Possible unsound safe function

[charlesxsh]

version 0.21.16:

src/frame/element_wise_helper.rs

pub struct TempBuffer {
    pub layout: Layout,
    pub buffer: *mut u8,
}

impl Default for TempBuffer {
    fn default() -> Self {
        TempBuffer { layout: Layout::new::<()>(), buffer: std::ptr::null_mut() }
    }
}

impl TempBuffer {
    pub fn ensure(&mut self, size: usize, alignment: usize) {
        unsafe {
            if size > self.layout.size() || alignment > self.layout.align() {
                let size = size.max(self.layout.size());
                let alignment = alignment.max(self.layout.align());
                if !self.buffer.is_null() {
                    std::alloc::dealloc(self.buffer, self.layout);
                }
                self.layout = Layout::from_size_align_unchecked(size, alignment);
                self.buffer = std::alloc::alloc(self.layout);
                assert!(!self.buffer.is_null());
            }
        }
    }
}

For the function ensure, std::alloc::dealloc requires:

• ptr is a block of memory currently allocated via this allocator and,
• layout is the same layout that was used to allocate that block of
  memory.
  Reference: https://doc.rust-lang.org/alloc/alloc/trait.GlobalAlloc.html#tymethod.dealloc

However, public field layout and buffer make it hard to hold these requirements.
If you want, I can submit a PR for this and along with #1871