10 files declare their use of ContactInterface in CRM Core 8

Activity.php in modules/crm_core_activity/src/Entity/Activity.php

ActivityInterface.php in modules/crm_core_activity/src/ActivityInterface.php

DateFieldHandler.php in modules/crm_core_match/src/Plugin/crm_core_match/field/DateFieldHandler.php

DefaultMatchingEngine.php in modules/crm_core_match/src/Plugin/crm_core_match/engine/DefaultMatchingEngine.php

FieldHandlerBase.php in modules/crm_core_match/src/Plugin/crm_core_match/field/FieldHandlerBase.php

FieldHandlerInterface.php in modules/crm_core_match/src/Plugin/crm_core_match/field/FieldHandlerInterface.php

MatchEngineInterface.php in modules/crm_core_match/src/Plugin/crm_core_match/engine/MatchEngineInterface.php

Matcher.php in modules/crm_core_match/src/Entity/Matcher.php

MatcherConfigInterface.php in modules/crm_core_match/src/Matcher/MatcherConfigInterface.php

NameFieldHandler.php in modules/crm_core_match/src/Plugin/crm_core_match/field/NameFieldHandler.php