How do you choose between trait objects and generics in Rust?

1 What are trait objects?

Trait objects are a way to use dynamic dispatch in Rust. Dynamic dispatch means that the compiler does not know at compile time which implementation of a trait to use for a given value, but instead it determines it at run time based on the actual type of the value. Trait objects are created by using the dyn keyword before a trait name, such as dyn Display or dyn Iterator. Trait objects can be stored in variables, passed as arguments, or returned from functions, as long as they are behind a pointer, such as a reference, a Box, or a Rc. Trait objects are useful when you need to work with heterogeneous collections of types that share a trait, or when you want to defer the decision of which type to use until run time.

2 What are generics?

Generics are a way to use static dispatch in Rust. Static dispatch means that the compiler knows at compile time which implementation of a trait to use for a given value, and it generates specialized code for each type that is used. Generics are created by using angle brackets after a type name, a function name, or a trait name, such as Vec<T>, fn foo<T: Display>(x: T), or impl<T: Display> Display for MyType<T>. Generics can also be stored in variables, passed as arguments, or returned from functions, without any pointers. Generics are useful when you want to write code that can work with any type that meets some constraints, or when you want to optimize the performance of your code by avoiding the overhead of dynamic dispatch.

3 How to compare trait objects and generics?

When comparing trait objects and generics, there are different trade-offs to consider in terms of flexibility, performance, and complexity. Trait objects are more flexible than generics, as they allow the use of any type that implements the trait, regardless of what is known at compile time. However, generics are more performant than trait objects due to the avoidance of dynamic dispatch overhead. Generics also have a greater complexity than trait objects, as they require more syntax, concepts, and constraints. On the other hand, trait objects are simpler to use as they only require the dyn keyword, object safety rules, and pointer types.