为什么这个迭代器需要一个生命周期，我该如何给它一个生命周期？

Question

I'm trying to get the following to compile:

fn read<'a>(tokens: impl Iterator<Item=&'a str>) -> impl Iterator<Item=String> {
  tokens.map(|t| t.to_owned())
}

error[E0482]: lifetime of return value does not outlive the function call
 --> src/lib.rs:1:53
  |
1 | fn read<'a>(tokens: impl Iterator<Item=&'a str>) -> impl Iterator<Item=String> {
  |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: the return value is only valid for the lifetime `'a` as defined on the function body at 1:9
 --> src/lib.rs:1:9
  |
1 | fn read<'a>(tokens: impl Iterator<Item=&'a str>) -> impl Iterator<Item=String> {
  |         ^^

Playground link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=55460a8b17ff10e30b56a9142338ec19

What's strange to me is that the return value is concerned with lifetimes at all; its contents are owned and passed directly outward.

I came across this blog post:https://blog.katona.me/2019/12/29/Rust-Lifetimes-and-Iterators/

but neither of these worked:

fn read<'a>(tokens: impl Iterator<Item=&'a str>) -> impl Iterator<Item=String> + '_ {

fn read<'a>(tokens: impl Iterator<Item=&'a str>) -> impl Iterator<Item=String> + 'a {

I suspect I'm misunderstanding some key aspect of Iterators here. Any suggestions?

Answer 1

I solved it by adding + 'a to both Iterators:

fn read<'a>(tokens: impl Iterator<Item=&'a str> + 'a) -> impl Iterator<Item=String> + 'a {
  tokens.map(|t| t.to_owned())
}