style: Use alias for StyleDisplay and StyleDisplayMode.

Map these two types to their original rust type in rust-bindgen.

Differential Revision: https://phabricator.services.mozilla.com/D10141

components/style/gecko/media_features.rs

@@ -179,7 +179,7 @@ fn eval_device_orientation(device: &Device, value: Option<Orientation>) -> bool
 }
 
 /// Values for the display-mode media feature.
-#[derive(Clone, Copy, Debug, FromPrimitive, Parse, ToCss)]
+#[derive(Clone, Copy, Debug, FromPrimitive, Parse, PartialEq, ToCss)]
 #[repr(u8)]
 #[allow(missing_docs)]
 pub enum DisplayMode {
@@ -191,16 +191,10 @@ pub enum DisplayMode {
 
 /// https://w3c.github.io/manifest/#the-display-mode-media-feature
 fn eval_display_mode(device: &Device, query_value: Option<DisplayMode>) -> bool {
-    let query_value = match query_value {
-        Some(v) => v,
-        None => return true,
-    };
-
-    let gecko_display_mode =
-        unsafe { bindings::Gecko_MediaFeatures_GetDisplayMode(device.document()) };
-
-    // NOTE: cbindgen guarantees the same representation.
-    gecko_display_mode as u8 == query_value as u8
+    match query_value {
+        Some(v) => v == unsafe { bindings::Gecko_MediaFeatures_GetDisplayMode(device.document()) },
+        None => true,
+    }
 }
 
 /// https://drafts.csswg.org/mediaqueries-4/#grid

components/style/properties/gecko.mako.rs

@@ -3031,9 +3031,8 @@ fn static_assert() {
 <%self:impl_trait style_struct_name="Box" skip_longhands="${skip_box_longhands}">
     #[inline]
     pub fn set_display(&mut self, v: longhands::display::computed_value::T) {
-        // unsafe: cbindgen ensures the representation is the same.
-        self.gecko.mDisplay = unsafe { transmute(v) };
-        self.gecko.mOriginalDisplay = unsafe { transmute(v) };
+        self.gecko.mDisplay = v;
+        self.gecko.mOriginalDisplay = v;
     }
 
     #[inline]
@@ -3053,14 +3052,12 @@ fn static_assert() {
         v: longhands::display::computed_value::T,
         _is_item_or_root: bool
     ) {
-        // unsafe: cbindgen ensures the representation is the same.
-        self.gecko.mDisplay = unsafe { transmute(v) };
+        self.gecko.mDisplay = v;
     }
 
     #[inline]
     pub fn clone_display(&self) -> longhands::display::computed_value::T {
-        // unsafe: cbindgen ensures the representation is the same.
-        unsafe { transmute(self.gecko.mDisplay) }
+        self.gecko.mDisplay
     }
 
     ${impl_simple('_moz_appearance', 'mAppearance')}