Macro pin_init::init_stack

macro_rules! init_stack {
    ($var:ident = $init:expr) => { ... };
}

Create and pin-initialize a new variable on the stack.

init_stack!(p = NeedPin::new);
// Now `p` is a `Result<Pin<&mut NeedPin>, Infallible>`.

Can be used together with init_pin!:

#[pin_init]
struct ManyPin {
    #[pin]
    a: NeedPin,
    b: usize,
}
init_stack!(p = init_pin!(ManyPin {
    a: NeedPin::new,
    b: 0,
}));

The initializers should not panic, and should use Err to report failures. If the initializer fails, because we cannot tell whether the value is initialized or not (or even just partially initialized), drop guarantee cannot be kept, this macro will abort the process.