Question to GC experts: NO_SCAN for a part of the block

[Stanislav Blinov via Digitalmars-d]

On Monday, 5 June 2017 at 13:11:25 UTC, Steven Schveighoffer 
wrote:

> adding a range marks it as having pointers to scan, AND stores 
> a reference to that array, so it won't be GC collected (nor 
> will anything it points to). The intention is for it to be used 
> on non-GC memory, like C malloc'd memory, where it doesn't 
> matter that the GC is pointing at it.
>

Huh?

https://dlang.org/phobos/core_memory.html#.GC.addRange :

> Note that p[0 .. sz] is treated as an opaque range of memory 
> assumed to be suitably managed by the caller. In particular, if 
> p points into a GC-managed memory block, addRange does not mark 
> this block as live.

Is that paragraph wrong?

> I would say that you are better off allocating 2 arrays -- one 
> with NO_SCAN where you put your non-pointer-containing data, 
> and one without the flag to put your other data. This is 
> similar to your "selective" function, but instead of allocating 
> 1 array, with a tuple of slices into it, just allocate 2 arrays 
> and return the tuple of those 2 arrays.

Then it's just the obvious() function. The whole point of the 
exercise is to make one GC allocation instead of N :) But still 
GC, so as not to put additional responsibility on the caller.