How to use a char[] buffer in D

[Andrew Chapman via Digitalmars-d-learn]

Hi everyone, just wanting some help with optimisation if anyone 
is kind enough :-)

I have a loop that iterates potentially millions of times, and 
inside that loop I have code that appends some strings together, 
e.g.:

string key = s1 ~ "_" ~ s2;

I discovered that due to the memory allocation required, this 
slows the execution significantly.

s1 is always a two character string, e.g "AA", and s2 is always a 
single character.

What I want to do is something like this:

Outside the loop:

char[4] buffer;
buffer[2] = '_';

Then inside the loop

buffer[0] = s1[0];
buffer[1] = s1[1];
buffer[3] = s2[0];

This works OK, however, I then need to use the buffer value to 
check for an existing value in a hashmap / associative array.

Code such as:

if(buffer in myHash) {

}

throws an access violation.  A string value works without error.  
Is there a way for me to use a buffer AND use it in functions 
expecting strings?

I can use idup() on the char[] to make a string, but again we're 
allocating memory which I'd rather avoid.

Thanks sincerely in advance,
Cheers,
Andrew.