m04-zero-cost

What the source does

• FilesystemNone observedNo reads or writes in the source
• NetworkNone observedNo outbound calls in the source
• ShellNone observedNo commands run in the source
• DependenciesPinnedNothing floats
• SecretsNone foundNo keys in source

Five audit layers

Gates applied: no behavioural pass, no license.

Audit 0.4.1 · source sha f4ff8c82542d · 29 Oct 2026Report an issue or request a re-scan

• Publisherzhanghandong GitHub organizationOwner of the repository this skill is read from
• Source folderzhanghandong/rust-skills/skills/m04-zero-costThe exact folder we audited
• LicenceNot declaredAs the repository states it
• Commit pinned for this verdictf4ff8c82542dA new audit runs when the source changes
• Repository statusNot archivedLast push 66 days ago
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its description: 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,…

---
name: m04-zero-cost
description: "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"
user-invocable: false
---

# 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:

1. **What abstraction is needed?**
   - Same behavior, different types → trait
   - Different behavior, same type → enum
   - No abstraction needed → concrete type

2. **When is type known?**
   - Compile time → generics (static dispatch)
   - Runtime → trait objects (dynamic dispatch)

3. **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

```rust
// 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: Sized` bound
- Doesn't return `Self`
- Doesn't have generic methods
- Uses `where Self: Sized` for 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 |

Excerpt, first 4,654 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/zhanghandong/rust-skills
git -C rust-skills checkout f4ff8c82542d

Works in

1. 2026-10-29
   ✓SAFE B 89 / 100

   first audit · source f4ff8c82542d

Questions about m04-zero-cost

• github.com/zhanghandong/rust-skills/tree/main/skills/m04-zero-co — the audited source at commit f4ff8c82542d and every finding aboveObserved · 29 Oct 2026

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