I'm glad that Rust has no `auto`. I find this:

    fn map<U, T, F, I>(it: I) -> impl Iterator<Item=U> 
        where I: Iterator<Item=T>, U: From<T>
    {
        it.map(|t| From::from(t))
    }

infinitely more readable than

    fn map<U, T, F, I>(it: I) -> auto
        where I: Iterator<Item=T>, U: From<T>
    {
        it.map(|t| From::from(t)) 
    }

The type signature of the first one clearly tells me that the return type is an `Iterator<U>`, even though the actual type cannot be named because of the anonymous closure.

The second one leaves me guessing what the return type is.

If the actual type cannot be named, it is rarely the case that this is all there is to it. Usually, users are expected to use that type "somehow" (it is a `Range`?), and that means that there are some interfaces that these types implement.