drt_chain_vm/with_shared/
with_shared_mut_ref.rs

1use std::rc::Rc;
2
3/// Temporarily converts a mutable reference into a reference-counted smart pointer (`Rc`).
4///
5/// This only takes as long as the closure `f` is executed.
6///
7/// All subsequent Rc clones must be dropped before `f` terminates, otherwise the function will panic.
8///
9/// The `Clone` of the argument is not used, except to preserve memory consistency in case of failure.
10///
11/// See the `Shared` type for a safer implementation, which does not require `Clone`.
12pub fn with_shared_mut_ref<T, F, R>(t: &mut T, f: F) -> R
13where
14    T: Clone,
15    F: FnOnce(Rc<T>) -> R,
16{
17    unsafe {
18        // forcefully extract the owned object from the mut ref (unsafe)
19        let obj = std::ptr::read(t);
20
21        // wrap the owned object
22        let obj_rc = Rc::new(obj);
23
24        // the main action
25        let result = f(obj_rc.clone());
26
27        // unwrapping the owned object
28        match Rc::try_unwrap(obj_rc) {
29            Ok(obj) => {
30                // Rc unwrapped successfully
31                // no need to write the owned object back to the location given by the pointer t,
32                // because it could not have changed in the mean time, it is already there
33
34                // though readonly, the object might have changed via cells,
35                // so it needs to be copied back
36                std::ptr::write(t, obj);
37            },
38            Err(obj_rc) => {
39                // could not unwrap, this means there are still references to obj elsewhere
40                // to avoid memory corruption, we perform a clone of the contents
41                let obj = (*obj_rc).clone();
42                std::ptr::write(t, obj);
43                panic!("failed to recover owned object from Rc")
44            },
45        }
46
47        result
48    }
49}
50
51#[cfg(test)]
52mod test {
53    use std::cell::RefCell;
54
55    use super::with_shared_mut_ref;
56
57    #[test]
58    fn test_with_shared_mut_ref_1() {
59        let mut s = "test string".to_string();
60        let l = with_shared_mut_ref(&mut s, |s_rc| s_rc.len());
61        assert_eq!(s.len(), l);
62    }
63
64    #[test]
65    fn test_with_shared_mut_ref_2() {
66        let mut s = RefCell::new("test string".to_string());
67        with_shared_mut_ref(&mut s, |s_rc| {
68            s_rc.borrow_mut().push_str(" ... changed");
69        });
70        assert_eq!(s.borrow().as_str(), "test string ... changed");
71    }
72
73    #[test]
74    #[should_panic = "failed to recover owned object from Rc"]
75    fn test_with_shared_mut_ref_fail() {
76        let mut s = "test string".to_string();
77        let _illegally_extracted_rc = with_shared_mut_ref(&mut s, |s_rc| s_rc);
78    }
79}
drt_chain_vm/with_shared/with_shared_mut_ref.rs

drt_chain_vm/with_shared/
with_shared_mut_ref.rs
