Extra .tupleof field in structs with disabled postblit blocks non-GC-allocation trait

[Per Nordlöw]

On Wednesday, 9 May 2018 at 14:07:37 UTC, Per Nordlöw wrote:
> This prevents the trait `mustAddGCRangeOfStructOrUnion` [1] 
> from detecting when a container with manual memory management 
> doesn't have to be scanned by the GC as in, for instance,
>
>     enum NoGc;
>     struct S
>     {
>         @disable this(this); // disable S postlib
>         @NoGc int* _ptr;
>     }
>     static assert(!mustAddGCRangeOfStructOrUnion!S); // is 
> false when postblit of `S` is disabled
>
> [1] 
> https://github.com/nordlow/phobos-next/blob/master/src/gc_traits.d#L81

Can we statically check if the postblit has been disabled via

@disable this(this);

?

If so, we can temporarily modify the trait to exclude the last 
`void*` member of the `S.tuple`. Given that it's always added as 
the last member.