Add spec for heap specifier

media/specs/heap-specifier.md

@@ -0,0 +1,67 @@
+# Technical Specification for the heap specifier
+
+## Implementation steps:
+
+- [x] Add basic support
+- [x] Move all std data structures to use the heap specifier
+- [x] Add tests for this feature
+- [x] Generate default dtor for structs with heap fields where free is called on those fields
+- [x] Make copy ctor mandatory for structs with heap fields
+- [ ] Allow assignment from heap to non-heap pointers to create non-owning pointers
+- [ ] Perform a swap when assigning from heap to heap pointers
+
+## Syntax
+
+```spice
+// As struct field
+type TestStruct struct {
+    heap int* ptr
+}
+
+// As local variable
+f<int> foo() {
+    heap int* ptr = sAlloc(sizeof(type int));
+    *ptr = 5;
+    return ptr;
+}
+```
+
+## Functionality
+
+### General introduction
+
+The `heap` specifier can only be applied to pointer variables/fields and is used to indicate that the memory for the
+variable/field was or is going to be allocated on the heap. It acts similar to a `unique_ptr` would in C++, but builtin
+into the language.
+
+The heap specifier can be used in the following ways:
+
+- As a field type specifier in a struct
+- As a local variable type specifier
+- As a function return type specifier
+- As a parameter type specifier
+
+When the `heap` specifier is used for a struct field, the compiler will automatically generate a destructor for that
+struct, that will free the memory allocated for the field. The `heap` specifier can also be used in local variables
+and function return types. When a function returns a `heap` pointer, the caller is responsible for freeing the memory
+allocated for the pointer. The `heap` specifier can also be used in function parameters. When a function parameter is
+declared as `heap`, the callee is responsible for freeing the memory allocated for the parameter.
+
+### Assignment semantics
+
+When assigning a `heap` pointer to a non-heap pointer, the non-heap pointer becomes a non-owning pointer. This means
+that the non-heap pointer does not have to free its memory, as it does not own it. The memory is still owned by the
+`heap` pointer, and the memory will be freed when the `heap` pointer goes out of scope.
+
+When assigning a `heap` pointer to another `heap` pointer, the memory is not copied, but the ownership gets
+transferred to the left hand side variable. This means that the original `heap` pointer will get the value `nil` and is
+no longer responsible for freeing the memory. This is done to prevent double freeing of the memory.
+
+Assigning a non-heap pointer to a `heap` pointer is not allowed, as the non-heap pointer does not own the memory which
+it points to.
+
+### Struct implications
+
+When a struct has a field with the `heap` specifier, the compiler will automatically generate a destructor for the
+struct. The destructor will free the memory allocated for the `heap` field. The compiler will also make the copy
+constructor mandatory for the struct. This is done to prevent double freeing of the memory.