stylo: Add sugar over the bitfield accessors in nsIAtom.

ports/geckolib/glue.rs

@@ -264,7 +264,7 @@ pub extern "C" fn Servo_GetComputedValuesForAnonymousBox(parent_style_or_null: *
     let data = PerDocumentStyleData::borrow_mut_from_raw(raw_data);
     data.flush_stylesheets();
 
-    let atom = unsafe { Atom::from_static(pseudo_tag) };
+    let atom = Atom::from(pseudo_tag);
     let pseudo = PseudoElement::from_atom_unchecked(atom, /* anon_box = */ true);
 
     type Helpers = ArcHelpers<ServoComputedValues, ComputedValues>;
@@ -293,7 +293,7 @@ pub extern "C" fn Servo_GetComputedValuesForPseudoElement(parent_style: *mut Ser
         }
     };
 
-    let atom = unsafe { Atom::from_static(pseudo_tag) };
+    let atom = Atom::from(pseudo_tag);
     let pseudo = PseudoElement::from_atom_unchecked(atom, /* anon_box = */ false);
 
     // The stylist consumes stylesheets lazily.

ports/geckolib/string_cache/lib.rs

@@ -128,6 +128,20 @@ impl WeakAtom {
         String::from_utf16(self.as_slice()).unwrap()
     }
 
+    #[inline]
+    pub fn is_static(&self) -> bool {
+        unsafe {
+            (*self.as_ptr()).mIsStatic() != 0
+        }
+    }
+
+    #[inline]
+    pub fn len(&self) -> u32 {
+        unsafe {
+            (*self.as_ptr()).mLength()
+        }
+    }
+
     #[inline]
     pub fn as_ptr(&self) -> *mut nsIAtom {
         let const_ptr: *const nsIAtom = &self.0;
@@ -158,7 +172,7 @@ impl Atom {
     }
 
     #[inline]
-    pub unsafe fn from_static(ptr: *mut nsIAtom) -> Self {
+    unsafe fn from_static(ptr: *mut nsIAtom) -> Self {
         Atom(ptr as *mut WeakAtom)
     }
 }
@@ -199,8 +213,10 @@ impl Clone for Atom {
 impl Drop for Atom {
     #[inline]
     fn drop(&mut self) {
-        unsafe {
-            Gecko_ReleaseAtom(self.as_ptr());
+        if !self.is_static() {
+            unsafe {
+                Gecko_ReleaseAtom(self.as_ptr());
+            }
         }
     }
 }
@@ -281,8 +297,11 @@ impl From<String> for Atom {
 impl From<*mut nsIAtom> for Atom {
     #[inline]
     fn from(ptr: *mut nsIAtom) -> Atom {
+        debug_assert!(!ptr.is_null());
         unsafe {
-            Gecko_AddRefAtom(ptr);
+            if (*ptr).mIsStatic() == 0 {
+                Gecko_AddRefAtom(ptr);
+            }
             Atom(WeakAtom::new(ptr))
         }
     }