A type question abou Rust.

iterators.rs

// The uncommented version of increment_all() works.  What I'm trying to
// understand is why I can't handle the return type as shown in the commented
// out version, which does not compile.

// fn increment_all<I: Iterator<int>>(numbers: I) -> I /* or Iterator<int> */ {
fn increment_all<'r, I: Iterator<int>>(numbers: I) -> std::iter::Map<'r, int, int, I> {
    numbers.map(|n| n + 1)
}

fn main() {
    let before          = vec![1i, 2, 3];
    let after: Vec<int> = increment_all(before.iter().map(|&n| n)).collect();
    println!("{}", after);
}