Infinitely recursive `dyn`

[Jules-Bertholet]

As discovered by @theemathas at rust-lang/rust#142900 (comment), the following compiles:

trait Trait: Sync {}

impl Trait for &dyn Trait {}

static X: &dyn Trait = &X;

Perhaps this is too obscure even for the quiz, but I’m sure it’s possible to make a fun question based on this trick.