The purpose of D (GC rant, long)

[BCS]

How much does the runtime, generated code and Phobos depend on the GC?
Does it try to optimize out allocations or delete things it can?
For example, does this preform one allocation or several?

char[] foo = "abc", bar = "baz";
char[] ret = foo ~ bar ~ foo;

// ret.length = 9;
// ret[0..3] = foo;
// ret[3..6] = bar;
// ret[6..9] = foo;

and would these temporaries get deleted (or maybe not even created)?

if((foo ~ bar)[4]=='g') go();

//auto __tmp = foo ~ bar
//if(__tmp[4] == 'g') go();
//delete __tmp;

or

//if( (foo.length>4 && foo[4]=='g') || bar[4-foo.length]=='g') go();