What does move and Pin really do?

[Yandros]

It's not a complete definition (but I agree that is a very nice short way to describe the main point of using Pin ) though, since Pin interacts very heavily with Drop semantics: Pin implies not only that the pointee has a "fixed address" ... but also that such address must remain valid until the pointee is dropped. That is, Pin is also a way to opt-out of compile-time lifetimes / borrows while still keeping some invariants; c.f. my post about it:

@Yandros

/// The following code causes Undefined Behavior (Run with MIRI to confirm)
fn main ()
{
    use some_lib::{BackPtr, SmartInteger};

    let back_ptr = BackPtr::new();
    {
        let mut smart_integer = SmartInteger::new(42);
        {
            let at_smart_integer = unsafe {
                Pin::new_unchecked(&mut smart_integer)
            };
            at_smart_integer.register_self_to(&back_ptr);
            assert_eq!(back_ptr.deref_read(), Some(42)); // OK
        }
        mem::forget(smart_integer); // swap isn't even needed, forget suffices
    }
    dbg!(back_ptr.deref_read()); // UAF
}

• Playground

• Since someone having ownership of the value after the lifetime of the pinned reference can thus cause unsoundness without using unsafe , it means that the unsoundess must lie somewhere else: in the Pin::new_unchecked call.

Suffices to say, Pin is very complex abstraction; the property of "preventing ill-formed mempcys" can be more easily provided by immutable heap pointers than by these advanced Pinned pointers. Only use the latter if you do need some form of (restricted) mutation anyways, and/or if you want to support such stack pointers (what most other languages do not even dare do).