What kind of type signature would prove the first block of any directory in an ext4 filesystem image isn’t a hole?
I don't know if the type system proves it's not a hole, but the type system certainly seems to force consumers to contend with the possibility by surfacing the outcomes at the type system level. That's what the Either is doing in the example's return type, is it not?
fn get_or_create_inode(
&self,
ino: Ino
) -> Result<Either<ARef<Inode<T>>, inode::New<T>>>