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m03-mutability

Analyze mutable state decisions in Rust code for safe ownership and synchronization.

1.4k|110|Updated Jan 17, 2026
Search Tags:#rust#borrowing#arc#mutex#mutability#cell#refcell
One-click install
npx skills add https://github.com/actionbook/rust-skills --skill m03-mutability-actionbook
Or copy as Structured Prompt for Agent▼
Please help me install this Agent Skill.
Skill: m03-mutability
Source: https://github.com/actionbook/rust-skills/tree/main/skills/m03-mutability
Command: npx skills add https://github.com/actionbook/rust-skills --skill m03-mutability-actionbook

SYSTEM DOCUMENTATION & REQUIREMENTS

What problem does it solve?

This Skill helps Rust developers reason about and decide when to use mutable state, interior mutability, and synchronization primitives to preserve safety and correctness while avoiding borrow conflicts.

Core Features & Use Cases

  • Guidance on when mutability is necessary and how to structure ownership to minimize borrow errors.
  • Mappings from common errors (E0596, E0499, E0502) to design decisions and selection of appropriate primitives (Cell, RefCell, Mutex, RwLock, Arc).
  • Real-world scenarios such as shared configuration, caching, and concurrent state management to illustrate proper patterns.

Quick Start

Ask: How should I design safe shared mutable state in Rust for a struct accessed by multiple threads, and which primitives (Cell, RefCell, Mutex, RwLock, Arc) are appropriate?

Frequently Asked Questions about m03-mutability

High-intent search queries and answers about installing and using this skill.

FAQPage Schema
How do I design safe shared mutable state in Rust for multiple threads?▼

Use Arc for thread-safe shared ownership and pair it with Mutex or RwLock to synchronize mutable access, preventing data races while allowing concurrent reads or exclusive writes across threads.

When should I use RefCell instead of Cell for interior mutability in Rust?▼

Use RefCell when you need runtime-checked borrow rules for mutable references, and Cell when you only need to copy or replace values without borrowing, as Cell requires Copy types and has no runtime overhead.

Why does my Rust code fail to borrow a mutable reference and how do I fix it?▼

Rust borrow errors like E0596, E0499, or E0502 occur due to conflicting mutable borrows or lack of mutability, and can be resolved by restructuring ownership or applying interior mutability patterns with Cell, RefCell, or Mutex.

What is the best way to handle shared configuration and caching in Rust?▼

For shared configuration and caching in Rust, use RwLock to allow multiple concurrent readers with exclusive write access, or RefCell in single-threaded contexts to safely manage mutable state behind an immutable interface.

Do I need Arc and Mutex for single-threaded Rust mutability?▼

No, for single-threaded Rust mutability, Arc and Mutex are unnecessary; use Cell for simple Copy types or RefCell for runtime-checked borrowing, which provide interior mutability without synchronization overhead.

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