Choose Generics and Trait Objects Deliberately
RUST-CHOOSE-GENERICS-AND-TRAIT-OBJECTS-DELIBERATELY
Summary
Pick generics, stored type parameters, or trait objects for the variation they actually model. The choice affects compile cost, object safety, lifetimes, and caller ergonomics.
Rule
Use generics, stored generic parameters, and trait objects deliberately.
Why
These choices put variation in different places, so they create different costs.
A generic function lets each caller choose a concrete type. Monomorphization enables static dispatch and optimization, but can increase compile time and machine-code size. Bounds also become part of the caller's contract and error surface.
A stored type parameter makes the choice part of the containing type's identity. The parameter and its bounds then spread through fields, constructors, return types, and wrappers. This is useful when each value has a fixed implementation, but awkward when callers do not care or need heterogeneous storage.
A trait object erases the concrete type, allowing runtime selection and heterogeneous storage. The cost is pointer indirection, dynamic dispatch, ownership and lifetime bounds, and a dyn-compatible trait with fewer available method shapes.
The wrong choice can therefore spread complexity through an API or constrain its future evolution. Choose the shape that matches where variation belongs, not the one that merely appears flexible.
Helps
Helps APIs expose the right kind of variation without surprising callers with compile-time cost, object-safety constraints, allocation, or lifetime complexity.
Limits
Generics are usually right for caller-chosen types and zero-cost abstraction. Trait objects are usually right for heterogeneous storage, dynamic plugins, or smaller public signatures.
Agent Instruction
Choose generics, stored type parameters, and trait objects deliberately because they trade off
monomorphization, object safety, compile time, and ergonomics.Mechanisms
Compare call-site ergonomics, monomorphization, object safety, lifetime bounds, storage needs, and expected extension points before choosing the abstraction shape.