/// Create a `&mut UninitSlice` from a pointer and a length.
    ///
    /// # Safety
    ///
    /// The caller must ensure that `ptr` references a valid memory region owned
    /// by the caller representing a byte slice for the duration of `'a`.
    ///
    #[inline]
    pub unsafe fn from_raw_parts_mut<'a>(ptr: *mut u8, len: usize) -> &'a mut UninitSlice {
        let maybe_init: &mut [MaybeUninit<u8>] =
            core::slice::from_raw_parts_mut(ptr as *mut _, len);
        Self::from_slice(maybe_init)
    }

这里*mut _这是什么含义