Add get_mut methods to the RefCell and Cell

This is safe since the borrow checker ensures that we have the only
mutable reference to the struct, thus we can safely borrow its interior.

Tracking issue is #33444.

Author: tbu-

src/libcore/cell.rs

@@ -232,6 +232,18 @@ impl<T:Copy> Cell<T> {
     pub fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
         &self.value
     }
+
+    /// Returns a mutable reference to the underlying data.
+    ///
+    /// This call borrows `Cell` mutably (at compile-time) which guarantees
+    /// that we possess the only reference.
+    #[inline]
+    #[unstable(feature = "cell_get_mut", issue = "33444")]
+    pub fn get_mut(&mut self) -> &mut T {
+        unsafe {
+            &mut *self.value.get()
+        }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -455,6 +467,18 @@ impl<T: ?Sized> RefCell<T> {
     pub unsafe fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
         &self.value
     }
+
+    /// Returns a mutable reference to the underlying data.
+    ///
+    /// This call borrows `RefCell` mutably (at compile-time) so there is no
+    /// need for dynamic checks.
+    #[inline]
+    #[unstable(feature = "cell_get_mut", issue="33444")]
+    pub fn get_mut(&mut self) -> &mut T {
+        unsafe {
+            &mut *self.value.get()
+        }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]