Obviously a 10-million-element array doesn't get stack allocated. But if individual objects of some type are immutable, then they can be stack allocated, or maybe not allocated at all (kept in registers).
Edit: reading your other post, it seems like you may mean persistent data structures, a la Clojure, rather than immutable structures, which are quite different. The former would indeed always be heap-allocated (it's necessary since they are quite pointer-heavy). Immutable structures, on the other hand are detached from any particular location in memory.
Moreover, if the elements in an array are mutable, eg Java objects, then each one needs to be individually heap allocated with a vtable pointer and the array has to be an array of pointers to those individually allocated objects. For pointer-sized objects (say an object that has a single pointer-sized field), that takes 3x memory to store x objects, so that's already brutal, but worse is that since the objects are all individually allocated, the GC needs to look at every single one, and freeing the space is a fragmentation nightmare. If the objects are immutable (and the type is final; btw all concrete types are final in Julia), then you can store them inline with no overhead and GC can deal with them a single big block.
Btw, I had to vouch for you to undead your posts in order to reply. Looks like you got downvoted a bunch.
