Rust core language

<objective> This skill covers idiomatic Rust for stable 1.96.1, edition 2024: designing function signatures and data flow around ownership and borrowing, and using edition-2024 features (let chains, async closures) correctly.

It also covers correcting the recurring failure modes of machine-generated Rust: the clone tax (cloning to silence the borrow checker instead of redesigning ownership), unwrap/expect infestation, String vs &str misuse, indexed loops instead of iterators, and over-annotated lifetimes the compiler could elide.

Out of scope: error-type design (thiserror vs anyhow) belongs to rust-error-handling; async runtime questions belong to rust-async-concurrency; SOLID/file-layout rules belong to fuse-solid:solid-rust. </objective>

Rust Core Language

Idiomatic Rust for stable 1.96.1, edition 2024. This skill covers what the compiler will not catch for you: ownership design that avoids gratuitous cloning, and the recurring failure modes of machine-generated Rust.

Agent Workflow (MANDATORY)

1. fuse-ai-pilot:research-expert — confirm the current stable version and any feature's stabilization release before relying on it (versions move fast).
2. fuse-ai-pilot:explore-codebase — read the crate's edition in Cargo.toml and its existing ownership conventions before adding code.
3. After writing code, run fuse-ai-pilot:sniper and cargo clippy.

Never state a stabilization version from memory — verify against doc.rust-lang.org/releases.html.

Overview

Topic	When
Edition 2024 features	Setting up a crate, using let chains / async closures
Ownership & borrowing	Designing function signatures and data flow
LLM pitfalls	Reviewing or repairing generated Rust before merge

Critical Rules

1. Borrow before you own. Take &T / &str / &[T] in parameters unless the function must keep the value. See ownership-borrowing.md.
2. .clone() needs a reason. A clone to silence a borrow-checker error is a design smell (the "clone tax"). Redesign ownership first; clone only when a second owner genuinely exists.
3. No .unwrap() / .expect() on fallible values without a written justification. Grep for them before merge. Prefer ?, match, or a documented invariant.
4. Arc::clone(&x) over x.clone() for reference-counted handles — it makes the cheap pointer-copy explicit and distinguishes it from a deep clone.
5. Iterators over indexed loops. Prefer iter()/map()/filter()/collect() to for i in 0..len { v[i] } — no bounds-check noise, no off-by-one.
6. Let elision work. Do not annotate lifetimes the compiler can infer.

Verified Facts (source-checked)

• Edition 2024 stabilized in Rust 1.85.0 (2025-02-20), RFC #3501.
• Let chains (if let A = x && let B = y && cond) stabilized in 1.88.0 (2025-06-26), edition 2024 only — depends on the if let temporary-scope change.
• Async closures (async || { .. }) stabilized in 1.85.0.
• gen blocks are UNSTABLE — feature gate #![feature(gen_blocks)], tracking issue rust-lang/rust#117078, RFC #3513. Do NOT use on stable; the returned iterators are not yet fused and the syntax may still change.

Reference Guide

Concepts

Topic	Reference	Load when
Edition 2024	edition-2024.md	Configuring edition, using let chains / async closures, checking feature stability
Ownership & borrowing	ownership-borrowing.md	Choosing owned vs borrowed params, designing lifetimes, sharing with Rc/Arc
LLM pitfalls	llm-pitfalls.md	Reviewing generated Rust for clone tax, unwrap infestation, String/&str, indexed loops

Templates

Template	Load when
idiomatic-code.md	Need copy-paste examples of borrow-first signatures, iterator chains, and let chains

Validation Checklist

• Cargo.toml declares edition = "2024" if edition-2024 features are used
• No .clone() added purely to satisfy the borrow checker
• Every .unwrap()/.expect() has a justifying comment or is provably infallible
• Parameters take references unless ownership is required
• No lifetime annotations the compiler can elide
• cargo clippy clean, sniper passed