Rust Language Idioms

Work with the language, not against it. Embrace ownership, the borrow checker, and the type system.

---

Core Values

• Safety: Lean on the compiler's guarantees. Avoid unsafe unless absolutely necessary.
• Memory Efficiency: Zero-cost abstractions. Avoid unnecessary allocations.
• Ergonomics: APIs should be intuitive and hard to misuse.

---

API Design

Naming Conventions

Prefix	Semantics	Cost	Example
as_	Borrowed view, no allocation	Cheap	as_ref(), as_slice()
to_	Creates new owned value	Expensive	to_string(), to_vec()
into_	Consumes self, transforms	Cheap	into_iter(), into_inner()

Argument Flexibility

Accept generic bounds instead of concrete types:

// ❌ Inflexible
fn read_file(path: &PathBuf) -> Result<String>

// ✅ Accepts &str, String, PathBuf, &Path
fn read_file(path: impl AsRef<Path>) -> Result<String>

Common patterns:

• impl AsRef<Path> — for paths
• impl AsRef<str> — for string-like types
• impl Into<String> — when ownership is needed

Constructors

• Simple: T::new() for basic instantiation
• Configured: T::with_capacity(n) for single-option variants
• Complex: Use the Builder pattern for 3+ optional fields

let config = ConfigBuilder::new()
    .port(8080)
    .timeout(Duration::from_secs(30))
    .build()?;

Monomorphization Control

For widely-used generic functions, consider splitting:

// Public generic wrapper (thin)
pub fn process(path: impl AsRef<Path>) -> Result<()> {
    process_inner(path.as_ref())
}

// Private concrete implementation (bulk of logic)
fn process_inner(path: &Path) -> Result<()> {
    // ...
}

This reduces binary bloat and compile times.

---

Error Handling

The ? Operator

Use ? to propagate errors. It:

1. Unwraps Ok(value) or returns Err(e) early
2. Automatically converts via From trait

Library vs. Application Errors

Context	Approach	Crate
Libraries	Custom enum error types	thiserror
Applications	Boxed errors with context	anyhow

// Library: structured, matchable errors
#[derive(Debug, thiserror::Error)]
pub enum ParseError {
    #[error("invalid header at byte {0}")]
    InvalidHeader(usize),
    #[error("unexpected EOF")]
    UnexpectedEof,
}

// Application: rich context chains
fn main() -> anyhow::Result<()> {
    let config = load_config()
        .context("failed to load configuration")?;
    Ok(())
}

Parse, Don't Validate

Encode invariants in the type system:

// ❌ Stringly-typed, requires repeated validation
fn send_email(to: &str) -> Result<()>

// ✅ Invalid states are unrepresentable
fn send_email(to: EmailAddress) -> Result<()>

The constructor for EmailAddress validates once; all downstream code operates with a compile-time guarantee.

---

Iterators

Iterators are lazy and zero-cost. Build declarative pipelines:

let result: Vec<_> = items
    .iter()
    .filter(|x| x.is_valid())
    .map(|x| x.transform())
    .collect();

Key Adapters

Adapter	Purpose
map	Transform each element
filter	Keep elements matching predicate
filter_map	Filter and transform in one step
flat_map	Map then flatten nested iterators
zip	Combine two iterators into pairs
chain	Concatenate iterators
take / skip	Limit or offset elements
inspect	Debug: observe without modifying

Pitfalls

• Iterators are lazy—no work until consumed (collect, sum, for_each)
• Avoid side effects in map; use for_each or explicit loops
• Use inspect() to debug intermediate pipeline stages

---

Concurrency

Thread Safety Markers

Trait	Meaning
Send	Ownership can transfer between threads
Sync	References (&T) can be shared between threads

Most types are automatically Send + Sync. Types containing raw pointers or interior mutability may not be.

Common Primitives

• Mutex<T> — mutual exclusion
• RwLock<T> — multiple readers OR single writer
• Arc<T> — atomic reference counting for shared ownership
• mpsc::channel — message passing

Anti-Pattern: if let Deadlock (pre-2024 edition)

// ❌ Lock held through else block in older editions
if let Some(x) = mutex.lock().unwrap().get("key") {
    // ...
} else {
    mutex.lock().unwrap().insert("key", value); // DEADLOCK
}

// ✅ Extract lock explicitly
let guard = mutex.lock().unwrap();
let result = guard.get("key").cloned();
drop(guard); // Release before else

Fixed in Rust 2024 edition.

---

Type System

Newtypes

Wrap primitives to create distinct types:

struct UserId(u64);
struct PostId(u64);

// Now UserId and PostId are incompatible at compile time

#[must_use]

Apply to functions/types where ignoring the result is almost always a bug:

#[must_use = "this returns a new value and does not modify self"]
fn sorted(&self) -> Vec<T>

impl Trait in Return Position

Hide concrete types when they're implementation details:

fn make_iter() -> impl Iterator<Item = u32> {
    (0..10).filter(|x| x % 2 == 0)
}

---

Anti-Patterns

Anti-Pattern	Description	Remedy
Overengineering	Complex abstractions, premature unsafe, macro abuse	Prefer simple, direct solutions
Simplistic Design	Failing to leverage type system (stringly-typed, bool-blind)	Use enums/newtypes to encode invariants
Premature Optimization	Optimizing without profiling	Write clear code first; measure, then optimize
Documentation Neglect	Missing or stale docs	Doc-comments (///) with examples; run cargo doc

---

Blessed Crates

Domain	Crate	Notes
Serialization	serde + serde_json	De facto standard
Async Runtime	tokio	Most widely supported
CLI Parsing	clap	Feature-rich, fast
Error (Library)	thiserror	Derive for custom errors
Error (Application)	anyhow	Context chains, backtraces
HTTP Client	reqwest	Built on tokio
Logging	tracing	Structured, async-aware

---

Quick Reference

• No .unwrap() in library code — propagate Result
• No .clone() without justification — prefer references
• No panic! in library code — return errors
• Use clippy — cargo clippy -- -D warnings
• Format with rustfmt — cargo fmt
• Document public APIs — /// with examples

These idioms refine but are subordinate to the Code-Edit Constraints.