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

[KamilaBorowska]

This example failed to compile because of implicit Sized bound for A parameter that wasn't required by Trait.

[rust-highfive]

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

[rust-highfive]

r? @davidtwco

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[davidtwco]

I'm not sure that this is an improvement, but my understanding of what this example is intended to demonstrate might be incorrect.

I think this example is intended to demonstrate a failing example for this case - this is what the previous cases do, so changing this to a passing example might be not desirable.

As I understand it, this example attempts to demonstrate why a trait is no longer object-safe when it uses Self in a supertrait listing:

When a trait Foo has a supertrait Super<Self> then that can make Foo non-object-safe even if Super<T> is object-safe. Suppose Super<T> contains a method returning T - and Super<T> is still object-safe if it does - then when used as a supertrait (Super<Self>), the subtrait would be not be object-safe as it would have a method returning a unknown type (this is the "Method references the Self type in its parameters or return type" case from above).

I think the error's example is better suited to explaining that if it remains as a failing example than as a passing example, as in this PR.

That said, the next paragraph might be slightly incorrect as it references a "trait Foo" even though the example names the trait "Trait", so if you want to modify this PR to correct that instead, then that would be appreciated.

[GuillaumeGomez]

The first example is supposed to fail and is checked as such.

[KamilaBorowska]

@davidtwco @GuillaumeGomez

Is this how the example is supposed to fail?

trait Super<A> {}

trait Trait: Super<Self> {}

Gives the following error:

error[E0277]: the size for values of type `Self` cannot be known at compilation time
 --> src/lib.rs:3:1
  |
3 | trait Trait: Super<Self> {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `Self`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = help: consider adding a `where Self: std::marker::Sized` bound
note: required by `Super`
 --> src/lib.rs:1:1
  |
1 | trait Super<A> {}
  | ^^^^^^^^^^^^^^

Note that the error is not E0038, but rather E0277. The issue is that with trait Trait: Super<Self> {}, Self doesn't have Sized bound, which is required by A parameter in Super<A> (due to implicit Sized bound on generic parameters).

There are two ways to fix that, one is to put Sized bound on Trait, but that would essentially make this Self: Sized requirement (nothing special about that one), so I picked the other way of putting a T: ?Sized bound so that Self would be used as a type parameter in the supertrait listing.

[KamilaBorowska]

But yeah, now that I'm checking there is something weird with the description, although I think it should be "trait Trait" instead of "trait Foo" (a rather small change).

[davidtwco]

@davidtwco @GuillaumeGomez Is this how the example is supposed to fail?

I think it would probably be desirable for the example to error with the code that it is describing (that said, I don't maintain the error codes and the intent might just have been to have as simple an example as possible that explains the case).

Note that the error is not E0038, but rather E0277. The issue is that with trait Trait: Super<Self> {}, Self doesn't have Sized bound, which is required by A parameter in Super<A> (due to implicit Sized bound on generic parameters).

I think that this slightly modified version of the example is probably what we're after. It shows that Trait is not object-safe only because of the Super<Self> (it's otherwise empty).

[KamilaBorowska]

Yeah, I can add main function, no problem :).

[GuillaumeGomez]

👍

[GuillaumeGomez]

Thanks!

@bors: r+ rollup

[bors]

📌 Commit 3f9d834 has been approved by GuillaumeGomez