/rust-core-language

name: rust-core-language
description: Use when writing or reviewing idiomatic Rust — edition 2024 ownership/borrowing design, or fixing LLM pitfalls (clone tax, unwrap infestation, indexed loops). Not for error-type design (rust-error-handling) or SOLID layout (solid-rust).
versions:
  rust: "1.99.1"
  edition: "2024"
references: references/edition-2024.md, references/ownership-borrowing.md, references/llm-pitfalls.md, references/templates/idiomatic-code.md
related-skills: rust-error-handling, fuse-solid:solid-rust

<objective> This skill covers idiomatic Rust for stable 1.99.1, 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 **1.99.1**, **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](references/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**.
• **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`.

• **Rust 1.99.0** (2026-10-01): C-variadic function definitions stabilized; `#[my_macro] mod foo;`

(outlined modules inside a custom attribute/derive macro) stabilized; allow-by-default `raw_borrows_via_references` lint; `UnsafeCell` contents guaranteed accessible without `get`. **1.99.1** (2026-10-15) fixes an x86_64 FFI `bool` return-value miscompilation.

• **`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](references/edition-2024.md) | Configuring `edition`, using let chains / async closures, checking feature stability | | Ownership & borrowing | [ownership-borrowing.md](references/ownership-borrowing.md) | Choosing owned vs borrowed params, designing lifetimes, sharing with Rc/Arc | | LLM pitfalls | [llm-pitfalls.md](references/llm-pitfalls.md) | Reviewing generated Rust for clone tax, unwrap infestation, String/&str, indexed loops |

Templates

| Template | Load when | |----------|-----------| | [idiomatic-code.md](references/templates/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