converting a number into bit array

[Alex]

Given this,

´´´
import std.bitmanip;
import core.stdc.limits;

void main()
{
	BitArray ba;
	ba.length = size_t.sizeof * CHAR_BIT; // enough length, known at 
compile time
	size_t arbitrary; // = random size_t, not known at compile time

	// ba ???assign??? arbitrary;
         assert(cast(size_t[])ba == [arbitrary]);
}
´´´

how to convert a number to a BitArray as fast as possible, given 
that the BitArray is already allocated to the needed length?
Is bit checking the way to go, or is there a way to cast one to 
the other somehow?