rust: hrtimer: add `UnsafeHrTimerPointer`

    fn start(self, expires: Ktime) -> Self::TimerHandle;

}

/// Unsafe version of [`HrTimerPointer`] for situations where leaking the

/// [`HrTimerHandle`] returned by `start` would be unsound. This is the case for

/// stack allocated timers.

///

/// Typical implementers are pinned references such as [`Pin<&T>`].

///

/// # Safety

///

/// Implementers of this trait must ensure that instances of types implementing

/// [`UnsafeHrTimerPointer`] outlives any associated [`HrTimerPointer::TimerHandle`]

/// instances.

pub unsafe trait UnsafeHrTimerPointer: Sync + Sized {

    /// A handle representing a running timer.

    ///

    /// # Safety

    ///

    /// If the timer is running, or if the timer callback is executing when the

    /// handle is dropped, the drop method of [`Self::TimerHandle`] must not return

    /// until the timer is stopped and the callback has completed.

    type TimerHandle: HrTimerHandle;

    /// Start the timer after `expires` time units. If the timer was already

    /// running, it is restarted at the new expiry time.

    ///

    /// # Safety

    ///

    /// Caller promises keep the timer structure alive until the timer is dead.

    /// Caller can ensure this by not leaking the returned [`Self::TimerHandle`].

    unsafe fn start(self, expires: Ktime) -> Self::TimerHandle;

}

/// Implemented by [`HrTimerPointer`] implementers to give the C timer callback a

/// function to call.

// This is split from `HrTimerPointer` to make it easier to specify trait bounds.