DIP1000 scope inference proposal

[Walter Bright]

On 10/26/2022 1:03 AM, Walter Bright wrote:
> On 10/24/2022 6:35 PM, Steven Schveighoffer wrote:
>> In a `@trusted` function today, without dip1000, the above is perfectly 
>> reasonable and not invalid. Will dip1000 make it corrupt memory?
> 
> A very good question. Clearly, having code work when it is @safe, but cause 
> memory corruption when it is marked @trusted, is the wrong solution. This should 
> never happen. I'm not sure what the solution should be here.
> 

[Some more thinking about the problem]

The question is when is [1,2,3] allocated on the stack, and when is it allocated 
on the GC heap?

Some points:

1. in C it is allocated on the stack. D's behavior to allocate it on the heap is 
kinda surprising in that light, even though D had such literals before C did

2. allocating on the heap means it is unusable in @nogc code

3. when writing expressions, the only way to get it on the stack is to assign it 
to a scope variable, which is inconvenient and inefficient

4. it runs against the idea that the simpler code should be more efficient than 
the complex code

Therefore, I suggest the following:

     [1,2,3] is always allocated on the stack

     [1,2,3].dup is always allocated on the heap

and thus, its behavior is not dependent on inference.

How we transition to this, we'll have to figure out.