6 methods override MigrateSourceInterface::__toString()

ContentEntity::__toString in core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntity.php
EmbeddedDataSource::__toString in core/modules/migrate/src/Plugin/migrate/source/EmbeddedDataSource.php
EmptySource::__toString in core/modules/migrate/src/Plugin/migrate/source/EmptySource.php
Allows class to decide how it will react when it is treated like a string.
MigrateExternalTranslatedTestSource::__toString in core/modules/migrate/tests/modules/migrate_external_translated_test/src/Plugin/migrate/source/MigrateExternalTranslatedTestSource.php
SqlBase::__toString in core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
Prints the query string when the object is used as a string.
StubSourcePlugin::__toString in core/modules/migrate/tests/src/Unit/StubSourcePlugin.php

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