async and returning impl Trait

The return type of an async function captures all of its generic parameters, including any lifetimes. So here:

#![allow(unused)]
fn main() {
async fn example(v: &mut Vec<String>) -> String {
    "Hi :-)".to_string()
}
}

The future returned by the async fn implicitly reborrows the v input, and "carries" the same lifetime, just like the other examples we saw.

The same is true when you use return-position impl Trait (RPIT) in traits (RPITIT):

#![allow(unused)]
fn main() {
struct MyStruct {}
trait StringIter {
    fn iter(&self) -> impl Iterator<Item = String>;
}

impl StringIter for MyStruct {
    fn iter(&self) -> impl Iterator<Item = String> {
        ["Hi :-)"].into_iter().map(ToString::to_string)
    }
}

// Fails to compile as `iter` borrows `my`
fn example(my: MyStruct) {
    let iter = my.iter();
    let _move_my = my;
    for _ in iter {}
}
}

self will remained borrowed here for as long as the iterator is alive. Note that this is true even if it's not required by the body! The implicit lifetime capture is considered part of the API contract.

For both of these cases, all generic types are also captured.

Not that RPIT outside of traits does not implicitly capture lifetimes! At least, not as of this writing -- the plan is that RPIT outside of traits will act like RPITIT and implicitly capture all lifetimes in edition 2024 and beyond. But for now, they only implicitly capture type parameters.

#![allow(unused)]
fn main() {
use std::fmt::Display;
// Required: edition 2021 or before
fn no_capture(s: &str) -> impl Display {
    s.to_string()
}

// This wouldn't compile if `no_capture` reborrowed `*s`
fn check() {
    let mut s = "before".to_string();
    let d = no_capture(&s);
    s ="after".to_string();
    println!("{d}");
}
}

#![allow(unused)]
fn main() {
use std::fmt::Display;
// This fails on edition 2021 or before, because it tries to
// return a reborrow of `*s`, but that requires capturing the lifetime
fn no_capture(s: &str) -> impl Display {
    s
}
}

#![allow(unused)]
fn main() {
use std::fmt::Display;
// This allows it to work again (but `+ '_` is too restrictive
// for every situation, which is part of why edition 2021 will
// change the behavior of RPIT outside of traits)
//
//                                     vvvv
fn no_capture(s: &str) -> impl Display + '_ {
    s
}
}

A lot can be said about async and RPITs; more than can be covered here. But their implicit capturing nature is something to be aware of, given how invisible it is.