> I think it is possible to make a language that has both a GC and a borrow checker, treating the GC types as a third level next to the stack and the heap, wher

> I think it is possible to make a language that has both a GC and a borrow checker, treating the GC types as a third level next to the stack and the heap, where complex referencial cycles can be 'promoted to the GC' It's doable but there are a few issues with that whole idea. You need the ability to safely promote objects to GC-roots whenever they're being referenced by GC-unaware code, and demote them again afterwards. And if any GC object happens to control the lifecycle of any heap-allocated objects, it must have a finalizer that cleans them up RAII style to avoid resource leaks.