Initializing static array with contents of (static and dynamic) arrays

[Johannes Loher via Digitalmars-d-learn]

In a project I am currently working on, I have lot's of code of the
following form:

static immutable ubyte[4] sigma0 = [101, 120, 112,  97];
static immutable ubyte[4] sigma1 = [110, 100,  32,  51];
static immutable ubyte[4] sigma2 = [ 50,  45,  98, 121];
static immutable ubyte[4] sigma3 = [116, 101,  32, 107];

void func(in ubyte[32] key, in ubyte[16] n)
{
    ubyte[64] buf;
    buf[0..4] = sigma0;
    buf[4..20] = key[0..16];
    buf[20..24] = sigma1;
    buf[24..40] = n;
    buf[40..44] = sigma2;
    buf[44..60] = key[16..$];
    buf[60..64] = sigma3;

   /* do something with buf */
}

This looks really bad to me. I would like to initialize buf with the
corresponding values (the way it's done now, it is impossible to make
buf immutable...).

One option for this would be to use ~ to concatenate the arrays. But
this obviously results in GC allocations, which I want to avoid here,
because the functions get called very often.

Another option would be to to manually expand the arrays and initialize
buf with that:

ubyte[64] buf = [sigma0[0], sigma0[1], /* ... */, sigma3[3]];

This is obviously very annoying and error prone.

Whe searching for a solution, I found this thread about automatic
expanding of arrays:

http://forum.dlang.org/thread/hwellpcaomwbpnpofzlx@forum.dlang.org?page=1

This would result in the following code for me:

ubyte[64] buf = [
    expand!sigma0,
    expand!key[0..16],
    expand!sigma1,
    expand!n,
    expand!sigma2,
    expand!key[16..$],
    expand!sigma3
];

Is this the suggested solution (is it robust)? Is there anything like
this in Phobos?

Thanks for your help!