Rust: What is the difference between Fn traits and trait objects?

Fn traits — closure types. Trait objects — dynamic dispatch.

Fn traits:

• Fn, FnMut, FnOnce.
• Closure-specific.
• Static dispatch.

Trait objects:

• Any trait.
• Dynamic dispatch.
• Fat pointer.

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

Key: Fn for static, trait objects for dynamic.