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Rollup merge of rust-lang#69634 - GuillaumeGomez:clean-up-e0378, r=Dy…
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…lan-DPC

clean up E0378 explanation

r? @Dylan-DPC
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Dylan-DPC authored Mar 2, 2020
2 parents 2dd28f8 + ba49ed0 commit 68885ab
Showing 1 changed file with 22 additions and 20 deletions.
42 changes: 22 additions & 20 deletions src/librustc_error_codes/error_codes/E0378.md
Original file line number Diff line number Diff line change
@@ -1,10 +1,28 @@
The `DispatchFromDyn` trait was implemented on something which is not a pointer
or a newtype wrapper around a pointer.

Erroneous code example:

```compile-fail,E0378
#![feature(dispatch_from_dyn)]
use std::ops::DispatchFromDyn;
struct WrapperExtraField<T> {
ptr: T,
extra_stuff: i32,
}
impl<T, U> DispatchFromDyn<WrapperExtraField<U>> for WrapperExtraField<T>
where
T: DispatchFromDyn<U>,
{}
```

The `DispatchFromDyn` trait currently can only be implemented for
builtin pointer types and structs that are newtype wrappers around them
— that is, the struct must have only one field (except for`PhantomData`),
and that field must itself implement `DispatchFromDyn`.

Examples:

```
#![feature(dispatch_from_dyn, unsize)]
use std::{
Expand All @@ -20,6 +38,8 @@ where
{}
```

Another example:

```
#![feature(dispatch_from_dyn)]
use std::{
Expand All @@ -37,21 +57,3 @@ where
T: DispatchFromDyn<U>,
{}
```

Example of illegal `DispatchFromDyn` implementation
(illegal because of extra field)

```compile-fail,E0378
#![feature(dispatch_from_dyn)]
use std::ops::DispatchFromDyn;
struct WrapperExtraField<T> {
ptr: T,
extra_stuff: i32,
}
impl<T, U> DispatchFromDyn<WrapperExtraField<U>> for WrapperExtraField<T>
where
T: DispatchFromDyn<U>,
{}
```

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