Type parameters - The Rust Reference

# Type parameters

Within the body of an item that has type parameter declarations, the names of its type parameters are types:

    #![allow(unused)]
    fn main() {
    fn to_vec<A: Clone>(xs: &[A]) -> Vec<A> {
        if xs.is_empty() {
            return vec![];
        }
        let first: A = xs[0].clone();
        let mut rest: Vec<A> = to_vec(&xs[1..]);
        rest.insert(0, first);
        rest
    }
    }

Here, `first` has type `A`, referring to `to_vec`'s `A` type parameter; and `rest` has type `Vec<A>`, a vector with element type `A`.