d@安全不能交换域变量

原文

static struct Foo{
    void *ptr;

    void proxySwap1(scope ref typeof(this) rhs)scope pure nothrow @trusted @nogc{
        auto tmp = this.ptr;
        this.ptr = rhs.ptr;
        rhs.ptr = tmp;
    }
    void proxySwap2()(scope ref typeof(this) rhs)scope pure nothrow @trusted @nogc{
        this.proxySwap1(rhs);
    }
    void proxySwap3()(scope ref typeof(this) rhs)scope pure nothrow @trusted @nogc{
        auto tmp = this.ptr;
        this.ptr = rhs.ptr;
        rhs.ptr = tmp;
    }//有().
}

void main()@safe{

    scope Foo a;
    scope Foo b;

    a.proxySwap1(b);	//OK
    a.proxySwap2(b);	//OK
    a.proxySwap3(b);	//错误,把b赋值给更长生命期的a.
}

为何不编译?

proxySwap1对属性说谎了.它说rhs是域,但用

this.ptr = rhs.ptr;

逃逸了.由于@trusted,编译器未抛错误.
proxySwap2只是模板包装proxySwap1,
proxySwap3是模板函数,编译器推导rhs为return scope.
虽然@trusted允许逃逸局部变量,编译器仍在参数上推导scope, return scope或return ref.
在@safe调用者中产生错误.
在@safe中不能交换域变量,因为:

void main() @safe {
    scope Foo a;
	{
		int x;
		scope Foo b = Foo(&x);
		a.proxySwap3(b); // 同样a<=b,不行.
	}
	// 现在a是悬挂指针.
}