Impl Trait + lifetime in return position

[towry]

In this code: rustc-fn:

pub fn elements(&self) -> impl Iterator<Item = Region<'tcx>> + '_ {
    self.relation.elements().copied()
}

Q: Why is the lifetime '_ required?

I searched and found this: Generic parameter capture - Impl trait initiative which doesn't help and in the issue `impl Trait` Lifetime Elision · Issue #43396 · rust-lang/rust · GitHub niko said:

• Lifetimes can be elided, but if you just write impl Trait it does not capture &self . To capture, you must write impl Trait + '_ .

But I do not understand why need to capture &self.

[aegroto]

Are you sure this is copying all the data and not returning any even partial reference to self?

[2e71828]

The iterator you use is lazy, so the original object needs to stay alive to provide new results. You could instead use a non-lazy iterator, which would look something like this:

pub fn elements(&self) -> impl Iterator<Item = Region<'tcx>> {
    let v: Vec<_> = self.relation.elements().copied().collect();
    v.into_iter()
}