Swift leaves it in the hand of programmers and says "use weak references".

Otherwise traditional GC use a copying mechanism, what doesn't end up copied is dead.

The old reference counting GC in Nim had that extra copying pass when a type could have cycles.

That said, it's very easy to tell if a type can have cycles or not at compile-time, you just need to check if it refers to itself i.e. does the Node type has a Node field (or a field that can recursively contain a Node field).