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Clean up E0376 explanation

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This commit is contained in:
Guillaume Gomez 2020-03-01 12:03:38 +01:00
parent abc3073c92
commit dd142b1dc4
1 changed files with 11 additions and 10 deletions
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21
src/librustc_error_codes/error_codes/E0376.md
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@ -1,14 +1,6 @@
The type you are trying to impl `CoerceUnsized` for is not a struct.
`CoerceUnsized` can only be implemented for a struct. Unsized types are
already able to be coerced without an implementation of `CoerceUnsized`
whereas a struct containing an unsized type needs to know the unsized type
field it's containing is able to be coerced. An [unsized type][1]
is any type that the compiler doesn't know the length or alignment of at
compile time. Any struct containing an unsized type is also unsized.
`CoerceUnsized` was implemented on something that isn't a struct.
[1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
Example of erroneous code:
Erroneous code example:
```compile_fail,E0376
#![feature(coerce_unsized)]
@ -22,6 +14,15 @@ struct Foo<T: ?Sized> {
impl<T, U> CoerceUnsized<U> for Foo<T> {}
```
`CoerceUnsized` can only be implemented for a struct. Unsized types are
already able to be coerced without an implementation of `CoerceUnsized`
whereas a struct containing an unsized type needs to know the unsized type
field it's containing is able to be coerced. An [unsized type][1]
is any type that the compiler doesn't know the length or alignment of at
compile time. Any struct containing an unsized type is also unsized.
[1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
The `CoerceUnsized` trait takes a struct type. Make sure the type you are
providing to `CoerceUnsized` is a struct with only the last field containing an
unsized type.