Make iter::Empty<T> Send and Sync for any T
Continuing from #57682
It's quite funny, when I initially submitted this pull request, I said "Likely nobody will be using that property of `iter::empty`", but then a year later I got a compilation error because it wasn't `Send` and `Sync`.
Unfortunately, `PhantomData<fn() -> T>` still errors out. Oh well. I proposed `
struct PhantomFnWorkaround<T>(fn() -> T);`, but dtolnay did not like it, so using explicit implementations.
Dylan DPC
d276e6942e