DIP1000 scope inference

[Paul Backus]

On Tuesday, 25 October 2022 at 02:38:02 UTC, Steven Schveighoffer 
wrote:
> OK, what about this?
>
> ```d
> int[] mkarr() @trusted {
>     int[3] arr = [1, 2, 3];
>     int[] other = [4, 5, 6];
>
>     auto foo = other;
>     other = arr[];
>     return foo;
> }
> ```
>
> `other` is inferred as `scope` (along with `foo`), because it 
> touches `arr[]` later (but after it was pointing at what should 
> have been heap memory). So does that count as possible for 
> stack allocation, or is it still heap allocated?

When I compile the above with `@safe` and `-preview=dip1000`, I 
get

     Error: reference to local variable `arr` assigned to 
non-scope `other`

...using both DMD 2.100.2 and DMD master. So `scope` is not 
actually being inferred here, and the array is allocated on the 
heap.

My expectation is that `scope` will probably *never* be inferred 
for `other`, because doing multi-step inference like this 
requires dataflow analysis in the general case, which is 
something Walter wants to avoid (see discussion in [issue 
20674][1]). So I don't think you have anything to worry about.

Still, this is a good illustration of how silently changing the 
rules on people can have unintended consequences. If Walter ever 
*does* consider adding dataflow analysis, overly-aggressive 
"optimizations" like these could easily become obstacles in the 
way of that goal.

[1]: https://issues.dlang.org/show_bug.cgi?id=20674