Module match_cache 

Epoch-scoped cache of CSS selector match results.

Selector matching is the dominant per-element cost after the rule set has been built: every layout run walks the candidate rules of every element and runs ComplexSelector::matches again, even though the result only depends on the (immutable) DOM snapshot and the (immutable) rule set. Repeated layout runs over an unchanged document — scroll, resize, hover and animation frames — therefore repeat exactly the same matches.

SelectorMatchCache memoizes (selector, element) pairs. The key is the identity of the ComplexSelector allocation (Arc::as_ptr) rather than the selector text: rules sharing a selector group share one Arc, so the pointer is stable for as long as the owning [RuleSet] is reused, and this avoids hashing selector trees on every lookup.

Invalidation contract

The cache is scoped to one DOM-snapshot/rule-set/interaction epoch. SelectorMatchCache::clear MUST be called whenever any input that can affect selector matching changes:

• The rule set is rebuilt (new selector allocations).
• The DOM snapshot changes (dom_version bumps): structural changes, attribute changes, pseudo-class state changes (:checked, :focus, etc.).
• Interaction state changes (interaction_version bumps): :hover, :active, :focus-within, etc.

Entries carry the epoch they were created in. SelectorMatchCache::get_or_insert returns a miss for entries from a previous epoch, so even if clear is accidentally skipped, stale entries are never reused. clear also drops the backing map to bound memory growth.

SelectorMatchCache is single-threaded: it is owned by a layout worker and accessed only from that worker’s thread.

Structs

SelectorMatchCache

Memoizes selector match results for one DOM/rule-set epoch.

Constants

DEFAULT_CAPACITY

Default number of memoized (selector, element) results.