Flatten a range of static arrays

[ag0aep6g]

On 08.02.20 02:38, ag0aep6g wrote:
> Simplified, we're looking at this:
> 
> ----
> struct Joiner
> {
>      int[3] _items;
>      int[] _current;
> }
> void main() @safe
> {
>      Joiner j;
>      j._current = j._items[];
> }
> ----
> 
> I.e., a self-referential struct. Or most fundamentally:
> 
> ----
> struct Joiner
> {
>      Joiner* p;
> }
> void main() @safe
> {
>      Joiner j;
>      j.p = &j; /* error */
> }
> ----
> 
> `dmd -dip1000` complains about the marked line:
> 
>      Error: reference to local variable j assigned to non-scope j.p
> 
> What if I mark `j` as `scope`? Then I should be able to assign a 
> reference-to-local to `j.p`. Indeed, the error goes away, but another 
> takes its place:
> 
>      Error: cannot take address of scope local j in @safe function main
> 
> Right. That can't be allowed, because `scope` gives only one level of 
> protection, and `&j` would need two (one for `j` itself and one for the 
> thing it points at).

I went a bit overboard with that second reduction. The original struct 
is self-referential, but it's not recursive. The errors are the same for 
the first reduction. But it's not as clear that they're necessary.

In the first reduction, `j` might be `scope`, but `j._items` has no 
indirections. `scope` doesn't actually mean anything for it. It's just 
an `int[3]` on the stack. So taking its address could be allowed.

But (the current implementation of) DIP 1000 is apparently too 
conservative for that. It seems to treat pointers into a struct the same 
as pointers to the whole thing.