servo: Merge #15872 - Fix debug assertion failure in gecko CI (from servo:dedup); r=emilio a=merge

Source-Repo: https://github.com/servo/servo
Source-Revision: 2565242665645e6a218f138e2b95d5a29f296104
MozReview-Commit-ID: Kd6ZOrODsGg

--HG--
extra : source : 2e536a204cec3d485c8861a4bcca96324e3b7ae1

servo/components/style/properties/declaration_block.rs

@@ -288,10 +288,15 @@ impl PropertyDeclarationBlock {
         }
         let important_count = &mut self.important_count;
         let mut removed_at_least_one = false;
+        let longhands = &mut self.longhands;
         self.declarations.retain(|&(ref declaration, importance)| {
-            let remove = declaration.id().is_or_is_longhand_of(property);
+            let id = declaration.id();
+            let remove = id.is_or_is_longhand_of(property);
             if remove {
                 removed_at_least_one = true;
+                if let PropertyDeclarationId::Longhand(id) = id {
+                    longhands.remove(id)
+                }
                 if importance.important() {
                     *important_count -= 1
                 }
@@ -299,9 +304,8 @@ impl PropertyDeclarationBlock {
             !remove
         });
 
-        if let PropertyId::Longhand(id) = *property {
+        if let PropertyId::Longhand(_) = *property {
             debug_assert!(removed_at_least_one);
-            self.longhands.remove(id);
         }
         removed_at_least_one
     }