senior-rust-practices

What this skill does

# Senior Rust Development Practices

Battle-tested patterns for Rust workspace architecture, code organization, dependencies, and testing that scale from prototype to production.

## Git Worktree Workflow Compliance

**All coding work MUST happen in git worktrees.** Before making any code changes:

1. Create a worktree: `git worktree add ~/.claude/worktrees/$(basename $(pwd))/<task> -b feat/<task>`
2. Work in that directory
3. Use `/merge` to consolidate changes back to main

Never edit files directly in the main worktree.

## Completion Requirements

**Before completing ANY Rust task, you MUST:**

1. Run tests: `cargo test --workspace`
2. Run linting: `trunk check`
3. Fix any issues before declaring done

If trunk has formatting issues, run `trunk fmt` to auto-fix.

## Workspace Architecture

### Start from "One Product = One Repo = One Workspace"

Use a Rust workspace when you have:

- Multiple crates that ship together (binary + libraries)
- Shared tooling / CI
- Shared versioning policy

**Canonical workspace structure:**

```text
repo/
  Cargo.toml            # workspace root
  crates/
    core/               # pure domain logic (no IO)
    storage/            # DB, filesystem, etc.
    api/                # HTTP/GRPC handlers, DTOs
    cli/                # binary
  tools/                # optional: internal binaries (codegen, migration, etc.)
  tests/                # optional: black-box integration tests
```

### Keep Crates "Thin" and Boundaries "Hard"

**Layered architecture:**

- **core**: Pure logic, types, validation, algorithms. Minimal deps.
- **adapters**: IO boundaries (db, network, rpc, filesystem). Trait-based boundary, minimal leakage.
- **app / service**: Wiring (DI), config, runtime, orchestration.
- **bins**: CLI/daemon that just calls "app".

**Critical rule:** If `core` imports `tokio`, `reqwest`, or `sqlx`, you've already lost the separation.

### Default to a Small Number of Crates

Too many crates is busywork. Start with 2–5 max.

**Split only when:**

- Compile times are painful and boundaries are real
- You need separate release cadence
- You need different dependency profiles (no-std, wasm, etc.)

### Workspace Dependencies: Centralize Versions, Not Architecture

In root `Cargo.toml`, use workspace dependencies to keep versions aligned:

```toml
[workspace]
members = ["crates/*"]
resolver = "2"

[workspace.dependencies]
anyhow = "*"  # use latest
thiserror = "*"  # use latest
serde = { version = "*", features = ["derive"] }  # use latest
tokio = { version = "*", features = ["macros", "rt-multi-thread"] }  # use latest
```

In crate `Cargo.toml`:

```toml
[dependencies]
serde = { workspace = true }
```

This reduces version drift and security churn.

### Be Ruthless with Features

- Prefer additive features (enable more capabilities) vs "feature flags that change semantics"
- Put "heavy" deps behind features (db, http, metrics)
- Avoid default features that pull the world

**Pattern for optional dependencies:**

```toml
[dependencies]
sqlx = { workspace = true, optional = true }

[features]
db = ["dep:sqlx"]
```

### Enforce a Policy: MSRV + Toolchain

- Pin toolchain with `rust-toolchain.toml`
- Decide MSRV (minimum supported Rust version) and test it in CI
- Keep clippy/rustfmt consistent

## Code Organization

### Modules Should Match How You Reason, Not File-Per-Type

Organize by capability / domain, not by "models/handlers/utils" spaghetti.

**Good organization:**

```text
core/
  src/
    lib.rs
    payment/
      mod.rs
      validation.rs
      pricing.rs
    user/
      mod.rs
      id.rs
      rules.rs
```

**Avoid:**

```text
models.rs
handlers.rs
utils.rs
```

### Public API: Small Surface Area, Explicit Re-Exports

- Make most things `pub(crate)` by default
- Re-export a curated API from `lib.rs`

```rust
mod payment;
pub use payment::{Payment, PaymentError};
```

If everything is `pub` you've created an accidental framework.

### Avoid "Prelude" Unless You Truly Need It

Preludes tend to hide dependencies and make code review harder. Prefer explicit imports.

### Error Strategy: Pick One and Stick to It

**Common approach:**

- Library crates: `thiserror` for typed errors
- Binaries: `anyhow` at the top level

Don't leak `anyhow::Error` across library boundaries unless you explicitly want "opaque".

### Keep Async at the Edge

If you can keep core synchronous and pure, you gain:

- Simpler tests
- Portability
- Less lifetime/pinning headaches

## Dependency Hygiene

### Be Picky: Fewer Deps, Higher-Quality Deps

Every dependency adds:

- Build time
- Audit surface
- Semver risk

Prefer "boring" crates with strong maintenance.

### Use `cargo-deny` + `cargo-audit`

Make dependency issues visible early (licenses, advisories, duplicate versions).

### Don't Use `unwrap()` in Libraries

In binaries/tests it's fine (especially in test scaffolding). In libraries, return errors with context.

## Testing Strategy That Scales

Think "pyramid":

### 1. Unit Tests: Fast, Deterministic, Lots

- Put most tests close to code: `mod tests {}` in the same file for private access
- Test invariants and edge cases, not just happy paths
- Avoid hitting the filesystem/network in unit tests

### 2. Integration Tests: Black-Box the Public API

Use `crates/<crate>/tests/*.rs` for API-level tests.

- Treat it as "a consumer of the crate"
- Don't reach into private internals

### 3. End-to-End Tests: Few, But Real

If you have a service:

- Spin up dependencies (db) in CI (containers)
- Run a small set of scenario tests

### 4. Property Tests + Fuzzing When Correctness Matters

- `proptest` for invariants ("decode(encode(x)) == x")
- `cargo-fuzz` for parsers/decoders/inputs from outside

### 5. Doctests Are Underrated

Doctests enforce that examples compile and keep your public API honest.

## Logging and Tracing

### Never Use `println!` - Use Tracing Instead

**NEVER use `println!`, `eprintln!`, or `dbg!` for output.** Always use the `tracing` crate:

```rust
use tracing::{debug, info, warn, error, trace};

// Good - structured logging
info!("Processing request for user {user_id}");
debug!("Cache hit: {key}");
warn!("Retry attempt {attempt} of {max_retries}");
error!("Failed to connect: {err}");

// Bad - never do this
println!("Processing request for user {}", user_id);
dbg!(value);
```

**Why:**

- Structured logging with levels (filter noise in production)
- Spans for distributed tracing
- Configurable output (JSON, pretty, etc.)
- Zero-cost when disabled

### Use `test-log` for Tests

Always use `test_log::test` attribute for tests to capture tracing output:

```rust
use test_log::test;

#[test]
fn test_something() {
    info!("This will be visible when test fails or with --nocapture");
    assert!(true);
}

#[test(tokio::test)]
async fn test_async_something() {
    debug!("Async test with tracing");
}
```

Add to `Cargo.toml` (use latest versions):

```toml
[dev-dependencies]
test-log = { version = "*", features = ["trace"] }  # use latest
tracing-subscriber = { version = "*", features = ["env-filter"] }  # use latest
```

Run tests with visible logs: `RUST_LOG=debug cargo test -- --nocapture`

## Clippy Rules to Follow

### Inline Format Arguments (`clippy::uninlined_format_args`)

Always use variables directly in format strings instead of passing them as arguments:

```rust
// Good - variable inlined
let name = "world";
info!("Hello, {name}!");
format!("Value: {value}, Count: {count}")

// Bad - uninlined arguments
info!("Hello, {}!", name);
format!("Value: {}, Count: {}", value, count)
```

This improves readability and reduces potential argument ordering mistakes.

## CI / Quality Gates (Minimum Set)

```bash
cargo fmt --check
cargo clippy --all-targets --all-features -D warnings
cargo test --workspace --all-features
```

**Additional gates:**

- MSRV check (if you claim one)
- `cargo deny` / `cargo audit`
- (optional) `cargo llvm-cov` for coverage, but don't worship %

## Compile Times and Ergonomics

- Use `resolver =