build-rust

Rust Production Patterns

Production-grade patterns that separate competent from exceptional Rust developers.

Philosophy

1. Make illegal states unrepresentable — use types to eliminate bugs (Minsky 2010)
2. Parse, don't validate — transform unstructured data into typed structures (King 2019)
3. Zero-cost abstractions — high-level code that compiles to optimal machine code (Stroustrup 2012)
4. Explicit over implicit — no hidden allocations, no surprise behavior (Rust design principle)
5. Design away lifetime complexity — if fighting the borrow checker, reconsider data model (community convention)
6. Clone consciously — every .clone() is a decision about allocation (community convention)
7. Trust but verify safety — Rust prevents data races, not deadlocks (language guarantee)

Decision Frameworks

String Ownership

Heuristic from Steve Klabnik (Rust core team, 12+ years experience): "Following this rule will get you through 95% of situations."

Context	Use	Why
Struct fields	String	Owned data lives with struct
Function params	&str	Accept any string via deref
Return (from input)	&str	Zero-cost slice
Return (newly created)	String	Caller needs ownership
Conditional modification	Cow<'_, str>	Clone-on-write

Trait Objects vs Generics

Factor	Generics	dyn Trait
Performance	Faster (static dispatch)	Slower (vtable)
Binary size	Larger	Smaller
Heterogeneous collections	No	Yes

Rule: Default to generics. Use dyn for heterogeneous collections or plugin systems.

Error Handling Selection

Writing a library?
├── YES → thiserror (callers match on variants)
└── NO (application) → Need pretty diagnostics?
    ├── YES → color-eyre (CLI) or miette (source snippets)
    └── NO → anyhow

Async vs Threads

Workload	Choice	Rule
CPU-bound	Threads / spawn_blocking	Never block async workers
High-concurrency I/O	Async	Scales to millions
Simple concurrency	Threads	Avoid async complexity

Guideline: Keep work between .await points brief (microseconds to tens of milliseconds). Tokio uses cooperative scheduling with budget-based yielding since v0.2.14 — tasks that exceed budget get nudged to yield, but long-running sync work still starves the runtime (tokio preemption blog).

Production Gotchas

Well-established patterns from tokio documentation and production experience.

Blocking in Async

• Trap: Sync operations inside async tasks starve runtime (tokio shared-state tutorial)
• Detection: tokio-console shows tasks not yielding (see "task liveliness" metrics)
• Fix: spawn_blocking() for CPU work; async alternatives for I/O

// Wrong
async fn bad() {
    std::thread::sleep(Duration::from_secs(2)); // Blocks worker
}

// Correct
async fn good() {
    tokio::task::spawn_blocking(|| heavy_computation()).await.unwrap();
}

Mutex Across Await

• Trap: std::sync::Mutex guard held across .await deadlocks (tokio shared-state tutorial)
• Detection: Deadlock under load; compiles fine (Clippy lint await_holding_lock catches this)
• Fix: Drop guard before await, or tokio::sync::Mutex

// Deadlock risk
async fn bad(mutex: Arc<std::sync::Mutex<i32>>) {
    let guard = mutex.lock().unwrap();
    some_async_op().await; // Guard held!
}

// Safe: explicit drop
async fn good(mutex: Arc<std::sync::Mutex<i32>>) {
    {
        let mut guard = mutex.lock().unwrap();
        *guard += 1;
    } // Dropped before await
    some_async_op().await;
}

Cancellation Safety

• Trap: Futures dropped mid-operation leave invalid state (tokio docs: cancel-safety)
• Detection: Check API docs for "Cancel safety" section; read is safe, read_line is NOT
• Fix: Don't hold invalid state across await; use CancellationToken (tokio-util)
• Checkpoint: Before using select! or timeout, verify each branch's cancellation safety in docs

Feature Flag Unification

• Trap: Cargo unifies features globally; non-additive features break (Cargo reference: features)
• Detection: cargo tree --edges features (Cargo book)
• Fix: Features must be additive; use default-features = false
• Checkpoint: Before adding feature-gated dependencies, run cargo tree -e features -i <crate> to check resolution

Hidden Allocations

• Trap: clone(), to_string(), format!(), Vec growth (community pattern)
• Detection: DHAT, cargo-flamegraph, Clippy perf lints
• Fix: with_capacity(), SmallVec, Cow, shrink_to_fit()

Reference Cycles

• Trap: Rc<RefCell<T>> cycles leak memory (Rust Book ch15)
• Fix: Weak<T> for back-edges; consider arenas

Obsolete Patterns

Obsolete	Replacement	Reference
lazy_static!	std::sync::LazyLock	Rust 1.80
once_cell (most uses)	std::sync::OnceLock	Rust 1.70
async-std	smol (or tokio)	Deprecated March 2025
structopt	clap v4 derive	clap 3.0 release
async-trait (some cases)	Native async fn in traits	Rust 1.75
async closure workarounds	Native async || {} closures	Rust 1.85
ansi_term	nu-ansi-term	RUSTSEC-2021-0139
wee_alloc	Default allocator or Talc	RUSTSEC-2022-0054

Note: async-trait still needed for dyn Trait with async methods.

Type Design Patterns

Newtype Pattern

Compile-time type safety for IDs:

struct UserId(u64);
struct OrderId(u64);

fn process_user(id: UserId) { /* ... */ }
// process_user(OrderId(1)); // Won't compile!

Builder Pattern

struct ConfigBuilder {
    required_field: Option<String>,
    optional_field: Option<i32>,
}

impl ConfigBuilder {
    fn required_field(mut self, val: String) -> Self {
        self.required_field = Some(val);
        self
    }

    fn build(self) -> Result<Config, BuilderError> {
        Ok(Config {
            required_field: self.required_field.ok_or(BuilderError::MissingField)?,
            optional_field: self.optional_field.unwrap_or_default(),
        })
    }
}

Use typed-builder crate in production.

Typestate Pattern

Compile-time state machine enforcement:

struct Connection<State> { /* ... */ _state: PhantomData<State> }
struct Disconnected;
struct Connected;
struct Authenticated;

impl Connection<Disconnected> {
    fn connect(self) -> Connection<Connected> { /* ... */ }
}

impl Connection<Connected> {
    fn authenticate(self, creds: &str) -> Connection<Authenticated> { /* ... */ }
}

impl Connection<Authenticated> {
    fn query(&self, sql: &str) -> Result<Data, Error> { /* ... */ }
}
// Can't call query() on unauthenticated connection - won't compile

When to use: Required state transitions, protocol implementations.

Enums Over Booleans

// Bad
fn validate(data: &str, strict: bool) { /* ... */ }

// Good
enum Validation { Strict, Lenient }
fn validate(data: &str, mode: Validation) { /* ... */ }

Error Handling

Library Errors — thiserror

#[derive(Debug, thiserror::Error)]
pub enum Error {
    #[error("invalid input: {0}")]
    InvalidInput(String),
    #[error("network error")]
    Network(#[from] std::io::Error),
    #[error(transparent)]
    Other(#[from] anyhow::Error),
}

pub type Result<T> = std::result::Result<T, Error>;

Application Errors — anyhow

use anyhow::{Context, Result, bail, ensure};

fn process() -> Result<()> {
    let data = read_file()
        .with_context(|| format!("failed to read config"))?;

    ensure!(!data.is_empty(), "config file is empty");

    if invalid(&data) {
        bail!("invalid configuration format");
    }
    Ok(())
}

Rules:

• Never .unwrap() in library code
• Never .expect() without useful message

Production Patterns

Defensive Slice Matching

// Anti-pattern: decoupled check
if !users.is_empty() { let user = users[0]; }

// Better: coupled via match
match users.as_slice() {
    [] => handle_empty(),
    [single] => handle_one(single),
    [first, ..] => handle_multiple(first),
}

Simplify Lifetimes Through Architecture

Insight: "Data lives forever or for duration of event loop."

• Use Copy IDs instead of references
• Pass references top-down each frame
• Trade-off: Lookup indirection vs lifetime elimination

Temporary Mutability Scoping

let sorted = {
    let mut temp = get_data();
    temp.sort();
    temp  // Immutable from here
};

Extension Traits

• Suffix with Ext (RFC 445)
• Export in prelude for glob import

Specialized Domains

Load reference based on project context:

Detected	Load
clap, lexopt, CLI binary	cli.md
axum, tonic, sqlx, API/service	backend.md
leptos, dioxus, wasm-bindgen, browser WASM	frontend.md
tauri, egui, desktop/mobile app	native.md
#![no_std], cortex-m, embassy, rtic	embedded.md
pingora, rama, proxy, xds	data-plane.md
bindgen, cbindgen, cxx, PyO3, unsafe	ffi-unsafe.md
proc-macro = true, syn, quote	proc-macros.md
reqwest, HTTP client, protocols	networking.md
Crate selection questions	ecosystem.md
Project setup, CI, configs	tooling.md
Deep async patterns, tokio internals	async.md

Anti-Patterns

Don't	Do	Why
.unwrap() in libs	Return Result	Callers can't recover
.clone() to fix borrow checker	Redesign ownership	Hidden allocation, wrong model
Arc<Mutex<T>> everywhere	Channels, message passing	Deadlock risk, contention
String for everything	Newtypes, enums	Type safety
pub by default	pub(crate), minimal exposure	API surface control
Hold mutex across .await	Drop before await	Deadlock
lazy_static!	LazyLock	Deprecated
Block in async	spawn_blocking	Starves runtime

Quick Reference

Commands

cargo clippy -- -W clippy::pedantic  # Lints
cargo nextest run   # Faster tests    |  cargo deny check    # Deps
cargo tree --edges features          # Feature resolution
cargo bloat --release                # Binary size

Common Crates

Need	Crate
Errors (lib)	thiserror
Errors (app)	anyhow
Serialization	serde, serde_json, toml
CLI	clap (or lexopt for minimal)
Async	tokio
HTTP	reqwest (client), axum (server)
Logging	tracing
Testing	proptest, nextest, insta