rust-core-language

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.

Rust Core Language

Idiomatic Rust for stable 1.98.1, 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)

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

Borrow before you own. Take &T / &str / &[T] in parameters unless the function must keep the value. See ownership-borrowing.md.
.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.
No .unwrap() / .expect() on fallible values without a written justification. Grep for them before merge. Prefer ?, match, or a documented invariant.
Arc::clone(&x) over x.clone() for reference-counted handles — it makes the cheap pointer-copy explicit and distinguishes it from a deep clone.
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.
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.
Rust 1.97.0 (2026-07-09): v0 symbol mangling is the default; Cargo build.warnings (CARGO_BUILD_WARNINGS=deny in CI, no build-cache invalidation); linker output is shown via the linker_messages lint (warn by default); integer highest_one/lowest_one/bit_width.
Rust 1.98.0 (2026-08-20): {integer}::format_into + core::fmt::NumBuffer (std replacement for itoa); f32/f64 algebraic_* methods; str::strip_circumfix, str::substr_range, String::from_utf16le/from_utf16be.
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