Comment by alfiedotwtf a day ago
In my universe, `let` wouldn’t exist… instead there would only be 3 ways to declare variables:
1. global my_global_var: GlobalType = …
2. heap my_heap_var: HeapType = …
3. stack my_stack_var: StackType = …
So by having the location of allocation in the type itself, we no longer have to do boxing mental gymnastics
You may already know this, but let-bindings are not necessarily on the stack. The reference does say they are (it's important to remember that the reference is not normative), and it is often simpler to think of them that way, but in reality they don't have to be on the stack.
The compiler can perform all sorts of optimizations, and on most modern CPU architectures, it is better to shove as many values into registers as possible. If you don't take the address of a variable, you don't run out of registers, and you don't call other, non-inlined functions, then let-bindings (and function arguments/return values) need not ever spill onto the stack.
In some cases, values don't even get into registers. Small numeric constants (literals, consts, immutable lets) can simply be inlined as immediate values in the assembly/machine code. In the other direction, large constant arrays and strings don't spill onto the stack but rather the constant pool.
But what does "heap my_heap_var" actually mean, without a garbage collector? Who owns "my_heap_var" and when does it get deallocated? What does explicitly writing out the heap-ness of a variable ultimately provide, that Rust's existing type system with its many heap-allocated types (Box, Rc, Arc, Vec, HashMap, etc.) doesn't already provide?
> What does explicitly writing out the heap-ness of a variable ultimately provide, that Rust's existing type system with its many heap-allocated types (Box, Rc, Arc, Vec, HashMap, etc.) doesn't already provide?
To be honest, I was thinking more in terms of cognitive overload i.e. is all that Box boilerplate even needed if we were to treat all `heap my_heap = …” as box underneath? In other words, couldn’t we elide all that away:
let foo = Box::new(MyFoo::default ());
heap foo = MyFoo::default();
Doesn't Rust do this? `let` is always on the stack. If you want to allocate on the heap then you need a Box. So `let foo = Box::new(MyFoo::default ())` creates a Box on the stack that points to a MyFoo on the heap. So MyFoo is a stack type and Box<MyFoo> is a heap type. Or do you think there is value in defining MyFooStack and MyFooHeap separately to support both use cases?