Is there a builtin identity function in Rust?

Answering your question

After Rust 1.33, see the sibling answer.

Before Rust 1.33, there is no such function in stable Rust. You can create your own:

fn id<T>(v: T) -> T { v }

Although most people just inline the code, as you did.

After Rust 1.29, use Iterator::flatten:

let v = vec![Some(1), None, Some(2)];
for i in v.into_iter().flatten() {
    println!("{}", i);
}

Before Rust 1.29, use Iterator::flat_map:

let v = vec![Some(1), None, Some(2)];
for i in v.into_iter().flat_map(|o| o) {
    println!("{}", i);
}