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