Issue 5432 - Allow variable declaration inside while condition

[yebblies]

Implement two re-writes:
while(auto var = exp) body => for(typeof(exp) var; var = exp ;) body
while(type var = exp) body => for(type var; var = exp;) body

http://d.puremagic.com/issues/show_bug.cgi?id=5432

[WalterBright]

I do not believe these rewrites are correct. The for statement has a separate initialization and assignment. Initialization and assignment are very distinct operations, and the rewrites confuse the two. I'm not sure there even is a correct answer here. Is the variable created once, default initialized, and then assigned to each time, or is a new one created and initialized each time through the loop?

For the programmer who wants to declare a variable, a for loop does the job in a straightforward manner.

[tgehr]

The right answer is that the variable is created and initialized each loop iteration, because the loop condition is part of the loop.
Basically, the rewrites have a similar effect as the correct rewrite
while(auto var = condition) body =>

for(;;) {
    if(auto var = condition) body
    else break;
}

Which does not confuse anything. If you use a for loop for the same effect, you won't get type inference, which is not nice.

[ghost]

I fully agree with tgehr. The "while" loop simply doesn't have any concept of initialization. Period.

Rather, the argument to while is a condition which is, by design, checked every time. Thus if we insert a declaration into this condition, the variable is redeclared/reassigned every single time.

This is especially useful in code like:

while(T elem = queue.pop()) {
    // use elem
}

which is definitely not an obscure example. This is also allowed in C++ thus should not come as a surprise to most experienced C++ programmers, which is your biggest demographic as far as I understand your PR messages.