M04 Zero Cost Agent Skill

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: 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 |