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clean up E0378 explanation

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Guillaume Gomez 2020-03-02 13:10:24 +01:00
parent 01a8b5f26e
commit ba49ed01f0
1 changed files with 22 additions and 20 deletions
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42
src/librustc_error_codes/error_codes/E0378.md
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@ -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::{
@ -20,6 +38,8 @@ where
{}
```
Another example:
```
#![feature(dispatch_from_dyn)]
use std::{
@ -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>,
{}
```