stylo: Fix and ensure alignment of restyle hints. Test it on Travis.

Turns out eRestyle_LaterSiblings was not really matching, sigh...

ports/geckolib/lib.rs

@@ -26,6 +26,7 @@ mod snapshot;
 mod snapshot_helpers;
 #[allow(non_snake_case)]
 pub mod glue;
+mod sanity_checks;
 mod traversal;
 mod wrapper;
 

ports/geckolib/sanity_checks.rs

@@ -0,0 +1,44 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * 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/. */
+
+//! Different static asserts that ensure the build does what it's expected to.
+//!
+//! TODO: maybe cfg(test) this?
+
+#![allow(unused_imports)]
+
+use std::mem;
+
+macro_rules! check_enum_value {
+    ($a:expr, $b:expr) => {
+        unsafe {
+            mem::transmute::<[u32; $a as usize],
+                             [u32; $b as usize]>([0; $a as usize]);
+        }
+    }
+}
+
+// NB: It's a shame we can't do this statically with bitflags, but no
+// const-fn and no other way to access the numerical value :-(
+macro_rules! check_enum_value_non_static {
+    ($a:expr, $b:expr) => {
+        assert_eq!($a as usize, $b as usize);
+    }
+}
+
+#[test]
+fn assert_restyle_hints_match() {
+    use style::restyle_hints::*; // For flags
+    use gecko_bindings::structs::nsRestyleHint;
+
+    check_enum_value_non_static!(nsRestyleHint::eRestyle_Self, RESTYLE_SELF.bits());
+    // XXX This for Servo actually means something like an hypothetical
+    // Restyle_AllDescendants (but without running selector matching on the
+    // element). ServoRestyleManager interprets it like that, but in practice we
+    // should align the behavior with Gecko adding a new restyle hint, maybe?
+    //
+    // See https://bugzilla.mozilla.org/show_bug.cgi?id=1291786
+    check_enum_value_non_static!(nsRestyleHint::eRestyle_SomeDescendants, RESTYLE_DESCENDANTS.bits());
+    check_enum_value_non_static!(nsRestyleHint::eRestyle_LaterSiblings, RESTYLE_LATER_SIBLINGS.bits());
+}