Auto merge of #12955 - nox:try-borrow, r=emilio

Use try_borrow instead of borrow_state

<!-- Reviewable:start -->
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12955)
<!-- Reviewable:end -->

components/script/dom/bindings/cell.rs

@@ -6,7 +6,7 @@
 
 use dom::bindings::trace::JSTraceable;
 use js::jsapi::JSTracer;
-use std::cell::{BorrowState, Ref, RefCell, RefMut};
+use std::cell::{BorrowError, BorrowMutError, Ref, RefCell, RefMut};
 use util::thread_state;
 use util::thread_state::SCRIPT;
 
@@ -52,49 +52,6 @@ impl<T> DOMRefCell<T> {
         &mut *self.value.as_unsafe_cell().get()
     }
 
-    /// Is the cell mutably borrowed?
-    ///
-    /// For safety checks in debug builds only.
-    pub fn is_mutably_borrowed(&self) -> bool {
-        self.value.borrow_state() == BorrowState::Writing
-    }
-
-    /// Attempts to immutably borrow the wrapped value.
-    ///
-    /// The borrow lasts until the returned `Ref` exits scope. Multiple
-    /// immutable borrows can be taken out at the same time.
-    ///
-    /// Returns `None` if the value is currently mutably borrowed.
-    ///
-    /// # Panics
-    ///
-    /// Panics if this is called off the script thread.
-    pub fn try_borrow(&self) -> Option<Ref<T>> {
-        debug_assert!(thread_state::get().is_script());
-        match self.value.borrow_state() {
-            BorrowState::Writing => None,
-            _ => Some(self.value.borrow()),
-        }
-    }
-
-    /// Mutably borrows the wrapped value.
-    ///
-    /// The borrow lasts until the returned `RefMut` exits scope. The value
-    /// cannot be borrowed while this borrow is active.
-    ///
-    /// Returns `None` if the value is currently borrowed.
-    ///
-    /// # Panics
-    ///
-    /// Panics if this is called off the script thread.
-    pub fn try_borrow_mut(&self) -> Option<RefMut<T>> {
-        debug_assert!(thread_state::get().is_script());
-        match self.value.borrow_state() {
-            BorrowState::Unused => Some(self.value.borrow_mut()),
-            _ => None,
-        }
-    }
-
     /// Version of the above that we use during restyle while the script thread
     /// is blocked.
     pub fn borrow_mut_for_layout(&self) -> RefMut<T> {
@@ -149,4 +106,34 @@ impl<T> DOMRefCell<T> {
     pub fn borrow_mut(&self) -> RefMut<T> {
         self.try_borrow_mut().expect("DOMRefCell<T> already borrowed")
     }
+
+    /// Attempts to immutably borrow the wrapped value.
+    ///
+    /// The borrow lasts until the returned `Ref` exits scope. Multiple
+    /// immutable borrows can be taken out at the same time.
+    ///
+    /// Returns `None` if the value is currently mutably borrowed.
+    ///
+    /// # Panics
+    ///
+    /// Panics if this is called off the script thread.
+    pub fn try_borrow(&self) -> Result<Ref<T>, BorrowError<T>> {
+        debug_assert!(thread_state::get().is_script());
+        self.value.try_borrow()
+    }
+
+    /// Mutably borrows the wrapped value.
+    ///
+    /// The borrow lasts until the returned `RefMut` exits scope. The value
+    /// cannot be borrowed while this borrow is active.
+    ///
+    /// Returns `None` if the value is currently borrowed.
+    ///
+    /// # Panics
+    ///
+    /// Panics if this is called off the script thread.
+    pub fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError<T>> {
+        debug_assert!(thread_state::get().is_script());
+        self.value.try_borrow_mut()
+    }
 }

components/script/lib.rs

@@ -3,7 +3,6 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #![feature(as_unsafe_cell)]
-#![feature(borrow_state)]
 #![feature(box_syntax)]
 #![feature(conservative_impl_trait)]
 #![feature(const_fn)]
@@ -19,6 +18,7 @@
 #![feature(slice_patterns)]
 #![feature(stmt_expr_attributes)]
 #![feature(question_mark)]
+#![feature(try_borrow)]
 #![feature(try_from)]
 
 #![deny(unsafe_code)]

ports/cef/browser.rs

@@ -17,7 +17,7 @@ use wrappers::CefWrap;
 use compositing::windowing::{WindowNavigateMsg, WindowEvent};
 use glutin_app;
 use libc::c_int;
-use std::cell::{Cell, RefCell, BorrowState};
+use std::cell::{Cell, RefCell};
 use std::ptr;
 use std::rc::Rc;
 use std::sync::atomic::{AtomicIsize, Ordering};
@@ -183,24 +183,18 @@ impl ServoCefBrowserExtensions for CefBrowser {
     }
 
     fn send_window_event(&self, event: WindowEvent) {
-        self.downcast().message_queue.borrow_mut().push(event);
+        let browser = self.downcast();
 
-        loop {
+        if let Ok(mut servo_browser) = browser.servo_browser.try_borrow_mut() {
-            match self.downcast().servo_browser.borrow_state() {
+            servo_browser.handle_event(event);
-                BorrowState::Unused => {
+            while let Some(event) = browser.message_queue.borrow_mut().pop() {
-                    let event = match self.downcast().message_queue.borrow_mut().pop() {
+                servo_browser.handle_event(event);
-                        None => return,
-                        Some(event) => event,
-                    };
-                    self.downcast().servo_browser.borrow_mut().handle_event(event);
-                }
-                _ => {
-                    // We're trying to send an event while processing another one. This will
-                    // cause general badness, so queue up that event instead of immediately
-                    // processing it.
-                    break
-                }
             }
+        } else {
+            // If we fail to borrow mutably, this means we're trying to send an
+            // event while processing another one. This will cause general badness,
+            // we just queue up that event instead of immediately processing it.
+            browser.message_queue.borrow_mut().push(event);
         }
     }
 

ports/cef/lib.rs

@@ -2,12 +2,12 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-#![feature(borrow_state)]
 #![feature(box_syntax)]
 #![feature(core_intrinsics)]
 #![feature(filling_drop)]
 #![feature(link_args)]
 #![feature(plugin)]
+#![feature(try_borrow)]
 #![feature(unicode)]
 #![feature(unsafe_no_drop_flag)]