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Remove the return Box<dyn Trait> issue in async fn #88

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Apr 28, 2020
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Remove the return Box<dyn Trait> issue in async fn #88

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28 changes: 0 additions & 28 deletions src/07_workarounds/02_return_type.md
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Expand Up @@ -42,31 +42,3 @@ function signature with the message "expected `SomeType`, found `OtherType`"
usually indicate that one or more return sites are incorrect.

A fix to this issue is being tracked in [this bug](https://github.com/rust-lang/rust/issues/54326).

## `Box<dyn Trait>`

Similarly, because the return type from the function signature is not
propagated down correctly, values returned from `async fn` aren't correctly
coerced to their expected type.

In practice, this means that returning `Box<dyn Trait>` objects from an
`async fn` requires manually `as`-casting from `Box<MyType>` to
`Box<dyn Trait>`.

This code will result in an error:

```rust,edition2018,ignore
async fn x() -> Box<dyn std::fmt::Display> {
Box::new("foo")
}
```

This issue can be worked around by manually casting using `as`:

```rust,edition2018
async fn x() -> Box<dyn std::fmt::Display> {
Box::new("foo") as Box<dyn std::fmt::Display>
}
```

A fix to this issue is being tracked in [this bug](https://github.com/rust-lang/rust/issues/60424).