How can I reference a Pin struct field from another field?

help
7

You cannot write this struct without unsafe code, and Pin cannot reduce the amount of unsafe code you need. Here's how you can build the struct without Pin:

struct S {
    buf: *mut [u8],
    slice: Option<*const [u8]>,
}
impl Drop for S {
    fn drop(&mut self) {
        let b = unsafe { Box::from_raw(self.buf) };
        drop(b);
    }
}

impl S {
    pub fn new() -> Self {
        let buf = vec![0u8, 1, 2, 3].into_boxed_slice();
        let buf_ptr = Box::into_raw(buf);
        let slice = Some(buf_ptr as *const [u8]);

        S {
            buf: buf_ptr,
            slice,
        }
    }
    fn buf(&self) -> &[u8] {
        unsafe { &*self.buf }
    }
    fn slice(&self) -> Option<&[u8]> {
        unsafe { self.slice.map(|ptr| &*ptr) }
    }
}

fn main() {
    let s = S::new();
    assert_eq!(s.slice().unwrap(), s.buf());
}

playground

You may want to read this post of mine, which explains where Pin is useful, namely when two pieces of unsafe code need to communicate guarantees through unknown non-unsafe user code. When everything happens in a single module, Pin does not help you at all, and module privacy can provide all the things you need to write correct (but wildly unsafe) self-referential structs.

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