargo outdated

Cleaning up Cargo.toml Auditing security vulnerabilities -- use cargo audit

Optimizing build times by removing bloat Checking license compliance -- use cargo deny

Verifying deps in CI Need nightly-accurate analysis -- use cargo +nightly udeps

Context

• Cargo.toml: !find . -maxdepth 1 -name 'Cargo.toml'

• Workspace: !grep -q '[workspace]' Cargo.toml

• cargo-machete installed: !cargo machete --version

• Machete config: !find . -maxdepth 1 -name '.cargo-machete.toml'

• Workspace members: !grep -A 20 '^[workspace]' Cargo.toml

Execution

Execute this unused dependency analysis:

Step 1: Verify installation

Check if cargo-machete is installed (see Context above). If not installed, install it:

cargo install cargo-machete

Step 2: Run dependency analysis

Run cargo-machete with metadata for detailed output:

Single crate

cargo machete --with-metadata

Workspace (if Context shows workspace)

cargo machete --workspace --with-metadata

Step 3: Evaluate results

Review the output and classify each finding:

Finding Action

Genuinely unused dependency Remove with --fix or manually

Proc-macro dependency (e.g., serde derive) Add machete:ignore comment

Build.rs-only dependency Move to [build-dependencies]

Re-exported dependency Add machete:ignore comment with explanation

Step 4: Apply fixes

For confirmed unused dependencies, auto-remove them:

cargo machete --fix

For false positives, add ignore annotations in Cargo.toml :

serde = "1.0" # machete:ignore - used via derive macro

Or create .cargo-machete.toml for project-wide ignores:

[ignore] dependencies = ["serde", "log"]

Step 5: Verify build

After removing dependencies, confirm everything still compiles:

cargo check --all-targets cargo test --no-run

False Positive Handling

Scenario Solution

Proc-macro deps (e.g., serde derive) machete:ignore comment

Build.rs-only deps Move to [build-dependencies]

Re-exported deps machete:ignore comment with explanation

Example/bench-only deps Verify in [dev-dependencies]

Comparison with cargo-udeps

Feature cargo-machete cargo-udeps

Accuracy Good Excellent

Speed Very fast Slower

Rust version Stable Requires nightly

False positives Some Fewer

Use cargo-machete for fast CI checks. Use cargo-udeps for thorough audits:

cargo +nightly udeps --workspace --all-features

Agentic Optimizations

Context Command

Quick check cargo machete

Detailed check cargo machete --with-metadata

Workspace check cargo machete --workspace --with-metadata

Auto-fix cargo machete --fix

Verify after fix cargo check --all-targets

Accurate check (nightly) cargo +nightly udeps --workspace

Quick Reference

Flag Description

--with-metadata

Show detailed output with versions and locations

--fix

Auto-remove unused dependencies from Cargo.toml

--workspace

Check all workspace members

-p <crate>

Check specific workspace member

--exclude <crate>

Exclude specific workspace member

Troubleshooting

Problem Solution

Proc macro flagged as unused Add machete:ignore comment in Cargo.toml

Build.rs dep flagged Move to [build-dependencies]

Re-exported dep flagged Add machete:ignore with explanation

Need more accuracy Use cargo +nightly udeps

For CI integration patterns, configuration file examples, pre-commit setup, and Makefile integration, see REFERENCE.md.