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diff --git a/servo/components/style/sharing/checks.rs b/servo/components/style/sharing/checks.rs
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+++ b/servo/components/style/sharing/checks.rs
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+/* 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 https://mozilla.org/MPL/2.0/. */
+
+//! Different checks done during the style sharing process in order to determine
+//! quickly whether it's worth to share style, and whether two different
+//! elements can indeed share the same style.
+
+use crate::bloom::StyleBloom;
+use crate::context::SharedStyleContext;
+use crate::dom::TElement;
+use crate::sharing::{StyleSharingCandidate, StyleSharingTarget};
+use selectors::NthIndexCache;
+
+/// Determines whether a target and a candidate have compatible parents for
+/// sharing.
+pub fn parents_allow_sharing<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>,
+) -> bool
+where
+    E: TElement,
+{
+    // If the identity of the parent style isn't equal, we can't share. We check
+    // this first, because the result is cached.
+    if target.parent_style_identity() != candidate.parent_style_identity() {
+        return false;
+    }
+
+    // Siblings can always share.
+    let parent = target.inheritance_parent().unwrap();
+    let candidate_parent = candidate.element.inheritance_parent().unwrap();
+    if parent == candidate_parent {
+        return true;
+    }
+
+    // Cousins are a bit more complicated.
+    //
+    // The fact that the candidate is here means that its element does not anchor
+    // the relative selector. However, it may have considered relative selector(s)
+    // to compute its style, i.e. there's a rule `<..> :has(<..>) <..> candidate`.
+    // In this case, evaluating style sharing requires evaluating the relative
+    // selector for the target anyway.
+    if candidate.considered_relative_selector {
+        return false;
+    }
+
+    // If a parent element was already styled and we traversed past it without
+    // restyling it, that may be because our clever invalidation logic was able
+    // to prove that the styles of that element would remain unchanged despite
+    // changes to the id or class attributes. However, style sharing relies in
+    // the strong guarantee that all the classes and ids up the respective parent
+    // chains are identical. As such, if we skipped styling for one (or both) of
+    // the parents on this traversal, we can't share styles across cousins.
+    //
+    // This is a somewhat conservative check. We could tighten it by having the
+    // invalidation logic explicitly flag elements for which it ellided styling.
+    let parent_data = parent.borrow_data().unwrap();
+    let candidate_parent_data = candidate_parent.borrow_data().unwrap();
+    if !parent_data.safe_for_cousin_sharing() || !candidate_parent_data.safe_for_cousin_sharing() {
+        return false;
+    }
+
+    true
+}
+
+/// Whether two elements have the same same style attribute (by pointer identity).
+pub fn have_same_style_attribute<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>,
+) -> bool
+where
+    E: TElement,
+{
+    match (target.style_attribute(), candidate.style_attribute()) {
+        (None, None) => true,
+        (Some(_), None) | (None, Some(_)) => false,
+        (Some(a), Some(b)) => &*a as *const _ == &*b as *const _,
+    }
+}
+
+/// Whether two elements have the same same presentational attributes.
+pub fn have_same_presentational_hints<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>,
+) -> bool
+where
+    E: TElement,
+{
+    target.pres_hints() == candidate.pres_hints()
+}
+
+/// Whether a given element has the same class attribute as a given candidate.
+///
+/// We don't try to share style across elements with different class attributes.
+pub fn have_same_class<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>,
+) -> bool
+where
+    E: TElement,
+{
+    target.class_list() == candidate.class_list()
+}
+
+/// Whether a given element has the same part attribute as a given candidate.
+///
+/// We don't try to share style across elements with different part attributes.
+pub fn have_same_parts<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>,
+) -> bool
+where
+    E: TElement,
+{
+    target.part_list() == candidate.part_list()
+}
+
+/// Whether a given element and a candidate match the same set of "revalidation"
+/// selectors.
+///
+/// Revalidation selectors are those that depend on the DOM structure, like
+/// :first-child, etc, or on attributes that we don't check off-hand (pretty
+/// much every attribute selector except `id` and `class`.
+#[inline]
+pub fn revalidate<E>(
+    target: &mut StyleSharingTarget<E>,
+    candidate: &mut StyleSharingCandidate<E>,
+    shared_context: &SharedStyleContext,
+    bloom: &StyleBloom<E>,
+    nth_index_cache: &mut NthIndexCache,
+) -> bool
+where
+    E: TElement,
+{
+    let stylist = &shared_context.stylist;
+
+    let for_element = target.revalidation_match_results(stylist, bloom, nth_index_cache);
+
+    let for_candidate = candidate.revalidation_match_results(stylist, bloom, nth_index_cache);
+
+    // This assert "ensures", to some extent, that the two candidates have
+    // matched the same rulehash buckets, and as such, that the bits we're
+    // comparing represent the same set of selectors.
+    debug_assert_eq!(for_element.len(), for_candidate.len());
+
+    for_element == for_candidate
+}
+
+/// Checks whether we might have rules for either of the two ids.
+#[inline]
+pub fn may_match_different_id_rules<E>(
+    shared_context: &SharedStyleContext,
+    element: E,
+    candidate: E,
+) -> bool
+where
+    E: TElement,
+{
+    let element_id = element.id();
+    let candidate_id = candidate.id();
+
+    if element_id == candidate_id {
+        return false;
+    }
+
+    let stylist = &shared_context.stylist;
+
+    let may_have_rules_for_element = match element_id {
+        Some(id) => stylist.may_have_rules_for_id(id, element),
+        None => false,
+    };
+
+    if may_have_rules_for_element {
+        return true;
+    }
+
+    match candidate_id {
+        Some(id) => stylist.may_have_rules_for_id(id, candidate),
+        None => false,
+    }
+}