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

impl Fn — static dispatch. dyn Fn — dynamic dispatch.

impl Fn:

• Compile-time.
• Monomorphized.
• Zero-cost.

dyn Fn:

• Runtime dispatch.
• Fat pointer.
• Flexible.

1// Static dispatch
2fn apply<F: Fn(i32) -> i32>(f: F, x: i32) -> i32 {
3    f(x)
4}
5
6// Dynamic dispatch
7fn apply_dyn(f: &dyn Fn(i32) -> i32, x: i32) -> i32 {
8    f(x)
9}

Key: impl Fn for performance, dyn Fn for flexibility.