We (Google) do both. Having recently done some coding in Swift to implement a little toy Hypervisor, I sorely missed these annotations. By and large the errors

We (Google) do both. Having recently done some coding in Swift to implement a little toy Hypervisor, I sorely missed these annotations. By and large the errors they catch are ones I'd be embarrassed to have pointed out in a code review; I don't mind nearly so much when the compiler catches me without my mutex locked.