Working version

README.md

@@ -1,2 +1,19 @@
 # access-cell
-A synchronous, single-threaded Mutex<T> anti-locking system
+A synchronous, single-threaded Mutex<T> anti-locking system
+
+```rust
+let value = Arc::new(AccessCell::new(0));
+
+value.access({
+    let value = value.clone();
+
+    move |_| {
+        // normally this would cause a deadlock
+        value.access(|v| {
+            *v = 10;
+        })
+    }
+});
+
+assert_eq!(*value.access_ref(), 10);
+```

src/lib.rs

@@ -1,14 +1,72 @@
-pub fn add(left: u64, right: u64) -> u64 {
+use std::{
-    left + right
+    cell::{Cell, UnsafeCell},
+    collections::VecDeque,
+    sync::Mutex,
+};
+
+pub struct AccessCell<T> {
+    value: UnsafeCell<T>,
+    running: Cell<bool>,
+    queue: Mutex<VecDeque<Box<dyn FnOnce(&mut T)>>>,
+}
+
+impl<T> AccessCell<T> {
+    pub fn new(value: T) -> Self {
+        Self {
+            value: UnsafeCell::new(value),
+            running: Cell::new(false),
+            queue: Mutex::new(VecDeque::new()),
+        }
+    }
+
+    pub fn access(&self, f: impl FnOnce(&mut T) + 'static) {
+        // already inside → enqueue
+        if self.running.get() {
+            self.queue.lock().unwrap().push_back(Box::new(f));
+
+            return;
+        }
+
+        // first entrant = executor
+        self.running.set(true);
+
+        // call current
+        f(self.access_mut());
+
+        // drain queued re-entrant calls
+        while let Some(job) = self.queue.lock().unwrap().pop_front() {
+            let value = self.access_mut();
+            job(value);
+        }
+
+        self.running.set(false);
+    }
+
+    pub fn access_mut(&self) -> &mut T {
+        unsafe { &mut *self.value.get() }
+    }
+
+    pub fn access_ref(&self) -> &T {
+        unsafe { &*self.value.get() }
+    }
 }
 
-#[cfg(test)]
 mod tests {
-    use super::*;
-
     #[test]
-    fn it_works() {
+    fn test_access() {
-        let result = add(2, 2);
+        let value = std::sync::Arc::new(crate::AccessCell::new(0));
-        assert_eq!(result, 4);
+
+        value.access({
+            let value = value.clone();
+
+            move |_| {
+                // normally this would cause a deadlock
+                value.access(|v| {
+                    *v = 10;
+                })
+            }
+        });
+
+        assert_eq!(*value.access_ref(), 10);
     }
 }