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m03-mutability — Claude Code skill: fixing Rust mutability errors

A Rust skill for mutability trouble: it walks from borrow-checker errors to a decision about the data, not just another mut.

Install into Claude Code · Skills CLI (Vercel) All methods and platforms ↓ 
npx skills add actionbook/rust-skills --skill m03-mutability -a claude-code
Open on GitHub

What m03-mutability does

The skill fires on E0596, E0499, E0502, on “cannot borrow as mutable” messages and on RefCell panics. Its first move is a question: does this value need to change at all, and who gets to change it? Sometimes the answer is a new value instead of a mutation.

Choosing interior mutability comes down to two facts in the reference tables: is the type Copy, and does the code cross threads. Cell<T> covers Copy types on one thread; RefCell<T> handles the rest without threads; Mutex<T> and RwLock<T> take the multithreaded cases. An anti-pattern table names the usual mistakes, including a lock taken inside a hot loop and RefCell sprinkled everywhere.

When borrow conflicts keep returning, the skill points to neighbouring topics on data structure design and concurrency, because a repeated E0499 often means the struct itself is wrong. It is not a slash command — the description triggers it automatically, so the SKILL.md file has to sit in your platform's skills directory.

What it helps with

  • work out an E0596 without blindly adding mut
  • pick between Cell, RefCell and Mutex for a given field
  • track down the borrow that makes RefCell panic at runtime
  • untangle overlapping borrows in E0499 and E0502
  • spot anti-patterns such as locking inside a hot loop

Example prompts for the agent

  • I'm getting E0502, a &mut while a & is alive. What should I restructure?
  • I need to mutate a non-Copy field through &self. RefCell, or something else?
  • My RefCell panics with already borrowed. How do I find the borrow that was never dropped?

How to install m03-mutability

1Skills CLI (Vercel)

npx skills add actionbook/rust-skills --skill m03-mutability -a claude-code

2Manually: put the files in place

~/.claude/skills/m03-mutability/SKILL.md

Copy the whole skill folder; for one project use .claude/skills/m03-mutability/

Step by step, with commands for each platform: How to install skills in Claude Code: five ways

Questions about m03-mutability

How do I install m03-mutability in Claude Code?

Run npx skills add actionbook/rust-skills --skill m03-mutability -a claude-code. The skill file lands at ~/.claude/skills/m03-mutability/SKILL.md.

What does m03-mutability do?

It helps with Rust mutability and borrow errors. Each error code leads to a design question about the data rather than a quick annotation fix.

Does m03-mutability run as a command?

No. It has user-invocable: false and triggers from its description when errors like E0596 or RefCell come up.

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