Is it possible to assumeSafeAppend malloced memory?

[monarch_dodra via Digitalmars-d-learn]

On Monday, 19 May 2014 at 06:08:18 UTC, Ali Çehreli wrote:
> We know that most of the time memory is allocated more than the 
> requested amount. Is there a way to take advantage of that 
> extra trailing space? (And potentially the pages that come 
> after that.)
>
> import core.memory;
>
> void main()
> {
>     const count = 1;
>
>     // I think there is extra capacity beyond the 'count' 
> elements
>     int* ptr = cast(int*)GC.malloc(count * int.sizeof);
>     int[] arr = ptr[0 .. count];
>
>     assert(arr.capacity == 0);
>     arr.assumeSafeAppend;
>     assert(arr.capacity == 0);    // still 0. :(
> }
>
> This issue puts std.array.array to a disadvantage compared to 
> proper slices because array() involves the following call 
> chain, the last of which does call GC.malloc:
>
>   trustedAllocateArray
>   uninitializedArray
>   arrayAllocImpl
>
> As a result, iota(10).array.assumeSafeAppend ends up having 0 
> capacity. :(
>
> Ali

Recently, a new function in druntime was added: "_d_newarrayU".

This void allocates a new array *with* appendable information. We 
can hope it will be given a more formal and public interface, and 
it would then be useable by array and/or Appender, to produce 
slices that have appendable data.