rust-zero-cost-abstractions

name	rust-zero-cost-abstractions
description	Guides generic vs trait object decisions in Rust. Use when choosing between static and dynamic dispatch, designing traits, handling E0277/E0038, or deciding between enum and dyn Trait.

Zero-Cost Abstractions

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?

---

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

---

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

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 time
Reduce compile time	dyn Trait	Less monomorphization
Small, known type set	enum	No indirection

Cost Comparison

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

---

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

Error Code Reference

Error	Cause	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

---

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

• type-driven-design — newtypes and type state
• performance — when dispatch cost matters
• concurrency — Send/Sync bounds on generics