Implement InstanceOf trait in Rust

instance-of.rs

use std::any::{Any, TypeId};

trait InstanceOf
where
  Self: Any
{
  fn instance_of<U: ?Sized + Any>(&self) -> bool {
    TypeId::of::<Self>() == TypeId::of::<U>()
  }
}
// Impment this trait for any type that implements `Any` (which is most types).
// impl<T: ?Sized + Any> InstanceOf for T {} 

impl InstanceOf for String {}
    
fn main() {
  let actually_a_string = some_unknown_function();
  let mut a_string = String::new();
    
  if actually_a_string.instance_of::<String>() {
    a_string = actually_a_string;
  }
  
  println!("{}", a_string);
}
    
fn some_unknown_function() -> String {
  "I'm actually a string!".into()
}