M03 Mutability

The m03-mutability skill diagnoses and resolves Rust mutability errors (E0596, E0499, E0502) by guiding developers to understand whether mutation is architecturally necessary and which mutability pattern fits their use case—from simple `&mut T` borrows to interior mutability types like `Cell`, `RefCell`, and `Mutex`. It serves Rust developers debugging borrow checker conflicts by transforming "add mut" impulses into design questions, then tracing issues upward to data structure problems or downward to the appropriate implementation pattern. The skill solves the common problem of developers treating mutability as a tactical syntax issue rather than a design decision about ownership and access control.

skills.sh installs: 773
GitHub stars: 1.3K
last updated: Jul 14, 2026

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Install

$ npx -y skills add zhanghandong/rust-skills --skill m03-mutability --agent claude-code

Installs into .claude/skills of the current project.

Files

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
What's the thread context?
- Single-thread → Cell/RefCell
- Multi-thread → Mutex/RwLock/Atomic