m03-mutability
CRITICAL: Use for mutability issues. Triggers: E0596, E0499, E0502, cannot borrow as mutable, already borrowed as immutable, mut, &mut, interior mutability, Cell, RefCell, Mutex, RwLock, 可变性, 内部可变性, 借用冲突
- Installs
- 3.4K
- Last 24h
- +19
- Trending rank
- #6195
- Security
- Not audited
Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/actionbook/rust-skills --skill m03-mutability- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads m03-mutability automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
SKILL.md
The instructions your agent reads when the skill is triggered.
Mutability
Layer 1: Language Mechanics
Core Question
Why does this data need to change, and who can change it?
Before adding interior mutability, understand:
- Is mutation essential or accidental complexity?
- Who should control mutation?
- Is the mutation pattern safe?
Error → Design Question
| Error | Don't Just Say | Ask Instead |
|---|---|---|
| E0596 | "Add mut" | Should this really be mutable? |
| E0499 | "Split borrows" | Is the data structure right? |
| E0502 | "Separate scopes" | Why do we need both borrows? |
| RefCell panic | "Use try_borrow" | Is runtime check appropriate? |
Thinking Prompt
Before adding mutability:
-
Is mutation necessary?
- Maybe transform → return new value
- Maybe builder → construct immutably
-
Who controls mutation?
- External caller →
&mut T - Internal logic → interior mutability
- Concurrent access → synchronized mutability
- External caller →
-
What's the thread context?
- Single-thread → Cell/RefCell
- Multi-thread → Mutex/RwLock/Atomic
Trace Up ↑
When mutability conflicts persist:
E0499/E0502 (borrow conflicts)
↑ Ask: Is the data structure designed correctly?
↑ Check: m09-domain (should data be split?)
↑ Check: m07-concurrency (is async involved?)
| Persistent Error | Trace To | Question |
|---|---|---|
| Repeated borrow conflicts | m09-domain | Should data be restructured? |
| RefCell in async | m07-concurrency | Is Send/Sync needed? |
| Mutex deadlocks | m07-concurrency | Is the lock design right? |
Trace Down ↓
From design to implementation:
"Need mutable access from &self"
↓ T: Copy → Cell<T>
↓ T: !Copy → RefCell<T>
"Need thread-safe mutation"
↓ Simple counters → AtomicXxx
↓ Complex data → Mutex<T> or RwLock<T>
"Need shared mutable state"
↓ Single-thread: Rc<RefCell<T>>
↓ Multi-thread: Arc<Mutex<T>>
Borrow Rules
At any time, you can have EITHER:
├─ Multiple &T (immutable borrows)
└─ OR one &mut T (mutable borrow)
Never both simultaneously.
Quick Reference
| Pattern | Thread-Safe | Runtime Cost | Use When |
|---|---|---|---|
&mut T | N/A | Zero | Exclusive mutable access |
Cell<T> | No | Zero | Copy types, no refs needed |
RefCell<T> | No | Runtime check | Non-Copy, need runtime borrow |
Mutex<T> | Yes | Lock contention | Thread-safe mutation |
RwLock<T> | Yes | Lock contention | Many readers, few writers |
Atomic* | Yes | Minimal | Simple types (bool, usize) |
Error Code Reference
| Error | Cause | Quick Fix |
|---|---|---|
| E0596 | Borrowing immutable as mutable | Add mut or redesign |
| E0499 | Multiple mutable borrows | Restructure code flow |
| E0502 | &mut while & exists | Separate borrow scopes |
Interior Mutability Decision
| Scenario | Choose |
|---|---|
| T: Copy, single-thread | Cell<T> |
| T: !Copy, single-thread | RefCell<T> |
| T: Copy, multi-thread | AtomicXxx |
| T: !Copy, multi-thread | Mutex<T> or RwLock<T> |
| Read-heavy, multi-thread | RwLock<T> |
| Simple flags/counters | AtomicBool, AtomicUsize |
Anti-Patterns
| Anti-Pattern | Why Bad | Better |
|---|---|---|
| RefCell everywhere | Runtime panics | Clear ownership design |
| Mutex for single-thread | Unnecessary overhead | RefCell |
| Ignore RefCell panic | Hard to debug | Handle or restructure |
| Lock inside hot loop | Performance killer | Batch operations |
Related Skills
| When | See |
|---|---|
| Smart pointer choice | m02-resource |
| Thread safety | m07-concurrency |
| Data structure design | m09-domain |
| Anti-patterns | m15-anti-pattern |
FAQ
What does the m03-mutability skill do?
CRITICAL: Use for mutability issues. Triggers: E0596, E0499, E0502, cannot borrow as mutable, already borrowed as immutable, mut, &mut, interior mutability, Cell, RefCell, Mutex, RwLock, 可变性, 内部可变性, 借用冲突
How do I install m03-mutability?
Run npx skills add https://github.com/actionbook/rust-skills --skill m03-mutability in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/m03-mutability/ (personal) or .claude/skills/m03-mutability/ (project).
Which AI agents can use m03-mutability?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is m03-mutability safe to install?
No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.
Who maintains m03-mutability?
It is published by actionbook in actionbook/rust-skills. It has 3,377 recorded installs.
More from actionbook/rust-skills
m15-anti-pattern
actionbook/rust-skills
Use when reviewing code for anti-patterns. Keywords: anti-pattern, common mistake, pitfall, code smell, bad practice, code review, is this an anti-pattern, better way to do this, common mistake to avoid, why is this bad, idiomatic way, beginner mistake, fighting borrow checker, clone everywhere, unwrap in production, should I refactor, 反模式, 常见错误, 代码异味, 最佳实践, 地道写法
coding-guidelines
actionbook/rust-skills
Use when asking about Rust code style or best practices. Keywords: naming, formatting, comment, clippy, rustfmt, lint, code style, best practice, P.NAM, G.FMT, code review, naming convention, variable naming, function naming, type naming, 命名规范, 代码风格, 格式化, 最佳实践, 代码审查, 怎么命名
m10-performance
actionbook/rust-skills
CRITICAL: Use for performance optimization. Triggers: performance, optimization, benchmark, profiling, flamegraph, criterion, slow, fast, allocation, cache, SIMD, make it faster, 性能优化, 基准测试
m07-concurrency
actionbook/rust-skills
CRITICAL: Use for concurrency/async. Triggers: E0277 Send Sync, cannot be sent between threads, thread, spawn, channel, mpsc, Mutex, RwLock, Atomic, async, await, Future, tokio, deadlock, race condition, 并发, 线程, 异步, 死锁
m06-error-handling
actionbook/rust-skills
CRITICAL: Use for error handling. Triggers: Result, Option, Error, ?, unwrap, expect, panic, anyhow, thiserror, when to panic vs return Result, custom error, error propagation, 错误处理, Result 用法, 什么时候用 panic
m01-ownership
actionbook/rust-skills
CRITICAL: Use for ownership/borrow/lifetime issues. Triggers: E0382, E0597, E0506, E0507, E0515, E0716, E0106, value moved, borrowed value does not live long enough, cannot move out of, use of moved value, ownership, borrow, lifetime, 'a, 'static, move, clone, Copy, 所有权, 借用, 生命周期
Popular backend & apis skills
find-skills
vercel-labs/skills
Helps users discover and install agent skills when they ask questions like "how do I do X", "find a skill for X", "is there a skill that can...", or express interest in extending capabilities. This skill should be used when the user is looking for functionality that might exist as an installable skill.
entra-app-registration
microsoft/azure-skills
Guides Microsoft Entra ID app registration, OAuth 2.0 authentication, and MSAL integration. USE FOR: create app registration, register Azure AD app, configure OAuth, set up authentication, add API permissions, generate service principal, MSAL example, console app auth, Entra ID setup, Azure AD authentication. DO NOT USE FOR: Key Vault secrets (use azure-keyvault-expiration-audit), general Azure resource security guidance.
google-agents-cli-adk-code
google/agents-cli
This skill should be used when the user wants to "write agent code", "build an agent with ADK", "add a tool", "create a callback", "define an agent", "use state management" — in a project that needs ADK (Agent Development Kit) API patterns and code examples. It provides a quick reference for agent types, tool definitions, orchestration patterns, callbacks, state management, the graph Workflow API, and reference recipes to study. Do NOT use for scaffolding (use google-agents-cli-scaffold) or deployment (use google-agents-cli-deploy).
embedded-captions
heygen-com/hyperframes
Add captions or subtitles to an existing single-subject talking-head video without editing the footage. Use for plain verbatim captions, cinematic captions embedded behind the subject, VFX captions, “炸/特效/酷炫字幕,” or a named identity from the 35-style catalog. Route by visual identity, not by backend engine. The quiet anchor rail is the default; embed every word only when the user explicitly wants a fully cinematic treatment. The workflow runs locally end to end, including transcription and subject matting; split multi-shot footage before applying it.
entra-agent-id
microsoft/azure-skills
Provision Microsoft Entra Agent Identity Blueprints, BlueprintPrincipals, and per-instance Agent Identities via Microsoft Graph, and configure OAuth 2.0 token exchange (fmipath, OBO, cross-tenant) including the Microsoft Entra SDK for AgentID sidecar. USE FOR: Agent Identity Blueprint, BlueprintPrincipal, agent OAuth, fmipath token exchange, agent OBO, Workload Identity Federation for agents, polyglot agent auth, Microsoft.Identity.Web.AgentIdentities. DO NOT USE FOR: standard Entra app registration (use entra-app-registration), Microsoft Foundry agent authoring (use microsoft-foundry).
neon
neondatabase/agent-skills
Overview of Neon, a complete set of cloud backend primitives around Lakebase Postgres: Auth, Object Storage, Functions, the AI Gateway, and Realtime. Start here to choose Neon for undecided login, files, APIs, and LLM calls, set up the CLI or MCP server, and follow the branch-first workflow. Use when building an app or backend on Neon, or when "Neon" or "Lakebase Postgres" is mentioned. Child skill neon-postgres wins for an existing DATABASEURL, SQL, schema, inspect, or search. Child skill neon-auth wins for login, users, sessions, identity routing, and Managed Better Auth setup. Child skill neon-realtime wins for live queries in reactive apps and agents. Also use for object storage, S3, buckets, serverless functions, function triggers, cron, AI gateway, LLM calls, realtime, logs, Loki, Grafana, observability, postgres, database, backend, Claimable Neon, neon.new, or a no-signup database.