RFC: patch statement

[Dejan Lekic via Digitalmars-d]

I know people her do not like to see proposals that change (add 
stuff to) the language. However, I strongly feel that for the 
testing purposes D should provide means to patch any object (no 
matter whether it is final or not!). Therefore I wonder what 
people think of adding a `patch(obj) {}` or perhaps change the 
semantics of the `with(obj) {}` so unittest writers can modify 
the object and set values.

The patch keyword would work ONLY inside unittest {} blocks AND 
inside functions annotated with @test annotation.

Imagine we have:

int myFun(Person person) { /* some logic here */ }

unittest {
   auto p = new Person() /* does not really matter which 
constructor we use */
   patch(p) {
     // here we can modify ANY attribute, no matter whether it is 
private or public
     p.fname = "Nikola"
     p.sname = "Tesla"
   }
   auto res = myFun(p)
   // do some assertions here
}

Similarly:

@test
void test_myFun() {
   // same code as in the unittest above.
}

I do not even know if patch() {} statement is even possible, that 
is the whole point of writing this, so people can enlighten me... 
:)

As I said in the introduction paragraph, for this purpose the 
semantics of the with statement could be changed, but I prefer a 
different keyword (patch) to be honest.