Auto merge of #18660 - upsuper:var-sub-inherit, r=emilio

Don't use inherited custom properties for substitution

<!-- Please describe your changes on the following line: -->
Custom properties from parent node has been considered in early stage of cascading them (in [`custom_properties::cascade`](01adcf69d7/components/style/custom_properties.rs (L461-L469))), and there is nothing in the spec stating that substitution should take inherited values into consideration when the value is invalid in the current node. Actually, there is a web-platform test explicitly checks that inherited value is not taken when the value is computed to invalid.

This would fix [bug 1402217](https://bugzilla.mozilla.org/show_bug.cgi?id=1402217).

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/18660)
<!-- Reviewable:end -->

components/style/custom_properties.rs

@@ -503,7 +503,7 @@ pub fn finish_cascade(specified_values_map: Option<OrderedMap<&Name, BorrowedSpe
                       -> Option<Arc<CustomPropertiesMap>> {
     if let Some(mut map) = specified_values_map {
         remove_cycles(&mut map);
-        Some(Arc::new(substitute_all(map, inherited)))
+        Some(Arc::new(substitute_all(map)))
     } else {
         inherited.clone()
     }
@@ -555,8 +555,7 @@ fn remove_cycles(map: &mut OrderedMap<&Name, BorrowedSpecifiedValue>) {
 }
 
 /// Replace `var()` functions for all custom properties.
-fn substitute_all(specified_values_map: OrderedMap<&Name, BorrowedSpecifiedValue>,
-                  inherited: &Option<Arc<CustomPropertiesMap>>)
+fn substitute_all(specified_values_map: OrderedMap<&Name, BorrowedSpecifiedValue>)
                   -> CustomPropertiesMap {
     let mut custom_properties_map = CustomPropertiesMap::new();
     let mut invalid = HashSet::new();
@@ -566,7 +565,7 @@ fn substitute_all(specified_values_map: OrderedMap<&Name, BorrowedSpecifiedValue
         // If this value is invalid at computed-time it won’t be inserted in computed_values_map.
         // Nothing else to do.
         let _ = substitute_one(
-            name, value, &specified_values_map, inherited, None,
+            name, value, &specified_values_map, None,
             &mut custom_properties_map, &mut invalid);
     }
 
@@ -580,7 +579,6 @@ fn substitute_all(specified_values_map: OrderedMap<&Name, BorrowedSpecifiedValue
 fn substitute_one(name: &Name,
                   specified_value: &BorrowedSpecifiedValue,
                   specified_values_map: &OrderedMap<&Name, BorrowedSpecifiedValue>,
-                  inherited: &Option<Arc<CustomPropertiesMap>>,
                   partial_computed_value: Option<&mut ComputedValue>,
                   custom_properties_map: &mut CustomPropertiesMap,
                   invalid: &mut HashSet<Name>)
@@ -604,7 +602,7 @@ fn substitute_one(name: &Name,
             &mut input, &mut position, &mut partial_computed_value,
             &mut |name, partial_computed_value| {
                 if let Some(other_specified_value) = specified_values_map.get(&name) {
-                    substitute_one(name, other_specified_value, specified_values_map, inherited,
+                    substitute_one(name, other_specified_value, specified_values_map,
                                    Some(partial_computed_value), custom_properties_map, invalid)
                 } else {
                     Err(())
@@ -615,13 +613,8 @@ fn substitute_one(name: &Name,
             partial_computed_value.push_from(position, &input, last_token_type);
             partial_computed_value
         } else {
-            // Invalid at computed-value time. Use the inherited value.
-            if let Some(inherited_value) = inherited.as_ref().and_then(|i| i.values.get(name)) {
-                inherited_value.clone()
-            } else {
-                invalid.insert(name.clone());
-                return Err(())
-            }
+            invalid.insert(name.clone());
+            return Err(())
         }
     } else {
         // The specified value contains no var() reference