Idiomatic Rust

Idiomatic Rust

This skill is about expression: what form does a Rust reader expect to see?

The shape of idiomatic Rust

Prefer expressions over statements: a match that returns a value beats one that assigns to a let mut in every arm, and a chain that produces the answer beats a flag variable set in a loop and checked after it — let the type system carry invariants instead of runtime checks. let ... else keeps an early return at the top instead of drifting the happy path rightward under a nested if let; matches! is the boolean test on a pattern. If-let chains compose conditions where the toolchain supports them — a recent edition, so check the repo MSRV first.

Iterators

Reach for the iterator pipeline before the index loop — for i in 0..v.len() { ... v[i] ... } almost always has an iterator equivalent, and the iterator version fails to compile on bad bounds instead of panicking at runtime. Collect into the type you want, not a Vec you then convert:

// Reads like a translation.
let mut names = Vec::new();
for user in &users {
    names.push(user.name.clone());
}

// Reads like Rust.
let names: Vec<String> = users.iter().map(|u| u.name.clone()).collect();

collect also targets HashMap, HashSet, String, and Result<Vec<_>, _> — that last is how ? composes with iteration.

use std::num::ParseIntError;

fn parse_all(lines: &[&str]) -> Result<Vec<i64>, ParseIntError> {
    lines.iter().map(|line| line.parse::<i64>()).collect::<Result<Vec<_>, _>>()
}

Prefer the plain for loop when the body has real side effects or an early exit a combinator would obscure — clarity beats iterator purity.

Conversions

When one type can be built from another, implement From — Into comes free through the blanket impl, so never write both directions by hand. When the conversion can fail, implement TryFrom instead of From plus a panic or a sentinel value.

struct Celsius(f64);
struct Fahrenheit(f64);

impl From<Celsius> for Fahrenheit {
    fn from(c: Celsius) -> Self {
        Fahrenheit(c.0 * 9.0 / 5.0 + 32.0)
    }
}

Derives before hand-written impls

Reach for #[derive(Debug, Clone, PartialEq, Default, Hash)] before writing it by hand — a derived impl is generated from the actual fields, so it stays in sync as the struct changes. Write the manual version only when the derive would be wrong:

• A Default that must preserve an invariant the field-wise default breaks (a Percentage summing to 100, not zero per field).
• A Debug that must redact a secret field (an API key, a password) rather than print it.
• PartialEq where logically-equal values can differ in a field that shouldn't count (a cached hash, a timestamp).

For an enum, #[derive(Default)] with #[default] on the intended variant is the same rule — BOILERPLATE.md in this directory has the pair.

Newtypes

A newtype costs nothing at runtime and makes a swapped-argument call a type error. Reach for one wherever a bare u64, String, or f64 stands in for a domain concept (an ID, a currency amount) rather than a number.

Naming

Names earn every word: Bookings, not BookingService — Service, Manager, Helper, Util, Data, and Info mark a name nobody finished. The as_/to_/into_ prefixes promise a cost — free view, allocation, consumption. No get_ on a field accessor; acronyms are words (HttpClient, Uuid); a helper that does not need Self is a free function, not an impl block used as a namespace. The tiers table and worked pairs live in NAMING.md.

Boilerplate that is actually a design signal

Not all repetition is a derive away from disappearing — five near-identical match arms, or a wrapper with ten forwarding methods, can point at a missing abstraction. See BOILERPLATE.md for the pairs, including the Deref trap.

Deferrals

This skill governs expression, not decision-making: the clone-versus-borrow call is /ownership-not-clone, error type shape is /rust-errors, and making an illegal state unrepresentable is /type-driven-design.

Verification

Same behavior, different shape — run the standard trio:

cargo clippy --all-targets   # add -- -D warnings only if the repo has no lint config
cargo fmt --check
cargo test

Run the trio at the lint level the repo configures; treat clippy::pedantic findings as proposals, not violations, unless the repo opted into that group.