Rust: What is the difference between impl Trait and dyn Trait?

impl Trait — static dispatch. dyn Trait — dynamic dispatch.

impl Trait:

• Compile-time.
• Monomorphized.
• Faster.

dyn Trait:

• Runtime.
• Fat pointer.
• Flexible.

1// impl Trait
2fn get_greeting(name: &str) -> impl std::fmt::Display {
3    format!("Hello, {}!", name)
4}
5
6// dyn Trait
7fn get_greeting(name: &str) -> Box<dyn std::fmt::Display> {
8    Box::new(format!("Hello, {}!", name))
9}

When to use:

• impl Trait: simple cases.
• dyn Trait: heterogeneous collections.

Key: impl for static, dyn for dynamic.