Why is adding a lifetime to a trait with the plus operator (Iterator + 'a) needed?

Question

I'm applying a closure on the iterator and I want to use stable, so I want to return a boxed Iterator. The obvious way to do so is the following:

struct Foo;

fn into_iterator(myvec: &Vec<Foo>) -> Box<dyn Iterator<Item = &Foo>> {
    Box::new(myvec.iter())
}

This fails because the borrow checker cannot infer the appropriate lifetimes.

After some research, I've found What is the correct way to return an Iterator (or any other trait)?, which brought me to adding + 'a:

fn into_iterator<'a>(myvec: &'a Vec<Foo>) -> Box<dyn Iterator<Item = &'a Foo> + 'a> {
    Box::new(myvec.iter())
}

But I don't understand

• What this does
• And why it is needed here

Answer 1

There is one thing that is easily overlooked: if you have a trait Bar and you want to have a boxed trait object Box<dyn Bar>, the compiler automatically adds a 'static lifetime bound (as specified in RFC 599). This means that Box<dyn Bar> and Box<dyn Bar + 'static> are equivalent!

In your case, the compiler automatically adds the static bound such that this ...

fn into_iterator(myvec: &Vec<Foo>) -> Box<dyn Iterator<Item = &Foo>>

... is equivalent to that:

fn into_iterator(myvec: &Vec<Foo>) -> Box<dyn Iterator<Item = &Foo> + 'static>

Now lifetime elision rules kick in and "connect" the two lifetime-slots, such that the above code is equivalent to:

fn into_iterator<'a>(myvec: &'a Vec<Foo>) -> Box<dyn Iterator<Item = &'a Foo> + 'static>

But the type Iter<'a, Foo> (the specific iterator type for Vec<Foo>) obviously does not satisfy the bound 'static (because it is borrowing the Vec<Foo>)! So we have to tell the compiler that we don't want the default 'static bound by specifying our own lifetime bound:

fn into_iterator<'a>(myvec: &'a Vec<Foo>) -> Box<dyn Iterator<Item = &Foo> + 'a>

Now the compiler knows that the trait object is only valid for the lifetime 'a. Note that we don't explicitly need to annotate the lifetime of the associated Item type! Lifetime elision rules take care of that.