make Sized predicates coinductive

[lcnr]

🤷

needs either an MCP/RFC and wg-traits signoff. While I think this is sound, I am not too knowledgeable about coinduction so I might be wrong here.

r? @nikomatsakis

[nikomatsakis]

I'm going to want to talk about the motivation =)

[lcnr]

I still want to add this as a test, so that I don't forget. I expect the error here to change from a predicate cycle to "recursive type Foo<()> has infinite size"

trait A { type Assoc; }

impl A for () {
    type Assoc = Foo<()>;
}
struct Foo<T: A>(T::Assoc);

fn main() {}

[lcnr]

also

trait A { type Assoc: ?Sized; }

impl A for () {
    type Assoc = Foo<()>;
}
struct Foo<T: A>(T::Assoc);

fn main() {
    let x: Foo<()>;
}

[JohnCSimon]

triage: @lcnr - is this still blocked?

[nikomatsakis]

@lcnr is taking a bit of a break from Rust at the moment -- we could close this, but it's also ok to leave it open.

[lcnr]

don't have the capacity to work on this rn

[marmeladema]

Would this allow to compile:

use std::borrow::Cow;

#[derive(Clone)]
enum Test<'a> {
    Int(u8),
    Array(Cow<'a, [Test<'a>]>),
}

?