3 files changed, 16 insertions, 7 deletions
diff --git a/components/script/dom/bindings/codegen/CodegenRust.py b/components/script/dom/bindings/codegen/CodegenRust.py
index f418a1ba569..73d9f7f3ed0 100644
--- a/components/script/dom/bindings/codegen/CodegenRust.py
+++ b/components/script/dom/bindings/codegen/CodegenRust.py
@@ -1334,8 +1334,11 @@ def MemberCondition(pref, func):
     """
     assert pref is None or isinstance(pref, str)
     assert func is None or isinstance(func, str)
+    assert func is None or pref is None
     if pref:
         return 'Condition::Pref("%s")' % pref
+    if func:
+        return 'Condition::Func(%s)' % func
     return "Condition::Satisfied"
 
 
diff --git a/components/script/dom/bindings/guard.rs b/components/script/dom/bindings/guard.rs
index 15b7c418a31..40faebbd4f2 100644
--- a/components/script/dom/bindings/guard.rs
+++ b/components/script/dom/bindings/guard.rs
@@ -4,6 +4,7 @@
 
 //! Machinery to conditionally expose things.
 
+use js::jsapi::{HandleObject, JSContext};
 use util::prefs::get_pref;
 
 /// A container with a condition.
@@ -22,8 +23,10 @@ impl<T: Clone + Copy> Guard<T> {
     }
 
     /// Expose the value if the condition is satisfied.
-    pub fn expose(&self) -> Option<T> {
-        if self.condition.is_satisfied() {
+    ///
+    /// The passed handle is the object on which the value may be exposed.
+    pub unsafe fn expose(&self, cx: *mut JSContext, obj: HandleObject) -> Option<T> {
+        if self.condition.is_satisfied(cx, obj) {
             Some(self.value)
         } else {
             None
@@ -33,6 +36,8 @@ impl<T: Clone + Copy> Guard<T> {
 
 /// A condition to expose things.
 pub enum Condition {
+    /// The condition is satisfied if the function returns true.
+    Func(unsafe fn(*mut JSContext, HandleObject) -> bool),
     /// The condition is satisfied if the preference is set.
     Pref(&'static str),
     /// The condition is always satisfied.
@@ -40,9 +45,10 @@ pub enum Condition {
 }
 
 impl Condition {
-    fn is_satisfied(&self) -> bool {
+    unsafe fn is_satisfied(&self, cx: *mut JSContext, obj: HandleObject) -> bool {
         match *self {
             Condition::Pref(name) => get_pref(name).as_boolean().unwrap_or(false),
+            Condition::Func(f) => f(cx, obj),
             Condition::Satisfied => true,
         }
     }
diff --git a/components/script/dom/bindings/interface.rs b/components/script/dom/bindings/interface.rs
index 371e70372ac..590e57ba8e0 100644
--- a/components/script/dom/bindings/interface.rs
+++ b/components/script/dom/bindings/interface.rs
@@ -220,7 +220,7 @@ pub unsafe fn create_callback_interface_object(
     rval.set(JS_NewObject(cx, ptr::null()));
     assert!(!rval.ptr.is_null());
     for guard in constants {
-        if let Some(specs) = guard.expose() {
+        if let Some(specs) = guard.expose(cx, rval.handle()) {
             define_constants(cx, rval.handle(), specs);
         }
     }
@@ -369,7 +369,7 @@ unsafe fn create_object(
     define_guarded_methods(cx, rval.handle(), methods);
     define_guarded_properties(cx, rval.handle(), properties);
     for guard in constants {
-        if let Some(specs) = guard.expose() {
+        if let Some(specs) = guard.expose(cx, rval.handle()) {
             define_constants(cx, rval.handle(), specs);
         }
     }
@@ -381,7 +381,7 @@ pub unsafe fn define_guarded_methods(
         obj: HandleObject,
         methods: &[Guard<&'static [JSFunctionSpec]>]) {
     for guard in methods {
-        if let Some(specs) = guard.expose() {
+        if let Some(specs) = guard.expose(cx, obj) {
             define_methods(cx, obj, specs).unwrap();
         }
     }
@@ -393,7 +393,7 @@ pub unsafe fn define_guarded_properties(
         obj: HandleObject,
         properties: &[Guard<&'static [JSPropertySpec]>]) {
     for guard in properties {
-        if let Some(specs) = guard.expose() {
+        if let Some(specs) = guard.expose(cx, obj) {
             define_properties(cx, obj, specs).unwrap();
         }
     }