`get_mut` is unsound

[qwaz-theori]

use self_reference::{OptionRef, SelfReference};

fn main() {
    let mut self_ref: SelfReference<&(), OptionRef<Box<usize>>> =
        SelfReference::new(&(), || Box::leak(Box::new(None)).as_ref());
    {
        let x = Box::new(1234usize);
        // Should be disallowed
        *self_ref.get_mut() = Some(&x);
    }
    // Prints dangling pointer
    println!("{:p}", self_ref.get_ref().unwrap().as_ref());
}

get_mut() can shorten the lifetime 'x in R::Type<'x>. This is unsound because T is invariant in &mut T.

https://doc.rust-lang.org/nomicon/subtyping.html#variance

MIRI log

error: Undefined Behavior: trying to retag from <3097> for Unique permission at alloc1467[0x0], but that tag does not exist in the borrow stack for this location
  --> /home/qwaz/.cargo/registry/src/index.crates.io-6f17d22bba15001f/self-reference-0.2.2/src/utils.rs:44:12
   |
44 |     return value;
   |            ^^^^^
   |            |
   |            trying to retag from <3097> for Unique permission at alloc1467[0x0], but that tag does not exist in the borrow stack for this location
   |            this error occurs as part of retag at alloc1467[0x0..0x8]
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <3097> was created by a Unique retag at offsets [0x0..0x8]
  --> src/main.rs:9:10
   |
9  |         *self_ref.get_mut() = Some(&x);
   |          ^^^^^^^^^^^^^^^^^^
help: <3097> was later invalidated at offsets [0x0..0x8] by a Unique retag
  --> src/main.rs:9:10
   |
9  |         *self_ref.get_mut() = Some(&x);
   |          ^^^^^^^^^^^^^^^^^^
   = note: BACKTRACE (of the first span):
   = note: inside `self_reference::utils::detach_lifetime_get_mut::<self_reference::OptionRef<std::boxed::Box<usize>>>` at /home/qwaz/.cargo/registry/src/index.crates.io-6f17d22bba15001f/self-reference-0.2.2/src/utils.rs:44:12: 44:17
   = note: inside `self_reference::SelfReference::<'_, &(), self_reference::OptionRef<std::boxed::Box<usize>>>::get_mut` at /home/qwaz/.cargo/registry/src/index.crates.io-6f17d22bba15001f/self-reference-0.2.2/src/lib.rs:144:18: 144:69
note: inside `main`
  --> src/main.rs:9:10
   |
9  |         *self_ref.get_mut() = Some(&x);
   |          ^^^^^^^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error