#[inline]
    #[must_use]
    #[unstable(feature = "waker_from_fn_ptr", issue = "146055")]
    pub const fn from_fn_ptr(f: fn()) -> Self {
        // SAFETY: Unsafe is used for transmutes, pointer came from `fn()` so it
        //         is sound to transmute it back to `fn()`.
        static VTABLE: RawWakerVTable = unsafe {
            RawWakerVTable::new(
                |this| RawWaker::new(this, &VTABLE),
                |this| transmute::<*const (), fn()>(this)(),
                |this| transmute::<*const (), fn()>(this)(),
                |_| {},
            )
        };
        let raw = RawWaker::new(f as *const (), &VTABLE);

        // SAFETY: `clone` is just a copy, `drop` is a no-op while `wake` and
        //         `wake_by_ref` just call the function pointer.
        unsafe { Self::from_raw(raw) }
    }