[PATCH 02 of 10] rust-cpython: move borrow_mut() to PySharedRefCell

[Yuya Nishihara]

# HG changeset patch
# User Yuya Nishihara <yuya at tcha.org>
# Date 1568470639 -32400
#      Sat Sep 14 23:17:19 2019 +0900
# Node ID 34a355c69f643f15ff46f4524c75137c08a8de3b
# Parent  e7f643679476b013e86b9a2b75f704d64eec4293
rust-cpython: move borrow_mut() to PySharedRefCell

PySharedRefCell() will host almost all py_shared public functions. This change
is the first step.

borrow_mut() can be safely implemented since PySharedRefCell knows its inner
object is managed by its own py_shared_state.

diff --git a/rust/hg-cpython/src/ref_sharing.rs b/rust/hg-cpython/src/ref_sharing.rs
--- a/rust/hg-cpython/src/ref_sharing.rs
+++ b/rust/hg-cpython/src/ref_sharing.rs
@@ -125,9 +125,15 @@ impl<T> PySharedRefCell<T> {
         self.inner.as_ptr()
     }
 
-    pub unsafe fn borrow_mut(&self) -> RefMut<T> {
-        // must be borrowed by self.py_shared_state(py).borrow_mut().
-        self.inner.borrow_mut()
+    // TODO: maybe this should be named as try_borrow_mut(), and use
+    // inner.try_borrow_mut(). The current implementation panics if
+    // self.inner has been borrowed, but returns error if py_shared_state
+    // refuses to borrow.
+    pub fn borrow_mut<'a>(
+        &'a self,
+        py: Python<'a>,
+    ) -> PyResult<PyRefMut<'a, T>> {
+        self.py_shared_state.borrow_mut(py, self.inner.borrow_mut())
     }
 }
 
@@ -216,6 +222,7 @@ macro_rules! py_shared_ref {
         $leaked: ident,
     ) => {
         impl $name {
+            // TODO: remove this function in favor of inner(py).borrow_mut()
             fn borrow_mut<'a>(
                 &'a self,
                 py: Python<'a>,
@@ -224,8 +231,7 @@ macro_rules! py_shared_ref {
                 // assert $data_member type
                 use crate::ref_sharing::PySharedRefCell;
                 let data: &PySharedRefCell<_> = self.$data_member(py);
-                data.py_shared_state
-                    .borrow_mut(py, unsafe { data.borrow_mut() })
+                data.borrow_mut(py)
             }
 
             /// Returns a leaked reference and its management object.