Hypothetically, (I'm not sure how widespread this technique is), some (Java) compilers turn heap allocations into stack allocations when they can prove that transitively an allocated object does not become reachable from other threads.

This general technique "escape analysis", i.e. "a reference to this object /escapes/ a given reachability type[1]", can be used to transform general heap allocations into thread local or stack allocations.

[1] Let me define reachability type as {can be accessed from anything, can be accessed from this thread only but is of indefinite lifetime, can be accessed from this thread only and it's reference is not take by any object allocated onto my local heap, ...}