Remove DerefMut impl for JS::MutableHandle (#574)

* Remove DerefMut impl for JS::MutableHandle

Signed-off-by: Greg Morenz <[email protected]>

* Add a new `JS::MutableHandle::as_mut` method

Signed-off-by: Greg Morenz <[email protected]>

---------

Signed-off-by: Greg Morenz <[email protected]>

Author: gmorenz

mozjs-sys/src/jsimpls.rs

@@ -27,7 +27,6 @@ use crate::jsval::{JSVal, UndefinedValue};
 use std::marker::PhantomData;
 use std::mem;
 use std::ops::Deref;
-use std::ops::DerefMut;
 use std::ptr;
 
 impl<T> Deref for JS::Handle<T> {
@@ -46,12 +45,6 @@ impl<T> Deref for JS::MutableHandle<T> {
     }
 }
 
-impl<T> DerefMut for JS::MutableHandle<T> {
-    fn deref_mut<'a>(&'a mut self) -> &'a mut T {
-        unsafe { &mut *self.ptr }
-    }
-}
-
 impl Default for jsid {
     fn default() -> Self {
         VoidId()
@@ -119,6 +112,13 @@ impl<T> JS::MutableHandle<T> {
     {
         unsafe { *self.ptr = v }
     }
+
+    /// The returned pointer is aliased by a pointer that the GC will read
+    /// through, and thus `&mut` references created from it must not be held
+    /// across GC pauses.
+    pub fn as_ptr(self) -> *mut T {
+        self.ptr
+    }
 }
 
 impl JS::HandleValue {