17 files declare their use of ResultSetInterface in Search API 8

BackendTestBase.php in tests/src/Kernel/BackendTestBase.php

Highlight.php in src/Plugin/search_api/processor/Highlight.php

Index.php in src/Entity/Index.php

IndexInterface.php in src/IndexInterface.php

NumberFieldBoostTest.php in tests/src/Kernel/Processor/NumberFieldBoostTest.php

ProcessingResultsEvent.php in src/Event/ProcessingResultsEvent.php

ProcessorInterface.php in src/Processor/ProcessorInterface.php

ProcessorPluginBase.php in src/Processor/ProcessorPluginBase.php

QueryHelper.php in src/Utility/QueryHelper.php

QueryHelperInterface.php in src/Utility/QueryHelperInterface.php

ResultsTrait.php in tests/src/Kernel/ResultsTrait.php

SearchApiQuery.php in src/Plugin/views/query/SearchApiQuery.php

search_api_test_excerpt_field.module in tests/search_api_test_excerpt_field/search_api_test_excerpt_field.module

Contains hook implementations for the Search API Excerpt Field Test module.

search_api_test_hooks.module in tests/search_api_test_hooks/search_api_test_hooks.module

Tests all the hooks defined by the Search API module.

search_api_test_hooks.search_api.inc in tests/search_api_test_hooks/search_api_test_hooks.search_api.inc

Tests all the hooks defined by the Search API module.

TestProcessor.php in tests/search_api_test/src/Plugin/search_api/processor/TestProcessor.php

UnsavedIndexConfiguration.php in src/UnsavedIndexConfiguration.php