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| Auditing for unused dependencies | Checking for outdated deps -- use cargo outdated |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-macheteStep 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-metadataStep 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 --fixFor false positives, add ignore annotations in Cargo.toml:
serde = "1.0" # machete:ignore - used via derive macroOr 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-runFalse 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-featuresAgentic 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.