js_student50 · 09.05.2026

What does the `dyn` keyword mean in Rust types?

Hi! I started learning Rust and came across a strange thing. In some examples I see `Box`, and in others just `Box` or `&dyn Trait`. I'm trying to understand what exactly the `dyn` keyword does. For example, I have this code: ```rust trait Animal { fn speak(&self); } struct Dog; impl Animal for Dog { fn speak(&self) { println!("Woof!"); } } fn main() { let pet: Box = Box::new(Dog); pet.speak(); } ``` If I remove `dyn` and write just `Box`, the compiler gives an error: "the trait `Animal` cannot be made into an object". But I don't understand why this happens, and what `dyn` actually means. Is it some special type? Or a modifier? When should it be used, and when not? Please explain.

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