When can structures be stored in the heap, and value types in the stack?

Structures are always stored on the stack when they are local variables within a function.

Structures can be stored on the heap in the following cases:

1. When they are properties of a class. Instances of classes are always on the heap, and their properties (including structures) are stored together with the class instance on the heap.
2. When they are captured by a closure, and the closure stores them on the heap.
3. When they are inside other types that are stored on the heap, such as arrays or dictionaries stored on the heap.

Value types (structures, enumerations, basic types like Int, Bool, etc.) are always stored on the stack when they are local variables. Value types cannot be stored on the stack in any form if they are a property of a class or an element of a container stored on the heap.