rust-cpython: mark unsafe functions as such

It wasn't trivial to fix leak_immutable() to be safe since we have to
allow immutable operations (e.g. iter()) on the leaked reference. So
let's mark it unsafe for now. Callers must take care of the returned
object to guarantee the memory safety.

I'll revisit this later. I think $leaked<T: 'static> could have a function
that converts itself into $leaked<U: 'static> with a given FnOnce(&T) -> &U,
where T is $inner_struct, and U is $iterator_type for example.

rust/hg-cpython/src/dirstate/dirs_multiset.rs

@@ -86,7 +86,7 b' py_class!(pub class Dirs |py| {'
             })
     }
     def __iter__(&self) -> PyResult<DirsMultisetKeysIterator> {
-        let (leak_handle, leaked_ref) = self.leak_immutable(py)?;
+        let (leak_handle, leaked_ref) = unsafe { self.leak_immutable(py)? };
         DirsMultisetKeysIterator::create_instance(
             py,
             RefCell::new(Some(leak_handle)),

rust/hg-cpython/src/dirstate/dirstate_map.rs

@@ -319,7 +319,7 b' py_class!(pub class DirstateMap |py| {'
     }
 
     def keys(&self) -> PyResult<DirstateMapKeysIterator> {
-        let (leak_handle, leaked_ref) = self.leak_immutable(py)?;
+        let (leak_handle, leaked_ref) = unsafe { self.leak_immutable(py)? };
         DirstateMapKeysIterator::from_inner(
             py,
             Some(leak_handle),
@@ -328,7 +328,7 b' py_class!(pub class DirstateMap |py| {'
     }
 
     def items(&self) -> PyResult<DirstateMapItemsIterator> {
-        let (leak_handle, leaked_ref) = self.leak_immutable(py)?;
+        let (leak_handle, leaked_ref) = unsafe { self.leak_immutable(py)? };
         DirstateMapItemsIterator::from_inner(
             py,
             Some(leak_handle),
@@ -337,7 +337,7 b' py_class!(pub class DirstateMap |py| {'
     }
 
     def __iter__(&self) -> PyResult<DirstateMapKeysIterator> {
-        let (leak_handle, leaked_ref) = self.leak_immutable(py)?;
+        let (leak_handle, leaked_ref) = unsafe { self.leak_immutable(py)? };
         DirstateMapKeysIterator::from_inner(
             py,
             Some(leak_handle),
@@ -434,7 +434,7 b' py_class!(pub class DirstateMap |py| {'
     }
 
     def copymapiter(&self) -> PyResult<CopyMapKeysIterator> {
-        let (leak_handle, leaked_ref) = self.leak_immutable(py)?;
+        let (leak_handle, leaked_ref) = unsafe { self.leak_immutable(py)? };
         CopyMapKeysIterator::from_inner(
             py,
             Some(leak_handle),
@@ -443,7 +443,7 b' py_class!(pub class DirstateMap |py| {'
     }
 
     def copymapitemsiter(&self) -> PyResult<CopyMapItemsIterator> {
-        let (leak_handle, leaked_ref) = self.leak_immutable(py)?;
+        let (leak_handle, leaked_ref) = unsafe { self.leak_immutable(py)? };
         CopyMapItemsIterator::from_inner(
             py,
             Some(leak_handle),

rust/hg-cpython/src/ref_sharing.rs

@@ -58,7 +58,12 b' impl PySharedState {'
     /// Return a reference to the wrapped data with an artificial static
     /// lifetime.
     /// We need to be protected by the GIL for thread-safety.
-    pub fn leak_immutable<T>(
+    ///
+    /// # Safety
+    ///
+    /// This is highly unsafe since the lifetime of the given data can be
+    /// extended. Do not call this function directly.
+    pub unsafe fn leak_immutable<T>(
         &self,
         py: Python,
         data: &PySharedRefCell<T>,
@@ -72,10 +77,14 b' impl PySharedState {'
         }
         let ptr = data.as_ptr();
         self.leak_count.replace(self.leak_count.get() + 1);
-        unsafe { Ok(&*ptr) }
+        Ok(&*ptr)
     }
 
-    pub fn decrease_leak_count(&self, _py: Python, mutable: bool) {
+    /// # Safety
+    ///
+    /// It's unsafe to update the reference count without knowing the
+    /// reference is deleted. Do not call this function directly.
+    pub unsafe fn decrease_leak_count(&self, _py: Python, mutable: bool) {
         self.leak_count
             .replace(self.leak_count.get().saturating_sub(1));
         if mutable {
@@ -123,6 +132,8 b" pub struct PyRefMut<'a, T> {"
 }
 
 impl<'a, T> PyRefMut<'a, T> {
+    // Must be constructed by PySharedState after checking its leak_count.
+    // Otherwise, drop() would incorrectly update the state.
     fn new(
         _py: Python<'a>,
         inner: RefMut<'a, T>,
@@ -152,7 +163,9 b" impl<'a, T> Drop for PyRefMut<'a, T> {"
     fn drop(&mut self) {
         let gil = Python::acquire_gil();
         let py = gil.python();
-        self.py_shared_state.decrease_leak_count(py, true);
+        unsafe {
+            self.py_shared_state.decrease_leak_count(py, true);
+        }
     }
 }
 
@@ -223,7 +236,7 b' macro_rules! py_shared_ref {'
             /// It's up to you to make sure that the management object lives
             /// longer than the leaked reference. Otherwise, you'll get a
             /// dangling reference.
-            fn leak_immutable<'a>(
+            unsafe fn leak_immutable<'a>(
                 &'a self,
                 py: Python<'a>,
             ) -> PyResult<($leaked, &'static $inner_struct)> {
@@ -247,7 +260,9 b' macro_rules! py_shared_ref {'
         }
 
         impl $leaked {
-            fn new(py: Python, inner: &$name) -> Self {
+            // Marked as unsafe so client code wouldn't construct $leaked
+            // struct by mistake. Its drop() is unsafe.
+            unsafe fn new(py: Python, inner: &$name) -> Self {
                 Self {
                     inner: inner.clone_ref(py),
                 }
@@ -258,9 +273,10 b' macro_rules! py_shared_ref {'
             fn drop(&mut self) {
                 let gil = Python::acquire_gil();
                 let py = gil.python();
-                self.inner
-                    .py_shared_state(py)
-                    .decrease_leak_count(py, false);
+                let state = self.inner.py_shared_state(py);
+                unsafe {
+                    state.decrease_leak_count(py, false);
+                }
             }
         }
     };
@@ -346,7 +362,7 b' macro_rules! py_shared_iterator_impl {'
 ///     data py_shared_state: PySharedState;
 ///
 ///     def __iter__(&self) -> PyResult<MyTypeItemsIterator> {
-///         let (leak_handle, leaked_ref) = self.leak_immutable(py)?;
+///         let (leak_handle, leaked_ref) = unsafe { self.leak_immutable(py)? };
 ///         MyTypeItemsIterator::create_instance(
 ///             py,
 ///             RefCell::new(Some(leak_handle)),