std.digest toHexString

[Carl Sturtivant via Digitalmars-d-learn]

What's going on here?
```
import std.digest.md, std.stdio;

void main() {
     string ans = hex("qwertyuiop");
     writeln(ans);
}

string hex(string arg) {
     string ans = md5Of(arg).toHexString();
     writeln(ans);
     return ans;
}
```
This compiles, and when run writes out corrupt nonsense from the 
writeln in main, while just before that writing out 
6EEA9B7EF19179A06954EDD0F6C05CEB from the function hex.

If I replace md5Of(arg).toHexString() with 
toHexString(md5Of(arg)) it won't compile, claiming that 
"toHexString.d(11): Error: function expected before (), not 
module toHexString of type void". Yet the examples here
http://dlang.org/phobos/std_digest_md.html
use toHexString freely in this way. I thought I was using 
toHexString by UFCS, in the earlier example, but this appears to 
be untrue.

The return from the function hex above somehow corrupts the 
string returned, suggesting deallocation is occurring in some 
fashion. So I replaced md5Of(arg).toHexString() with 
md5Of(arg).toHexString().dup and the problem vanished. Now 
6EEA9B7EF19179A06954EDD0F6C05CEB is printed out twice.