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Auditing for unused dependencies	Checking for outdated deps -- use cargo outdated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