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Rollup merge of #65200 - xfix:patch-20, r=GuillaumeGomez

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Add ?Sized bound to a supertrait listing in E0038 error documentation

This example failed to compile because of implicit `Sized` bound for `A` parameter that wasn't required by `Trait`.
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Tyler Mandry 2019-10-11 15:09:47 -07:00 committed by GitHub
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14
src/librustc/error_codes.rs
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@ -259,8 +259,8 @@ trait Foo {
This is similar to the second sub-error, but subtler. It happens in situations This is similar to the second sub-error, but subtler. It happens in situations
like the following: like the following:
```compile_fail ```compile_fail,E0038
trait Super<A> {} trait Super<A: ?Sized> {}
trait Trait: Super<Self> { trait Trait: Super<Self> {
} }
@ -270,17 +270,21 @@ struct Foo;
impl Super<Foo> for Foo{} impl Super<Foo> for Foo{}
impl Trait for Foo {} impl Trait for Foo {}
fn main() {
let x: Box<dyn Trait>;
}
``` ```
Here, the supertrait might have methods as follows: Here, the supertrait might have methods as follows:
``` ```
trait Super<A> { trait Super<A: ?Sized> {
fn get_a(&self) -> A; // note that this is object safe! fn get_a(&self) -> &A; // note that this is object safe!
} }
``` ```
If the trait `Foo` was deriving from something like `Super<String>` or If the trait `Trait` was deriving from something like `Super<String>` or
`Super<T>` (where `Foo` itself is `Foo<T>`), this is okay, because given a type `Super<T>` (where `Foo` itself is `Foo<T>`), this is okay, because given a type
`get_a()` will definitely return an object of that type. `get_a()` will definitely return an object of that type.