Citation for !Sized and trait default implementations

[cole-miller]

Thanks for putting this together, I think it'll be really useful.

If you still need a citation for this under dyn Trait:

• Leads to situation where the only implementation for unsized types are default function bodies

you could perhaps link to this thread: https://internals.rust-lang.org/t/feature-idea-generic-trait-methods-on-trait-objects-using-the-default-implementation/15174. (Which I see you posted in.)