Why no (auto foo = bar) in while loops?

[Andrej Mitrovic]

Here's some code that iterates through "parents" of some class object
until it finds an object with no parent (where parent is null):

import std.stdio;

class Foo
{
    Foo parent;
    int state;

    this (int state) { this.state = state; }
}

void main()
{
    auto foo          = new Foo(0);
    foo.parent        = new Foo(1);
    foo.parent.parent = new Foo(2);

    while (true)
    {
        if (auto par = foo.parent)
        {
            writeln(par.state);
            foo = par;
        }
        else
        {
            break;
        }
    }
}

(syntax-highlighted: http://codepad.org/8yHRmICh)

But I was hoping I could simplify this by doing:

    while (auto par = foo.parent)
    {
        writeln(par.state);
        foo = par;
    }

However that doesn't work, I get back:
expression expected, not 'auto'

Is there a limitation on why this couldn't work or can this be added
to the language?