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Clean up E0733 explanation #74781

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Aug 2, 2020
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Clean up E0733 explanation #74781

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14 changes: 9 additions & 5 deletions src/librustc_error_codes/error_codes/E0733.md
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Original file line number Diff line number Diff line change
@@ -1,4 +1,6 @@
Recursion in an `async fn` requires boxing. For example, this will not compile:
An [`async`] function used recursion without boxing.

Erroneous code example:

```edition2018,compile_fail,E0733
async fn foo(n: usize) {
Expand All @@ -8,8 +10,8 @@ async fn foo(n: usize) {
}
```

To achieve async recursion, the `async fn` needs to be desugared
such that the `Future` is explicit in the return type:
To perform async recursion, the `async fn` needs to be desugared such that the
`Future` is explicit in the return type:

```edition2018,compile_fail,E0720
use std::future::Future;
Expand All @@ -36,5 +38,7 @@ fn foo_recursive(n: usize) -> Pin<Box<dyn Future<Output = ()>>> {
}
```

The `Box<...>` ensures that the result is of known size,
and the pin is required to keep it in the same place in memory.
The `Box<...>` ensures that the result is of known size, and the pin is
required to keep it in the same place in memory.

[`async`]: https://doc.rust-lang.org/std/keyword.async.html