Update section on "existential type" to "opaque type"

[varkor]

Updates the guide as per the changes in rust-lang/rfcs#2515.

[Centril]

cc rust-lang/rust#63063

[mark-i-m]

Ping @varkor there are some comments waiting for you

[varkor]

Addressed the comments.

[mark-i-m]

Thanks!

[kpp]

Shouldn't it be struct Foo; ?

[varkor]

Yes it should! Feel free to open a PR with a fix :)

Edit: I misread. See comment below.

[kpp]

Then the whole example should be

fn foo() -> impl Bar {
    struct Foo;
    impl Bar for Foo { /* stuff */ }
    Foo
}

?

[varkor]

Sorry, I misread earlier. Really, this code snippet should be read in the context of the previous one. But that's confusing, so maybe we should add the definition of Foo here too. So the complete snippet would be:

type Foo = impl Bar;

struct Struct;

impl Bar for Struct { /* stuff */ }

fn foo() -> Foo {
    Struct
}