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

Thinking Prompt

Before adding trait bounds:

1. What abstraction is needed?
2. Same behavior, different types → trait
3. Different behavior, same type → enum

4. No abstraction needed → concrete type

5. When is type known?

6. Compile time → generics (static dispatch)

7. Runtime → trait objects (dynamic dispatch)

8. What's the trade-off priority?

9. Performance → generics
10. Compile time → trait objects
11. 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?)

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

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

Decision Guide

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

Related Skills