13 files declare their use of MigrateSourceInterface

ContentEntity.php in core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntity.php
ContentEntityTest.php in core/modules/migrate_drupal/tests/src/Kernel/Plugin/migrate/source/ContentEntityTest.php
forum.module in core/modules/forum/forum.module
Provides discussion forums.
migrate.api.php in core/modules/migrate/migrate.api.php
Hooks provided by the Migrate module.
MigratePrepareRowTestHooks.php in core/modules/migrate/tests/modules/migrate_prepare_row_test/src/Hook/MigratePrepareRowTestHooks.php
MigrateSkipAllRowsTestHooks.php in core/modules/migrate/tests/modules/migrate_skip_all_rows_test/src/Hook/MigrateSkipAllRowsTestHooks.php
MigrateSourceTest.php in core/modules/migrate/tests/src/Unit/MigrateSourceTest.php
MigrateStubTest.php in core/modules/migrate/tests/src/Unit/MigrateStubTest.php
migrate_prepare_row_test.module in core/modules/migrate/tests/modules/migrate_prepare_row_test/migrate_prepare_row_test.module
migrate_skip_all_rows_test.module in core/modules/migrate/tests/modules/migrate_skip_all_rows_test/migrate_skip_all_rows_test.module
MigrationStateUnitTest.php in core/modules/migrate_drupal/tests/src/Unit/MigrationStateUnitTest.php
MigrationTest.php in core/modules/migrate/tests/src/Unit/MigrationTest.php
SourcePluginBase.php in core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php

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