Impl Trait lets you promise less

Let's break down and compare the two function signatures:

fn f1() -> std::vec::IntoIter<u32> {
    /*...*/
}

fn f2() -> impl Iterator<Item = u32> {
    /*...*/
}

fn f1() -> std::vec::IntoIter<u32> {
    /*...*/
}

Return Type: The function f1 returns a std::vec::IntoIter<u32>.
std::vec::IntoIter<u32>: This type is a concrete type representing the iterator produced by consuming a Vec<u32>. When you call into_iter() on a vector, you get an IntoIter that yields elements of type u32.
Usage: This return type is explicit, meaning that the caller knows exactly what type of iterator they are getting. This can be useful when the specific type is important for the caller.