Garbage collected trace objects (`dyn Trace`)

[andersk]

The current Trace trait is not object-safe, so it can’t be used with trait objects:

trait Foo: Trace {
    fn foo(&self);
}

#[derive(Trace)]
struct AnyFoo(dyn Foo);

Could it be made object-safe by turning the associated constants into functions and moving the V: GcVisitor type parameter of Trace::visit<V> up to Trace<V>::visit?

[Techcable]

Oh man ☹️ Based on my experiments today, this is hard to do right.

I really don't want to parameterize Trace like Trace<V: GcVisitor>.

I've played around with having a separate set of DynTrace and DynGcVisitor taits (kind of like erased-serde) but its hard to write blanket impls over regular Trace/GcVisitor traits because they expect statically visible type info.

I'm starting to think maybe I should go closer to the approach you suggested and modify Trace and GcVisitor to be object safe directly.
I really want to keep support for NEEDS_TRACE, but that is hard to do since associated constants aren't supported on trait objects 😦 There is no way to add a special where Self: Sized bound. Maybe I can move it to a separate trait and then specialize?

Kind of just throwing out random suggestions here. This project is still a little experimental 😄 😉

This is related to #16 because both arrays and trait objects are ?Sized.

[Techcable]

The new Pointee type and the nightly ptr_metadata API may significantly change this. (See rust-lang/rust#81513)

I'm hesitant to refactor the API design to depend so heavily on a nightly feature, however that may be the only way I can reasonably implement this feature.

In practice, however, zerogc is already limited to nightly because the simple collector requires nightly (although this is mostly for convenience at this point).