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| Auditing for unused dependencies | Checking for outdated deps -- use |
| Cleaning up Cargo.toml | Auditing security vulnerabilities -- use |
| Optimizing build times by removing bloat | Checking license compliance -- use |
| Verifying deps in CI | Need nightly-accurate analysis -- use |
Context
- Cargo.toml: !
test -f Cargo.toml && echo "EXISTS" || echo "MISSING"
- Workspace: !
grep -q '\[workspace\]' Cargo.toml 2>/dev/null && echo "YES" || echo "NO"
- cargo-machete installed: !
cargo machete --version 2>/dev/null || echo "NOT INSTALLED"
- Machete config: !
test -f .cargo-machete.toml && echo "EXISTS" || echo "MISSING"
- Workspace members: !
grep -A 20 '^\[workspace\]' Cargo.toml 2>/dev/null | grep -o '"[^"]*"' 2>/dev/null
Execution
Execute this unused dependency analysis:
Step 1: Verify installation
Check if cargo-machete is installed (see Context above). If not installed, install it:
bash
cargo install cargo-machete
Step 2: Run dependency analysis
Run cargo-machete with metadata for detailed output:
bash
# 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 or manually |
| Proc-macro dependency (e.g., serde derive) | Add comment |
| Build.rs-only dependency | Move to |
| Re-exported dependency | Add comment with explanation |
Step 4: Apply fixes
For confirmed unused dependencies, auto-remove them:
For false positives, add ignore annotations in
:
toml
serde = "1.0" # machete:ignore - used via derive macro
Or create
for project-wide ignores:
toml
[ignore]
dependencies = ["serde", "log"]
Step 5: Verify build
After removing dependencies, confirm everything still compiles:
bash
cargo check --all-targets
cargo test --no-run
False Positive Handling
| Scenario | Solution |
|---|
| Proc-macro deps (e.g., serde derive) | comment |
| Build.rs-only deps | Move to |
| Re-exported deps | comment with explanation |
| Example/bench-only deps | Verify in |
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:
bash
cargo +nightly udeps --workspace --all-features
Agentic Optimizations
| Context | Command |
|---|
| Quick check | |
| Detailed check | cargo machete --with-metadata
|
| Workspace check | cargo machete --workspace --with-metadata
|
| Auto-fix | |
| Verify after fix | cargo check --all-targets
|
| Accurate check (nightly) | cargo +nightly udeps --workspace
|
Quick Reference
| Flag | Description |
|---|
| Show detailed output with versions and locations |
| Auto-remove unused dependencies from Cargo.toml |
| Check all workspace members |
| Check specific workspace member |
| Exclude specific workspace member |
Troubleshooting
| Problem | Solution |
|---|
| Proc macro flagged as unused | Add comment in Cargo.toml |
| Build.rs dep flagged | Move to |
| Re-exported dep flagged | Add with explanation |
| Need more accuracy | Use |
For CI integration patterns, configuration file examples, pre-commit setup, and Makefile integration, see REFERENCE.md.