Only apply old mentions rules if no m.mentions is present

This is required by the spec, however no such condition is actually
present in the rules... To work around that, we just add our own, but we
should write an MSC to fix that eventually.

lib/structs/pushrules.cpp

@@ -248,6 +248,7 @@ struct PushRuleEvaluator::OptimizedRules
                     return false;
                 }
 
+                // We have some internal rules, which just match on a field being present.
                 return true;
             }
         };
@@ -464,7 +465,17 @@ PushRuleEvaluator::PushRuleEvaluator(const Ruleset &rules_)
   : rules(std::make_unique<OptimizedRules>())
 {
     auto add_conditions_to_rule = [](OptimizedRules::OptimizedRule &rule,
-                                     const std::vector<PushCondition> &conditions) {
+                                     const std::vector<PushCondition> &conditions,
+                                     std::string_view rule_id) {
+        // These rules should only be enabled, if there is an m.mentions property, but the spec
+        // doesn't actually add such a condition, so let's do it ourselves...
+        if (rule_id == ".m.rule.contains_display_name" || rule_id == ".m.rule.roomnotif" ||
+            rule_id == ".m.rule.contains_user_name") {
+            OptimizedRules::OptimizedRule::PatternCondition c;
+            c.field = "content.m\\.mentions";
+            rule.patterns.push_back(std::move(c));
+        }
+
         for (const auto &cond : conditions) {
             if (cond.kind == "event_match") {
                 OptimizedRules::OptimizedRule::PatternCondition c;
@@ -542,7 +553,7 @@ PushRuleEvaluator::PushRuleEvaluator(const Ruleset &rules_)
         OptimizedRules::OptimizedRule rule;
         rule.actions = rule_.actions;
 
-        if (!add_conditions_to_rule(rule, rule_.conditions))
+        if (!add_conditions_to_rule(rule, rule_.conditions, rule_.rule_id))
             continue;
 
         rules->override_.push_back(std::move(rule));
@@ -555,7 +566,7 @@ PushRuleEvaluator::PushRuleEvaluator(const Ruleset &rules_)
         OptimizedRules::OptimizedRule rule;
         rule.actions = rule_.actions;
 
-        if (!add_conditions_to_rule(rule, rule_.conditions))
+        if (!add_conditions_to_rule(rule, rule_.conditions, rule_.rule_id))
             continue;
 
         rules->underride.push_back(std::move(rule));
@@ -602,7 +613,7 @@ PushRuleEvaluator::PushRuleEvaluator(const Ruleset &rules_)
           PushCondition{.kind = "event_match", .key = "content.body", .pattern = rule_.pattern},
         };
 
-        if (!add_conditions_to_rule(rule, conditions))
+        if (!add_conditions_to_rule(rule, conditions, rule_.rule_id))
             continue;
 
         rules->content.push_back(std::move(rule));