m04-zero-cost

Compile-time versus runtime polymorphism: generics, trait objects, and zero-cost abstraction patterns. Layer 1: Language Mechanics Do we need compile-time or runtime polymorphism? Before choosing between generics and trait objects: Before adding trait bounds: What abstraction is needed? When is type known? What's the trade-off priority? When type system fights back: From design to implementation: A trait is object-safe if it: - Distinguishes static dispatch (generics, impl Trait ) from dynamic dispatch ( dyn Trait ), with decision guidance on when each is appropriate based on type knowledge, performance priorities, and collection heterogeneity - Maps common type system errors (E0277, E0308, E0599, E0038) to underlying design questions rather than mechanical fixes - Covers object safety constraints, monomorphization trade-offs, and anti-patterns like over-generics or unnecessary dyn indirection - Includes syntax reference, quick decision table, and trace-up/trace-down patterns linking to related domain and type-driven design skills - Is the type known at compile time? - Is a heterogeneous collection needed? - What's the performance priority?