Cargo & crates

One tool runs the entire workflow: build, test, doc, publish, format, lint. crates.io is the central registry. Cargo.toml declares dependencies; Cargo.lock pins exact versions.

1 · The commands you'll use daily

Command	What it does
`cargo new myapp`	Scaffold a new binary project (use `--lib` for library).
`cargo run`	Build (if needed) and run the binary.
`cargo build [--release]`	Build. `--release` optimises; default is fast-compile debug.
`cargo test`	Run all tests, including doc tests.
`cargo check`	Type-check only. Faster than full build.
`cargo doc --open`	Build HTML docs and open them.
`cargo fmt`	Format the whole tree to canonical style.
`cargo clippy`	The lint pass. Often catches subtle issues.
`cargo add serde`	Add a dependency to `Cargo.toml`.
`cargo update`	Refresh `Cargo.lock` within the version ranges in `Cargo.toml`.

2 · Cargo.toml — the manifest

toml Cargo.toml

1234567891011121314

[package]
name    = "myapp"
version = "0.1.0"
edition = "2021"

[dependencies]
serde      = { version = "1", features = ["derive"] }
tokio      = { version = "1", features = ["full"] }
reqwest    = "0.12"
anyhow     = "1"
thiserror  = "1"

[dev-dependencies]
proptest = "1"

Semver caret. "1" means "anything 1.x.y but not 2.0". For exact pins, use "=1.2.3". Cargo.lock stores the resolved versions and should be checked in for binaries, ignored for libraries.

3 · Workspaces — many crates, one repo

toml Cargo.toml (workspace root)

[workspace]
members = ["api", "core", "cli"]
resolver = "2"

[workspace.dependencies]
serde = { version = "1", features = ["derive"] }
tokio = { version = "1", features = ["full"] }

Shared target/, shared lockfile. Inside each sub-crate, depend on workspace deps with serde = { workspace = true }.

4 · Crates worth knowing

Crate	For
`serde` + `serde_json`	JSON, YAML, etc. The serialization standard.
`tokio`	Async runtime. The default choice.
`reqwest`	HTTP client.
`axum` / `actix-web`	HTTP servers.
`clap`	CLI argument parsing.
`anyhow` + `thiserror`	Errors. Pair them.
`tracing`	Structured logging and spans.
`sqlx` / `diesel`	Database access.
`rayon`	Easy data parallelism — replace `.iter()` with `.par_iter()`.

5 · Common mistakes

Forgetting feature flags. Many crates ship with minimal features by default — tokio needs features = ["full"] for the macros.
Running cargo build when you meant --release. Debug builds are easily 10× slower at runtime.
Committing target/. It's huge. Make sure it's in .gitignore (cargo new does this).
Ignoring clippy. It catches real bugs, not just style.

6 · When it clicks

cargo check is your inner loop, not cargo run.
You add a crate with cargo add foo without thinking about it.
cargo clippy runs in CI on every PR.

Next · Day 7 · Concept 19

Testing & benchmarks

#[test] inside the module. cargo test runs everything. doc tests too.

Next concept

Found this useful?