Make unrooted_must_root a bit more aggressive.

Basically, instead of trying to check for specific kinds of statements,
just check the types of all local variables.

Also included are some commented-out proposals for some slightly more
aggressive lints which might be useful (but trigger a little too
frequently at the moment).

components/script/dom/bindings/js.rs

@@ -310,11 +310,13 @@ impl<T: Reflectable> MutNullableHeap<JS<T>> {
 
     /// Retrieve a copy of the inner optional `JS<T>` as `LayoutJS<T>`.
     /// For use by layout, which can't use safe types like Temporary.
+    #[allow(unrooted_must_root)]
     pub unsafe fn get_inner_as_layout(&self) -> Option<LayoutJS<T>> {
         ptr::read(self.ptr.get()).map(|js| js.to_layout())
     }
 
     /// Get a rooted value out of this object
+    #[allow(unrooted_must_root)]
     pub fn get(&self) -> Option<Root<T>> {
         unsafe {
             ptr::read(self.ptr.get()).map(|o| o.root())

components/script/dom/canvasrenderingcontext2d.rs

@@ -431,6 +431,7 @@ impl CanvasRenderingContext2DMethods for CanvasRenderingContext2D {
         self.ipc_renderer.send(CanvasMsg::Canvas2d(Canvas2dMsg::SaveContext)).unwrap();
     }
 
+    #[allow(unrooted_must_root)]
     // https://html.spec.whatwg.org/multipage/#dom-context-2d-restore
     fn Restore(&self) {
         let mut saved_states = self.saved_states.borrow_mut();

components/script/dom/webglrenderingcontext.rs

@@ -109,6 +109,7 @@ impl WebGLRenderingContext {
         })
     }
 
+    #[allow(unrooted_must_root)]
     pub fn new(global: GlobalRef, canvas: &HTMLCanvasElement, size: Size2D<i32>, attrs: GLContextAttributes)
                -> Option<Root<WebGLRenderingContext>> {
         match WebGLRenderingContext::new_inherited(global, canvas, size, attrs) {