diff --git a/lib/DAV/PropFindPlugin.php b/lib/DAV/PropFindPlugin.php
index b605de4b3..5556a57de 100644
--- a/lib/DAV/PropFindPlugin.php
+++ b/lib/DAV/PropFindPlugin.php
@@ -52,7 +52,7 @@ public function propFind(PropFind $propFind, INode $node): void {
 			);
 			$propFind->handle(
 				self::GROUP_FOLDER_ID_PROPERTYNAME,
-				fn () => $node->getFolderId()
+				fn (): int => $node->getFolderId()
 			);
 		}
 	}
diff --git a/lib/Versions/VersionsBackend.php b/lib/Versions/VersionsBackend.php
index a585813b4..441ab47d8 100644
--- a/lib/Versions/VersionsBackend.php
+++ b/lib/Versions/VersionsBackend.php
@@ -58,9 +58,8 @@ private function getFolderIdForFile(FileInfo $file): int {
 		} elseif ($storage->instanceOfStorage(GroupFolderStorage::class)) {
 			/** @var GroupFolderStorage $storage */
 			return $storage->getFolderId();
-		} else {
-			throw new \LogicException('groupfolder version backend called for non groupfolder file');
 		}
+		throw new \LogicException('groupfolder version backend called for non groupfolder file');
 	}
 
 	public function getVersionFolderForFile(FileInfo $file): Folder {
diff --git a/rector.php b/rector.php
index 93218586b..042f75631 100644
--- a/rector.php
+++ b/rector.php
@@ -17,5 +17,5 @@
 	])
 	->withPreparedSets(
 		deadCode: true,
-	)
-	->withTypeCoverageLevel(0);
+		typeDeclarations: true,
+	);
diff --git a/tests/ACL/ACLCacheWrapperTest.php b/tests/ACL/ACLCacheWrapperTest.php
index 415203e6e..45d5768d1 100644
--- a/tests/ACL/ACLCacheWrapperTest.php
+++ b/tests/ACL/ACLCacheWrapperTest.php
@@ -23,9 +23,8 @@ class ACLCacheWrapperTest extends TestCase {
 	private $aclManager;
 	/** @var ICache|\PHPUnit_Framework_MockObject_MockObject */
 	private $source;
-	/** @var ACLCacheWrapper */
-	private $cache;
-	private $aclPermissions = [];
+	private ?ACLCacheWrapper $cache = null;
+	private array $aclPermissions = [];
 
 	protected function setUp(): void {
 		parent::setUp();
@@ -39,7 +38,7 @@ protected function setUp(): void {
 		$this->cache = new ACLCacheWrapper($this->source, $this->aclManager, false);
 	}
 
-	public function testHideNonRead() {
+	public function testHideNonRead(): void {
 		$this->source->method('getFolderContentsById')
 			->willReturn([
 				new CacheEntry([
diff --git a/tests/ACL/ACLManagerTest.php b/tests/ACL/ACLManagerTest.php
index 349c9987a..d793f3c3d 100644
--- a/tests/ACL/ACLManagerTest.php
+++ b/tests/ACL/ACLManagerTest.php
@@ -41,10 +41,10 @@ protected function setUp(): void {
 		$this->dummyMapping = $this->createMapping('dummy');
 
 		$this->ruleManager->method('getRulesForFilesByPath')
-			->willReturnCallback(function (IUser $user, int $storageId, array $paths) {
+			->willReturnCallback(function (IUser $user, int $storageId, array $paths): array {
 				// fill with empty in case no rule was found
 				$rules = array_fill_keys($paths, []);
-				$actualRules = array_filter($this->rules, function (string $path) use ($paths) {
+				$actualRules = array_filter($this->rules, function (string $path) use ($paths): bool {
 					return array_search($path, $paths) !== false;
 				}, ARRAY_FILTER_USE_KEY);
 
diff --git a/tests/ACL/ACLScannerTest.php b/tests/ACL/ACLScannerTest.php
index dba331836..4634c28b0 100644
--- a/tests/ACL/ACLScannerTest.php
+++ b/tests/ACL/ACLScannerTest.php
@@ -29,7 +29,7 @@ private function getAclManager(array $rules): ACLManager {
 		return $manager;
 	}
 
-	public function testScanAclStorage() {
+	public function testScanAclStorage(): void {
 		$baseStorage = new Temporary([]);
 		$baseStorage->mkdir('foo');
 		$baseStorage->mkdir('foo/bar');
diff --git a/tests/ACL/ACLStorageWrapperTest.php b/tests/ACL/ACLStorageWrapperTest.php
index 35b63fe89..473b9ad7b 100644
--- a/tests/ACL/ACLStorageWrapperTest.php
+++ b/tests/ACL/ACLStorageWrapperTest.php
@@ -12,17 +12,14 @@
 use OCA\GroupFolders\ACL\ACLManager;
 use OCA\GroupFolders\ACL\ACLStorageWrapper;
 use OCP\Constants;
-use OCP\Files\Storage\IStorage;
 use Test\TestCase;
 
 class ACLStorageWrapperTest extends TestCase {
 	/** @var ACLManager|\PHPUnit_Framework_MockObject_MockObject */
 	private $aclManager;
-	/** @var IStorage */
-	private $source;
-	/** @var ACLStorageWrapper */
-	private $storage;
-	private $aclPermissions = [];
+	private ?Temporary $source = null;
+	private ?ACLStorageWrapper $storage = null;
+	private array $aclPermissions = [];
 
 	protected function setUp(): void {
 		parent::setUp();
@@ -40,7 +37,7 @@ protected function setUp(): void {
 		]);
 	}
 
-	public function testNoReadImpliesNothing() {
+	public function testNoReadImpliesNothing(): void {
 		$this->source->mkdir('foo');
 		$this->aclPermissions['foo'] = Constants::PERMISSION_ALL - Constants::PERMISSION_READ;
 
@@ -50,7 +47,7 @@ public function testNoReadImpliesNothing() {
 		$this->assertEquals(false, $this->storage->isSharable('foo'));
 	}
 
-	public function testOpenDir() {
+	public function testOpenDir(): void {
 		$this->source->mkdir('foo');
 		$this->source->touch('foo/file1');
 		$this->source->touch('foo/file2');
@@ -73,7 +70,7 @@ public function testOpenDir() {
 		$this->assertEquals($expected, $result);
 	}
 
-	public function testMove() {
+	public function testMove(): void {
 		$this->source->mkdir('foo');
 		$this->source->touch('file1');
 
diff --git a/tests/ACL/RuleManagerTest.php b/tests/ACL/RuleManagerTest.php
index 36a9b43ae..480acbe5b 100644
--- a/tests/ACL/RuleManagerTest.php
+++ b/tests/ACL/RuleManagerTest.php
@@ -24,8 +24,7 @@
 class RuleManagerTest extends TestCase {
 	/** @var \PHPUnit_Framework_MockObject_MockObject | IUserMappingManager */
 	private $userMappingManager;
-	/** @var RuleManager */
-	private $ruleManager;
+	private ?RuleManager $ruleManager = null;
 	/** @var \PHPUnit_Framework_MockObject_MockObject | IUser */
 	private $user;
 
@@ -42,7 +41,7 @@ protected function setUp(): void {
 		$this->userMappingManager = $this->createMock(IUserMappingManager::class);
 		$this->userMappingManager->expects($this->any())
 			->method('mappingFromId')
-			->willReturnCallback(function ($type, $id) {
+			->willReturnCallback(function ($type, $id): UserMapping {
 				if ($type === 'user') {
 					return new UserMapping($type, $id, 'The User');
 				} else {
@@ -54,7 +53,7 @@ protected function setUp(): void {
 		$this->ruleManager = new RuleManager(\OC::$server->getDatabaseConnection(), $this->userMappingManager, $this->eventDispatcher);
 	}
 
-	public function testGetSetRule() {
+	public function testGetSetRule(): void {
 		$mapping = new UserMapping('user', '1', 'The User');
 		$this->userMappingManager->expects($this->any())
 			->method('getMappingsForUser')
@@ -104,7 +103,7 @@ public function testGetSetRule() {
 		$this->ruleManager->deleteRule($rule);
 	}
 
-	public function testGetMultiple() {
+	public function testGetMultiple(): void {
 		$mapping1 = new UserMapping('test', '1');
 		$mapping2 = new UserMapping('test', '2');
 		$this->userMappingManager->expects($this->any())
@@ -131,7 +130,7 @@ public function testGetMultiple() {
 		$this->ruleManager->deleteRule($rule3);
 	}
 
-	public function testGetByPath() {
+	public function testGetByPath(): void {
 		$storage = new Temporary([]);
 		$storage->mkdir('foo');
 		$storage->mkdir('foo/bar');
@@ -166,7 +165,7 @@ public function testGetByPath() {
 		$this->ruleManager->deleteRule($rule2);
 	}
 
-	public function testGetByPathMore() {
+	public function testGetByPathMore(): void {
 		$storage = new Temporary([]);
 		$storage->mkdir('foo');
 		$paths = [];
@@ -207,7 +206,7 @@ public function testGetByPathMore() {
 		$this->ruleManager->deleteRule($rule);
 	}
 
-	public function testGetByParent() {
+	public function testGetByParent(): void {
 		$storage = new Temporary([]);
 		$storage->mkdir('foo');
 		$storage->mkdir('foo/bar');
diff --git a/tests/ACL/RuleTest.php b/tests/ACL/RuleTest.php
index 9f549e7c5..5876e3660 100644
--- a/tests/ACL/RuleTest.php
+++ b/tests/ACL/RuleTest.php
@@ -26,7 +26,7 @@ public function permissionsProvider() {
 	/**
 	 * @dataProvider permissionsProvider
 	 */
-	public function testApplyPermissions($input, $mask, $permissions, $expected) {
+	public function testApplyPermissions($input, $mask, $permissions, $expected): void {
 		$rule = new Rule($this->createMock(IUserMapping::class), 0, $mask, $permissions);
 		$this->assertEquals($expected, $rule->applyPermissions($input));
 	}
@@ -56,8 +56,8 @@ public function mergeRulesProvider() {
 	/**
 	 * @dataProvider mergeRulesProvider
 	 */
-	public function testMergeRules($inputs, $expectedMask, $expectedPermissions) {
-		$inputRules = array_map(function (array $input) {
+	public function testMergeRules($inputs, $expectedMask, $expectedPermissions): void {
+		$inputRules = array_map(function (array $input): Rule {
 			return new Rule($this->createMock(IUserMapping::class), 0, $input[0], $input[1]);
 		}, $inputs);
 
diff --git a/tests/Folder/FolderManagerTest.php b/tests/Folder/FolderManagerTest.php
index 89ca8fc1b..bc4080f38 100644
--- a/tests/Folder/FolderManagerTest.php
+++ b/tests/Folder/FolderManagerTest.php
@@ -51,7 +51,7 @@ protected function setUp(): void {
 		$this->clean();
 	}
 
-	private function clean() {
+	private function clean(): void {
 		$query = \OC::$server->getDatabaseConnection()->getQueryBuilder();
 		$query->delete('group_folders')->execute();
 
@@ -59,12 +59,12 @@ private function clean() {
 		$query->delete('group_folders_groups')->execute();
 	}
 
-	private function assertHasFolders($folders) {
+	private function assertHasFolders(array $folders): void {
 		$existingFolders = array_values($this->manager->getAllFolders());
-		usort($existingFolders, function ($a, $b) {
+		usort($existingFolders, function (array $a, array $b): int {
 			return strcmp($a['mount_point'], $b['mount_point']);
 		});
-		usort($folders, function ($a, $b) {
+		usort($folders, function (array $a, array $b): int {
 			return strcmp($a['mount_point'], $b['mount_point']);
 		});
 
@@ -89,7 +89,7 @@ private function assertHasFolders($folders) {
 		$this->assertEquals($folders, $existingFolders);
 	}
 
-	public function testCreateFolder() {
+	public function testCreateFolder(): void {
 		$this->manager->createFolder('foo');
 
 		$this->assertHasFolders([
@@ -97,7 +97,7 @@ public function testCreateFolder() {
 		]);
 	}
 
-	public function testSetMountpoint() {
+	public function testSetMountpoint(): void {
 		$folderId1 = $this->manager->createFolder('foo');
 		$this->manager->createFolder('bar');
 
@@ -109,7 +109,7 @@ public function testSetMountpoint() {
 		]);
 	}
 
-	public function testAddApplicable() {
+	public function testAddApplicable(): void {
 		$folderId1 = $this->manager->createFolder('foo');
 		$folderId2 = $this->manager->createFolder('bar');
 		$this->manager->addApplicableGroup($folderId1, 'g1');
@@ -154,7 +154,7 @@ public function testAddApplicable() {
 		);
 	}
 
-	public function testSetPermissions() {
+	public function testSetPermissions(): void {
 		$folderId1 = $this->manager->createFolder('foo');
 		$this->manager->addApplicableGroup($folderId1, 'g1');
 		$this->manager->addApplicableGroup($folderId1, 'g2');
@@ -182,7 +182,7 @@ public function testSetPermissions() {
 		);
 	}
 
-	public function testRemoveApplicable() {
+	public function testRemoveApplicable(): void {
 		$folderId1 = $this->manager->createFolder('foo');
 		$folderId2 = $this->manager->createFolder('bar');
 		$this->manager->addApplicableGroup($folderId1, 'g1');
@@ -225,7 +225,7 @@ public function testRemoveApplicable() {
 		);
 	}
 
-	public function testRemoveFolder() {
+	public function testRemoveFolder(): void {
 		$folderId1 = $this->manager->createFolder('foo');
 		$this->manager->createFolder('bar');
 
@@ -236,7 +236,7 @@ public function testRemoveFolder() {
 		]);
 	}
 
-	public function testRenameFolder() {
+	public function testRenameFolder(): void {
 		$folderId1 = $this->manager->createFolder('foo');
 		$this->manager->createFolder('other');
 
@@ -248,7 +248,7 @@ public function testRenameFolder() {
 		]);
 	}
 
-	public function testSetACL() {
+	public function testSetACL(): void {
 		$folderId1 = $this->manager->createFolder('foo');
 		$this->manager->createFolder('other');
 
@@ -267,7 +267,7 @@ public function testSetACL() {
 		]);
 	}
 
-	public function testGetFoldersForGroup() {
+	public function testGetFoldersForGroup(): void {
 		$folderId1 = $this->manager->createFolder('foo');
 		$this->manager->addApplicableGroup($folderId1, 'g1');
 		$this->manager->addApplicableGroup($folderId1, 'g2');
@@ -280,7 +280,7 @@ public function testGetFoldersForGroup() {
 		$this->assertEquals(2, $folder['permissions']);
 	}
 
-	public function testGetFoldersForGroups() {
+	public function testGetFoldersForGroups(): void {
 		$folderId1 = $this->manager->createFolder('foo');
 		$this->manager->addApplicableGroup($folderId1, 'g1');
 		$this->manager->addApplicableGroup($folderId1, 'g2');
@@ -309,13 +309,13 @@ protected function getUser($groups = []) {
 		return $user;
 	}
 
-	public function testGetFoldersForUserEmpty() {
+	public function testGetFoldersForUserEmpty(): void {
 		$folders = $this->manager->getFoldersForUser($this->getUser());
 		$this->assertEquals([], $folders);
 	}
 
 
-	public function testGetFoldersForUserSimple() {
+	public function testGetFoldersForUserSimple(): void {
 		$db = $this->createMock(IDBConnection::class);
 		/** @var FolderManager|\PHPUnit_Framework_MockObject_MockObject $manager */
 		$manager = $this->getMockBuilder(FolderManager::class)
@@ -338,7 +338,7 @@ public function testGetFoldersForUserSimple() {
 		$this->assertEquals([$folder], $folders);
 	}
 
-	public function testGetFoldersForUserMerge() {
+	public function testGetFoldersForUserMerge(): void {
 		$db = $this->createMock(IDBConnection::class);
 		/** @var FolderManager|\PHPUnit_Framework_MockObject_MockObject $manager */
 		$manager = $this->getMockBuilder(FolderManager::class)
@@ -374,7 +374,7 @@ public function testGetFoldersForUserMerge() {
 		], $folders);
 	}
 
-	public function testGetFolderPermissionsForUserMerge() {
+	public function testGetFolderPermissionsForUserMerge(): void {
 		$db = $this->createMock(IDBConnection::class);
 		/** @var FolderManager|\PHPUnit_Framework_MockObject_MockObject $manager */
 		$manager = $this->getMockBuilder(FolderManager::class)
diff --git a/tests/Trash/TrashBackendTest.php b/tests/Trash/TrashBackendTest.php
index 9c45ea428..119065783 100644
--- a/tests/Trash/TrashBackendTest.php
+++ b/tests/Trash/TrashBackendTest.php
@@ -105,7 +105,7 @@ private function createNoReadRule(string $userId, int $fileId): Rule {
 		);
 	}
 
-	public function testHideTrashItemAcl() {
+	public function testHideTrashItemAcl(): void {
 		$this->loginAsUser('manager');
 
 		$restricted = $this->managerUserFolder->newFile("{$this->folderName}/restricted.txt", 'content');
@@ -126,7 +126,7 @@ public function testHideTrashItemAcl() {
 		$this->logout();
 	}
 
-	public function testHideItemInDeletedFolderAcl() {
+	public function testHideItemInDeletedFolderAcl(): void {
 		$this->loginAsUser('manager');
 
 		$folder = $this->managerUserFolder->newFolder("{$this->folderName}/folder");
@@ -155,7 +155,7 @@ public function testHideItemInDeletedFolderAcl() {
 		$this->logout();
 	}
 
-	public function testHideDeletedTrashItemInDeletedFolderAcl() {
+	public function testHideDeletedTrashItemInDeletedFolderAcl(): void {
 		$this->loginAsUser('manager');
 
 		$folder = $this->managerUserFolder->newFolder("{$this->folderName}/restricted");
@@ -183,7 +183,7 @@ public function testHideDeletedTrashItemInDeletedFolderAcl() {
 		$this->logout();
 	}
 
-	public function testHideDeletedTrashItemInDeletedParentFolderAcl() {
+	public function testHideDeletedTrashItemInDeletedParentFolderAcl(): void {
 		$this->loginAsUser('manager');
 
 		$parent = $this->managerUserFolder->newFolder("{$this->folderName}/parent");