Efficient string concatenation?

[Jacek Furmankiewicz]

Since D strings are immutable (like in most other languages), 
string concatenation is usually pretty inefficient due to the 
need to create a new copy of the string every time.

I presume string concatenation using the typical array syntax can 
be optimized by the compiler to do all of this in one shot, e..g

string newString = string1 ~ string2 ~ string3;

but what happens if I need to concatenante a large string in a 
loop?

I tried looking through Phobos for a StringBuilder class (since 
that is the common solution in Java and C#), but did not find 
anything similar.

What is the D way of doing efficient string concatenation 
(especially if it spans multiple statements, e.g. while in a 
loop)?