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? |
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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>) { } // ownedError 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: Sizedbound - Doesn't return
Self - Doesn't have generic methods
- Uses
where Self: Sizedfor non-object-safe methods
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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 |
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Related Skills
| When | See |
|---|---|
| Type-driven design | m05-type-driven |
| Domain abstraction | m09-domain |
| Performance concerns | m10-performance |
| Send/Sync bounds | m07-concurrency |
