100-exercises-to-learn-rust/book/src/06_ticket_management/09_impl_trait_2.md

impl Trait in argument position

In the previous section, we saw how impl Trait can be used to return a type without specifying its name.
The same syntax can also be used in argument position:

fn print_iter(iter: impl Iterator<Item = i32>) {
    for i in iter {
        println!("{}", i);
    }
}

print_iter takes an iterator of i32s and prints each element.
When used in argument position, impl Trait is equivalent to a generic parameter with a trait bound:

fn print_iter<T>(iter: T) 
where
    T: Iterator<Item = i32>
{
    for i in iter {
        println!("{}", i);
    }
}

Downsides

As a rule of thumb, prefer generics over impl Trait in argument position.
Generics allow the caller to explicitly specify the type of the argument, using the turbofish syntax (::<>), which can be useful for disambiguation. That’s not the case with impl Trait.