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A captured variable in a closure may not live long enough.
Erroneous code example:
```compile_fail,E0373
fn foo() -> Box<Fn(u32) -> u32> {
let x = 0u32;
Box::new(|y| x + y)
}
```
This error occurs when an attempt is made to use data captured by a closure,
when that data may no longer exist. It's most commonly seen when attempting to
return a closure as shown in the previous code example.
Notice that `x` is stack-allocated by `foo()`. By default, Rust captures
closed-over data by reference. This means that once `foo()` returns, `x` no
longer exists. An attempt to access `x` within the closure would thus be
unsafe.
Another situation where this might be encountered is when spawning threads:
```compile_fail,E0373
fn foo() {
let x = 0u32;
let y = 1u32;
let thr = std::thread::spawn(|| {
x + y
});
}
```
Since our new thread runs in parallel, the stack frame containing `x` and `y`
may well have disappeared by the time we try to use them. Even if we call
`thr.join()` within foo (which blocks until `thr` has completed, ensuring the
stack frame won't disappear), we will not succeed: the compiler cannot prove
that this behavior is safe, and so won't let us do it.
The solution to this problem is usually to switch to using a `move` closure.
This approach moves (or copies, where possible) data into the closure, rather
than taking references to it. For example:
```
fn foo() -> Box<Fn(u32) -> u32> {
let x = 0u32;
Box::new(move |y| x + y)
}
```
Now that the closure has its own copy of the data, there's no need to worry
about safety.
This error may also be encountered while using `async` blocks:
```compile_fail,E0373,edition2018
use std::future::Future;
async fn f() {
let v = vec![1, 2, 3i32];
spawn(async { //~ ERROR E0373
println!("{:?}", v)
});
}
fn spawn<F: Future + Send + 'static>(future: F) {
unimplemented!()
}
```
Similarly to closures, `async` blocks are not executed immediately and may
capture closed-over data by reference. For more information, see
<https://rust-lang.github.io/async-book/03_async_await/01_chapter.html>.
|
