Dependencies

Prerequisites: Cargo In Depth
Next: Testing

This article covers how to find, add, and manage Rust dependencies — the equivalent of pip, PyPI, and requirements.txt.

---

Finding Packages

• crates.io — the official Rust package registry (like PyPI)
• lib.rs — better search UI and category browsing
• docs.rs — auto-generated API docs for every published crate version

Search from the terminal:

cargo search "json parser"

---

Adding Dependencies

# Add a dependency (updates Cargo.toml and fetches the crate)
cargo add serde --features derive
cargo add serde_json
cargo add tokio --features full

# Remove a dependency
cargo remove tokio

# Update all dependencies to latest compatible versions (respects semver)
cargo update

# Update one specific dependency
cargo update -p serde

# See the full dependency tree (like pipdeptree)
cargo tree

# Check for outdated dependencies (needs cargo-outdated)
cargo install cargo-outdated
cargo outdated

# Check for known security vulnerabilities
cargo install cargo-audit
cargo audit

---

Python to Rust Crate Analogues

Python library	Rust crate	Purpose
json	serde_json	JSON parse / serialise
pydantic	serde + derive	Struct ↔ JSON/YAML/etc.
requests	reqwest	HTTP client
asyncio	tokio	Async runtime
click / argparse	clap	CLI argument parsing
logging	tracing	Structured logging
datetime	chrono	Date and time
pathlib	std::path::PathBuf	File paths (stdlib)
re	regex	Regular expressions
uuid	uuid	UUID generation
python-dotenv	dotenvy	.env file loading
psycopg2	sqlx	PostgreSQL async
redis-py	redis	Redis client
pytest	built-in #[test] + cargo test	Testing
PyO3 / cffi	pyo3	Python ↔ Rust bridge

---

Specifying Version Requirements

Rust uses semantic versioning. In Cargo.toml:

[dependencies]
serde = "1"          # >= 1.0.0, < 2.0.0  (compatible release)
serde = "1.0"        # >= 1.0.0, < 1.1.0  (more restrictive)
serde = "=1.0.195"   # exact version
serde = ">=1.0, <2"  # explicit range
serde = "*"          # any version (avoid — too permissive)

Features

Some crates have optional features (like extras in Python):

[dependencies]
serde      = { version = "1", features = ["derive"] }   # enables #[derive(Serialize, Deserialize)]
tokio      = { version = "1", features = ["full"] }     # enables all tokio features
pyo3       = { version = "0.20", features = ["extension-module"] }

# Add a crate with a specific feature enabled
cargo add serde --features derive

---

Cargo.lock — Exact Reproducible Builds

Cargo.lock records the exact version of every transitive dependency, like pip freeze. Cargo updates it whenever you run cargo build with new or changed dependencies.

Convention:

• Binary projects (executables): commit Cargo.lock so every developer and CI run uses identical versions.
• Library crates (no fn main): .gitignore Cargo.lock — let downstream users pick compatible versions within your declared ranges.

logferry is a library (from Cargo's perspective), but it has a Python-facing binary build. Committing Cargo.lock is reasonable either way.

---

Dev and Build Dependencies

# Dev dependencies — used in tests and examples, not shipped to users
[dev-dependencies]
pretty_assertions = "1"   # nicer diff output in test failures
tempfile          = "3"   # temporary files and directories for tests

# Build dependencies — used by build.rs scripts only
[build-dependencies]
cc = "1"   # C compiler wrapper (for crates with C code)

---

Private and Path Dependencies

[dependencies]
# Local path — useful for multi-crate workspaces or developing two crates together
metrics_lib = { path = "../metrics_lib" }

# Git dependency — pin to a specific commit or tag
my_crate = { git = "https://github.com/user/my_crate", tag = "v1.2.0" }

---

Workspaces

For projects with multiple related crates (e.g. a binary + a library), use a Cargo workspace:

# Workspace-level Cargo.toml (at the repo root)
[workspace]
members = [
    "logferry",
    "logferry-cli",
    "metrics_lib",
]

# Build all workspace members
cargo build --workspace

# Test all workspace members
cargo test --workspace

---

See Also

• Testing — using dev dependencies in tests
• Cargo Cheatsheet — all cargo commands
• Python → Rust Cheatsheet — full library analogy table