Principled method of lookup-or-insert in associative arrays?

[Andrei Alexandrescu]

TDPL has an example that can be reduced as follows:

void main() {
   uint[string] map;
   foreach (line; stdin.byLine()) {
     ++map[line];
   }
}

byLine reuses its buffer so it exposes it as char[]. Therefore, 
attempting to use map[line] will fail. The program compiled and did the 
wrong thing because of a bug.

The challenge is devising a means to make the program compile and work 
as expected. Looking up an uint[string] with a char[] should work, and 
if the char[] is to be inserted, a copy should be made (via .idup or 
to!string). The rule needs to be well defined and reasonably general.

The effect is something like this:

void main() {
   uint[string] map;
   foreach (line; stdin.byLine()) {
     auto p = line in map;
     if (p) ++*p;
     else map[line.idup] = 1;
   }
}

Ideally the programmer would write the simple code (first snippet) and 
achieve the semantics of the more complex code (second snippet). Any ideas?

Andrei