类型参数

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

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.