Allow forcing unsafe blocks to be on a new line

[aclysma]

I'd like for unsafe blocks to be forced onto new lines, both before the unsafe and after the "unsafe {". (Possibly with an exception in simple let statements.)

The motivation for asking for this is that I have some rendering code that calls out to Vulkan. When I ran cargo fmt, I didn't like how the unsafe blocks got collapsed into a statement. I felt that it made it a bit less clear that there was unsafe code being called. There are lots of calls so it wouldn't be practical to decorate every call with #[rustfmt::skip].

Current Behavior

semaphors.push(unsafe { vkCreateSemaphore() });

let x = unsafe { get_x() };

Desired Behavior

semaphors.push(
    unsafe { 
        vkCreateSemaphore() 
    }
);

let x = unsafe { 
    get_x() 
};

Rationale

I'd prefer that unsafe code is as conspicuous as possible. While IDEs can certainly highlight the unsafe keyword, code is rendered in places other than IDEs, and a structural change in code layout is an additional form of visual feedback.

I ended up avoiding this undesired formatting by using a temporary variable to simplify the statement, and maybe that's the best solution, but it gave me pause to using auto-formatting and seemed like something worth considering.