Is there a reason parameters are Iterators instead of IntoIterator?

[FelixBenning]

If I understand what I am doing, then, if

pub fn mean<I>(it: I) -> f64 where
    I: Iterator,
    <I as Iterator>::Item: ToPrimitive, 

was changed to something like

fn mean<'a, I, T>(x:I)-> f64 where
    I: IntoIterator<Item= T>,
    T: Into<&'a f64>
{
   let it = x.into_iter();
   ...
}

You could also use mean on vectors. e.g.

mean(&vec![1,2,3])

as opposed to

mean(vec![1,2,3].iter().collect())

without losing any functionality for iterators themselves. But I am just learning Rust, so I might not understand some limitation of this approach.

[BurntSushi]

Yes, the API should use IntoIterator. This crate was actually written IntoIterator existed (IIRC), and I just never updated the APIs.

But you don't need vec![1, 2, 3].iter().collect(). Just &[1, 2, 3].iter() should work. Or at least, &[1, 2, 3].iter().cloned().