M04 Zero Cost

Before you slap another trait bound on that generic function, this prompts you to ask whether you actually need the abstraction. It maps common type errors like E0277 and E0308 to design questions: is this really compile time polymorphism, or should it be an enum, or just a concrete type? The quick reference tables are genuinely useful, showing you when static dispatch bloats your binary versus when dynamic dispatch costs you a vtable lookup. The trace up/down sections connect abstraction choices to other skills in the marketplace, which matters when your "simple" generic suddenly needs Send bounds or domain redesign. Honest take: most Rust codebases over-abstract early, and this at least makes you pause first.

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last updated: Aug 21, 2026

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Install

$ npx -y skills add actionbook/rust-skills --skill m04-zero-cost --agent claude-code

Installs into .claude/skills of the current project.

Files

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