m04-zero-cost
CRITICAL: Use for generics, traits, zero-cost abstraction. Triggers: E0277, E0308, E0599, generic, trait, impl, dyn, where, monomorphization, static dispatch, dynamic dispatch, impl Trait, trait bound not satisfied, 泛型, 特征, 零成本抽象, 单态化
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Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/actionbook/rust-skills --skill m04-zero-cost- 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 m04-zero-cost 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.
Zero-Cost Abstraction
Layer 1: Language Mechanics
Core Question
Do we need compile-time or runtime polymorphism?
Before choosing between generics and trait objects:
- Is the type known at compile time?
- Is a heterogeneous collection needed?
- What's the performance priority?
Error → Design Question
| Error | Don't Just Say | Ask Instead |
|---|---|---|
| E0277 | "Add trait bound" | Is this abstraction at the right level? |
| E0308 | "Fix the type" | Should types be unified or distinct? |
| E0599 | "Import the trait" | Is the trait the right abstraction? |
| E0038 | "Make object-safe" | Do we really need dynamic dispatch? |
Thinking Prompt
Before adding trait bounds:
-
What abstraction is needed?
- Same behavior, different types → trait
- Different behavior, same type → enum
- No abstraction needed → concrete type
-
When is type known?
- Compile time → generics (static dispatch)
- Runtime → trait objects (dynamic dispatch)
-
What's the trade-off priority?
- Performance → generics
- Compile time → trait objects
- Flexibility → depends
Trace Up ↑
When type system fights back:
E0277 (trait bound not satisfied)
↑ Ask: Is the abstraction level correct?
↑ Check: m09-domain (what behavior is being abstracted?)
↑ Check: m05-type-driven (should use newtype?)
| Persistent Error | Trace To | Question |
|---|---|---|
| Complex trait bounds | m09-domain | Is the abstraction right? |
| Object safety issues | m05-type-driven | Can typestate help? |
| Type explosion | m10-performance | Accept dyn overhead? |
Trace Down ↓
From design to implementation:
"Need to abstract over types with same behavior"
↓ Types known at compile time → impl Trait or generics
↓ Types determined at runtime → dyn Trait
"Need collection of different types"
↓ Closed set → enum
↓ Open set → Vec<Box<dyn Trait>>
"Need to return different types"
↓ Same type → impl Trait
↓ Different types → Box<dyn Trait>
Quick Reference
| Pattern | Dispatch | Code Size | Runtime Cost |
|---|---|---|---|
fn foo<T: Trait>() | Static | +bloat | Zero |
fn foo(x: &dyn Trait) | Dynamic | Minimal | vtable lookup |
impl Trait return | Static | +bloat | Zero |
Box<dyn Trait> | Dynamic | Minimal | Allocation + vtable |
Syntax Comparison
// Static dispatch - type known at compile time
fn process(x: impl Display) { } // argument position
fn process<T: Display>(x: T) { } // explicit generic
fn get() -> impl Display { } // return position
// Dynamic dispatch - type determined at runtime
fn process(x: &dyn Display) { } // reference
fn process(x: Box<dyn Display>) { } // owned
Error Code Reference
| Error | Cause | Quick Fix |
|---|---|---|
| E0277 | Type doesn't impl trait | Add impl or change bound |
| E0308 | Type mismatch | Check generic params |
| E0599 | No method found | Import trait with use |
| E0038 | Trait not object-safe | Use generics or redesign |
Decision Guide
| Scenario | Choose | Why |
|---|---|---|
| Performance critical | Generics | Zero runtime cost |
| Heterogeneous collection | dyn Trait | Different types at runtime |
| Plugin architecture | dyn Trait | Unknown types at compile |
| Reduce compile time | dyn Trait | Less monomorphization |
| Small, known type set | enum | No indirection |
Object Safety
A trait is object-safe if it:
- Doesn't have
Self: Sizedbound - Doesn't return
Self - Doesn't have generic methods
- Uses
where Self: Sizedfor non-object-safe methods
Anti-Patterns
| Anti-Pattern | Why Bad | Better |
|---|---|---|
| Over-generic everything | Compile time, complexity | Concrete types when possible |
dyn for known types | Unnecessary indirection | Generics |
| Complex trait hierarchies | Hard to understand | Simpler design |
| Ignore object safety | Limits flexibility | Plan for dyn if needed |
Related Skills
| When | See |
|---|---|
| Type-driven design | m05-type-driven |
| Domain abstraction | m09-domain |
| Performance concerns | m10-performance |
| Send/Sync bounds | m07-concurrency |
FAQ
What does the m04-zero-cost skill do?
CRITICAL: Use for generics, traits, zero-cost abstraction. Triggers: E0277, E0308, E0599, generic, trait, impl, dyn, where, monomorphization, static dispatch, dynamic dispatch, impl Trait, trait bound not satisfied, 泛型, 特征, 零成本抽象, 单态化
How do I install m04-zero-cost?
Run npx skills add https://github.com/actionbook/rust-skills --skill m04-zero-cost 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/m04-zero-cost/ (personal) or .claude/skills/m04-zero-cost/ (project).
Which AI agents can use m04-zero-cost?
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 m04-zero-cost 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 m04-zero-cost?
It is published by actionbook in actionbook/rust-skills. It has 3,404 recorded installs.
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