diff --git a/rector.php b/rector.php
index 1ac454a..f9db34a 100644
--- a/rector.php
+++ b/rector.php
@@ -11,7 +11,7 @@
         __DIR__ . '/tests',
     ])
     ->withPhpSets()
-    ->withTypeCoverageLevel(1)
+    ->withPreparedSets(typeDeclarations: true)
     ->withDeadCodeLevel(2)
     ->withCodeQualityLevel(10)
     ->withCodingStyleLevel(0)
diff --git a/src/Scanner/CodeScanner.php b/src/Scanner/CodeScanner.php
index d01caf3..532096d 100644
--- a/src/Scanner/CodeScanner.php
+++ b/src/Scanner/CodeScanner.php
@@ -95,7 +95,7 @@ protected function getFunctionHandler(ParsedFunction $function): ?callable
 
     protected function addComments(ParsedFunction $function, ?Translation $translation): ?Translation
     {
-        if (empty($this->commentsPrefixes) || empty($translation)) {
+        if (empty($this->commentsPrefixes) || $translation === null) {
             return $translation;
         }
 
@@ -110,7 +110,7 @@ protected function addComments(ParsedFunction $function, ?Translation $translati
 
     protected function addFlags(ParsedFunction $function, ?Translation $translation): ?Translation
     {
-        if (empty($translation)) {
+        if ($translation === null) {
             return $translation;
         }
 
diff --git a/tests/BasePoLoaderTestCase.php b/tests/BasePoLoaderTestCase.php
index 4029d72..da672a6 100644
--- a/tests/BasePoLoaderTestCase.php
+++ b/tests/BasePoLoaderTestCase.php
@@ -11,7 +11,7 @@ abstract class BasePoLoaderTestCase extends TestCase
 {
     abstract protected function createPoLoader(): Loader;
 
-    public function testPoLoader()
+    public function testPoLoader(): void
     {
         $loader = $this->createPoLoader();
         $translations = $loader->loadFile(__DIR__.'/assets/translations.po');
@@ -73,7 +73,7 @@ public function testPoLoader()
         $this->assertSame('bs', $translations->getLanguage());
     }
 
-    private function translation1(Translation $translation)
+    private function translation1(Translation $translation): void
     {
         $this->assertSame(
             'Ensure this value has at least %(limit_value)d character (it has %sd).',
@@ -87,7 +87,7 @@ private function translation1(Translation $translation)
         $this->assertSame(['', ''], $translation->getPluralTranslations());
     }
 
-    private function translation2(Translation $translation)
+    private function translation2(Translation $translation): void
     {
         $this->assertSame(
             'Ensure this value has at most %(limit_value)d character (it has %sd).',
@@ -101,7 +101,7 @@ private function translation2(Translation $translation)
         $this->assertSame(['', ''], $translation->getPluralTranslations());
     }
 
-    private function translation3(Translation $translation)
+    private function translation3(Translation $translation): void
     {
         $this->assertSame('%ss must be unique for %ss %ss.', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -109,7 +109,7 @@ private function translation3(Translation $translation)
         $this->assertCount(0, $translation->getPluralTranslations());
     }
 
-    private function translation4(Translation $translation)
+    private function translation4(Translation $translation): void
     {
         $this->assertSame('and', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -118,7 +118,7 @@ private function translation4(Translation $translation)
         $this->assertSame(['c-format'], $translation->getFlags()->toArray());
     }
 
-    private function translation5(Translation $translation)
+    private function translation5(Translation $translation): void
     {
         $this->assertSame('Value %sr is not a valid choice.', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -127,7 +127,7 @@ private function translation5(Translation $translation)
         $this->assertSame(['This is a extracted comment'], $translation->getExtractedComments()->toArray());
     }
 
-    private function translation6(Translation $translation)
+    private function translation6(Translation $translation): void
     {
         $this->assertSame('This field cannot be null.', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -137,7 +137,7 @@ private function translation6(Translation $translation)
         $this->assertSame(['C:/Users/Me/Documents/foo2.php' => [1]], $translation->getReferences()->toArray());
     }
 
-    private function translation7(Translation $translation)
+    private function translation7(Translation $translation): void
     {
         $this->assertSame('This field cannot be blank.', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -147,7 +147,7 @@ private function translation7(Translation $translation)
         $this->assertSame(['C:/Users/Me/Documents/foo1.php' => []], $translation->getReferences()->toArray());
     }
 
-    private function translation8(Translation $translation)
+    private function translation8(Translation $translation): void
     {
         $this->assertSame('Field of type: %ss', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -163,7 +163,7 @@ private function translation8(Translation $translation)
         );
     }
 
-    private function translation9(Translation $translation)
+    private function translation9(Translation $translation): void
     {
         $this->assertSame('Integer', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -174,7 +174,7 @@ private function translation9(Translation $translation)
         $this->assertSame(['a simple line comment is above'], $translation->getComments()->toArray());
     }
 
-    private function translation10(Translation $translation)
+    private function translation10(Translation $translation): void
     {
         $this->assertSame('{test1}', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -191,7 +191,7 @@ private function translation10(Translation $translation)
         );
     }
 
-    private function translation11(Translation $translation)
+    private function translation11(Translation $translation): void
     {
         $this->assertSame('{test2}', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -205,7 +205,7 @@ private function translation11(Translation $translation)
         );
     }
 
-    private function translation12(Translation $translation)
+    private function translation12(Translation $translation): void
     {
         $this->assertSame('Multibyte test', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -215,7 +215,7 @@ private function translation12(Translation $translation)
         $this->assertCount(0, $translation->getReferences());
     }
 
-    private function translation13(Translation $translation)
+    private function translation13(Translation $translation): void
     {
         $this->assertSame('Tabulation test', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -225,7 +225,7 @@ private function translation13(Translation $translation)
         $this->assertCount(0, $translation->getReferences());
     }
 
-    private function translation14(Translation $translation)
+    private function translation14(Translation $translation): void
     {
         $this->assertSame('%s has been added to your cart.', $translation->getOriginal());
         $this->assertSame('%s have been added to your cart.', $translation->getPlural());
@@ -257,10 +257,8 @@ public function stringDecodeProvider()
 
     /**
      * @dataProvider stringDecodeProvider
-     * @param mixed $source
-     * @param mixed $decoded
      */
-    public function testStringDecode($source, $decoded)
+    public function testStringDecode(string $source, string $decoded): void
     {
         $po = <<<EOT
 msgid "source"
@@ -270,7 +268,7 @@ public function testStringDecode($source, $decoded)
         $this->assertSame($decoded, $translations->find(null, 'source')->getTranslation());
     }
 
-    public function testMultilineDisabledTranslations()
+    public function testMultilineDisabledTranslations(): void
     {
         $po = <<<'EOT'
 #~ msgid "Last agent hours-description"
diff --git a/tests/CommentsTest.php b/tests/CommentsTest.php
index 64420f5..c272a00 100644
--- a/tests/CommentsTest.php
+++ b/tests/CommentsTest.php
@@ -8,7 +8,7 @@
 
 class CommentsTest extends TestCase
 {
-    public function testComments()
+    public function testComments(): void
     {
         $comments = new Comments();
 
@@ -36,7 +36,7 @@ public function testComments()
         $this->assertCount(1, $comments);
     }
 
-    public function testMergeComments()
+    public function testMergeComments(): void
     {
         $comments1 = new Comments('one', 'two', 'three');
         $comments2 = new Comments('three', 'four', 'five');
@@ -50,7 +50,7 @@ public function testMergeComments()
         $this->assertNotSame($merged, $comments2);
     }
 
-    public function testCreateFromState()
+    public function testCreateFromState(): void
     {
         $state = ['comments' => ['First comment', 'Second comment']];
         $comments = Comments::__set_state($state);
diff --git a/tests/FlagsTest.php b/tests/FlagsTest.php
index d0f7692..e6eeb96 100644
--- a/tests/FlagsTest.php
+++ b/tests/FlagsTest.php
@@ -8,7 +8,7 @@
 
 class FlagsTest extends TestCase
 {
-    public function testFlags()
+    public function testFlags(): void
     {
         $flags = new Flags();
 
@@ -41,7 +41,7 @@ public function testFlags()
         $this->assertCount(2, $flags);
     }
 
-    public function testMergeFlags()
+    public function testMergeFlags(): void
     {
         $flags1 = new Flags('one', 'two', 'three');
         $flags2 = new Flags('three', 'four', 'five');
@@ -61,7 +61,7 @@ public function testMergeFlags()
         $this->assertNotSame($merged, $flags2);
     }
 
-    public function testCreateFromState()
+    public function testCreateFromState(): void
     {
         $state = ['flags' => ['one', 'two']];
         $flags = Flags::__set_state($state);
diff --git a/tests/HeadersTest.php b/tests/HeadersTest.php
index df838c3..39d9d21 100644
--- a/tests/HeadersTest.php
+++ b/tests/HeadersTest.php
@@ -9,7 +9,7 @@
 
 class HeadersTest extends TestCase
 {
-    public function testHeaders()
+    public function testHeaders(): void
     {
         $headers = new Headers();
 
@@ -41,7 +41,7 @@ public function testHeaders()
         $this->assertCount(0, $headers);
     }
 
-    public function testDomain()
+    public function testDomain(): void
     {
         $headers = new Headers();
         $headers->setDomain('foo');
@@ -52,7 +52,7 @@ public function testDomain()
         $this->assertSame('foo', $headers->getDomain());
     }
 
-    public function testLanguage()
+    public function testLanguage(): void
     {
         $headers = new Headers();
         $headers->setLanguage('gl_ES');
@@ -63,7 +63,7 @@ public function testLanguage()
         $this->assertSame('gl_ES', $headers->getLanguage());
     }
 
-    public function testInvalidLanguage()
+    public function testInvalidLanguage(): void
     {
         $this->expectException(InvalidArgumentException::class);
 
@@ -71,7 +71,7 @@ public function testInvalidLanguage()
         $headers->setPluralForm(1, 'foo');
     }
 
-    public function testPluralForm()
+    public function testPluralForm(): void
     {
         $headers = new Headers();
         $headers->setPluralForm(2, '(n=1)');
@@ -82,7 +82,7 @@ public function testPluralForm()
         $this->assertSame([2, '(n=1)'], $headers->getPluralForm());
     }
 
-    public function testMergeHeaders()
+    public function testMergeHeaders(): void
     {
         $headers1 = new Headers(['X-Domain' => 'foo', 'Language' => 'gl_ES']);
         $headers2 = new Headers(['Translator' => 'Oscar Otero', 'Language' => 'ru']);
@@ -97,7 +97,7 @@ public function testMergeHeaders()
         $this->assertNotSame($merged, $headers2);
     }
 
-    public function testCreateFromState()
+    public function testCreateFromState(): void
     {
         $state = ['headers' => ['X-Domain' => 'foo']];
         $headers = Headers::__set_state($state);
diff --git a/tests/MergeTest.php b/tests/MergeTest.php
index bd064f9..4248e7b 100644
--- a/tests/MergeTest.php
+++ b/tests/MergeTest.php
@@ -93,7 +93,7 @@ private static function createPO(): Translations
         return $translations;
     }
 
-    public function testNoStrategy()
+    public function testNoStrategy(): void
     {
         $pot = self::createPOT();
         $po = self::createPO();
@@ -106,7 +106,7 @@ public function testNoStrategy()
     /**
      * We want to use the scanner to fetch new entries and complete them with PO files.
      */
-    public function testScanAndLoadStrategy()
+    public function testScanAndLoadStrategy(): void
     {
         $pot = self::createPOT();
         $po = self::createPO();
@@ -126,7 +126,7 @@ public function testScanAndLoadStrategy()
         $this->assertSnapshot(__FUNCTION__, $merged);
     }
 
-    private function assertSnapshot(string $name, Translations $translations, bool $forceCreate = false)
+    private function assertSnapshot(string $name, Translations $translations, bool $forceCreate = false): void
     {
         $file = __DIR__."/snapshots/{$name}.php";
         $array = $translations->toArray();
diff --git a/tests/MoGeneratorTest.php b/tests/MoGeneratorTest.php
index 4a0bd27..3687180 100644
--- a/tests/MoGeneratorTest.php
+++ b/tests/MoGeneratorTest.php
@@ -11,7 +11,7 @@
 
 class MoGeneratorTest extends TestCase
 {
-    public function testMoGenerator()
+    public function testMoGenerator(): void
     {
         $generator = (new MoGenerator())->includeHeaders();
         $loader = new MoLoader();
diff --git a/tests/MoLoaderTest.php b/tests/MoLoaderTest.php
index 30a21a3..900d431 100644
--- a/tests/MoLoaderTest.php
+++ b/tests/MoLoaderTest.php
@@ -9,7 +9,7 @@
 
 class MoLoaderTest extends TestCase
 {
-    public function testMoLoader()
+    public function testMoLoader(): void
     {
         $loader = new MoLoader();
         $translations = $loader->loadFile(__DIR__.'/assets/translations.mo');
@@ -54,7 +54,7 @@ public function testMoLoader()
         $this->assertSame('bs', $translations->getLanguage());
     }
 
-    private function translation0(Translation $translation)
+    private function translation0(Translation $translation): void
     {
         $this->assertSame('%s has been added to your cart.', $translation->getOriginal());
         $this->assertSame('%s have been added to your cart.', $translation->getPlural());
@@ -62,7 +62,7 @@ private function translation0(Translation $translation)
         $this->assertCount(1, $translation->getPluralTranslations());
     }
 
-    private function translation1(Translation $translation)
+    private function translation1(Translation $translation): void
     {
         $this->assertSame('%ss must be unique for %ss %ss.', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -70,7 +70,7 @@ private function translation1(Translation $translation)
         $this->assertCount(0, $translation->getPluralTranslations());
     }
 
-    private function translation2(Translation $translation)
+    private function translation2(Translation $translation): void
     {
         $this->assertSame('Field of type: %ss', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -78,7 +78,7 @@ private function translation2(Translation $translation)
         $this->assertCount(0, $translation->getPluralTranslations());
     }
 
-    private function translation3(Translation $translation)
+    private function translation3(Translation $translation): void
     {
         $this->assertSame('Integer', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -86,7 +86,7 @@ private function translation3(Translation $translation)
         $this->assertCount(0, $translation->getPluralTranslations());
     }
 
-    private function translation4(Translation $translation)
+    private function translation4(Translation $translation): void
     {
         $this->assertSame('Multibyte test', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -94,7 +94,7 @@ private function translation4(Translation $translation)
         $this->assertCount(0, $translation->getPluralTranslations());
     }
 
-    private function translation5(Translation $translation)
+    private function translation5(Translation $translation): void
     {
         $this->assertSame('Tabulation test', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -102,7 +102,7 @@ private function translation5(Translation $translation)
         $this->assertCount(0, $translation->getPluralTranslations());
     }
 
-    private function translation6(Translation $translation)
+    private function translation6(Translation $translation): void
     {
         $this->assertSame('This field cannot be blank.', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -110,7 +110,7 @@ private function translation6(Translation $translation)
         $this->assertCount(0, $translation->getPluralTranslations());
     }
 
-    private function translation7(Translation $translation)
+    private function translation7(Translation $translation): void
     {
         $this->assertSame('This field cannot be null.', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -118,7 +118,7 @@ private function translation7(Translation $translation)
         $this->assertCount(0, $translation->getPluralTranslations());
     }
 
-    private function translation8(Translation $translation)
+    private function translation8(Translation $translation): void
     {
         $this->assertSame('and', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -126,7 +126,7 @@ private function translation8(Translation $translation)
         $this->assertCount(0, $translation->getPluralTranslations());
     }
 
-    private function translation9(Translation $translation)
+    private function translation9(Translation $translation): void
     {
         $this->assertSame('{test1}', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
@@ -134,7 +134,7 @@ private function translation9(Translation $translation)
         $this->assertCount(0, $translation->getPluralTranslations());
     }
 
-    private function translation10(Translation $translation)
+    private function translation10(Translation $translation): void
     {
         $this->assertSame('{test2}', $translation->getOriginal());
         $this->assertNull($translation->getPlural());
diff --git a/tests/ParsedFunctionsAndCommentsTest.php b/tests/ParsedFunctionsAndCommentsTest.php
index 9bad631..452b70e 100644
--- a/tests/ParsedFunctionsAndCommentsTest.php
+++ b/tests/ParsedFunctionsAndCommentsTest.php
@@ -8,7 +8,7 @@
 
 class ParsedFunctionsAndCommentsTest extends TestCase
 {
-    public function testParsedFunction()
+    public function testParsedFunction(): void
     {
         $function = new ParsedFunction('__', 'template.php', 45);
 
diff --git a/tests/PoGeneratorTest.php b/tests/PoGeneratorTest.php
index 11da0a9..b6ee22e 100644
--- a/tests/PoGeneratorTest.php
+++ b/tests/PoGeneratorTest.php
@@ -10,7 +10,7 @@
 
 class PoGeneratorTest extends TestCase
 {
-    public function testPoLoader()
+    public function testPoLoader(): void
     {
         $generator = new PoGenerator();
         $translations = Translations::create('my-domain');
@@ -96,7 +96,7 @@ public function testPoLoader()
         $this->assertSame($expected, $result);
     }
 
-    public function stringEncodeProvider()
+    public function stringEncodeProvider(): array
     {
         return [
             ['"test"', 'test'],
@@ -109,10 +109,8 @@ public function stringEncodeProvider()
 
     /**
      * @dataProvider stringEncodeProvider
-     * @param mixed $encoded
-     * @param mixed $decoded
      */
-    public function testStringEncode($encoded, $decoded)
+    public function testStringEncode(string $encoded, string $decoded): void
     {
         $this->assertSame($encoded, PoGenerator::encode($decoded));
     }
diff --git a/tests/PoLoaderTest.php b/tests/PoLoaderTest.php
index 3708690..399f893 100644
--- a/tests/PoLoaderTest.php
+++ b/tests/PoLoaderTest.php
@@ -18,7 +18,7 @@ protected function createPoLoader(): Loader
      * @param mixed $source
      * @param mixed $decoded
      */
-    public function testStringDecode($source, $decoded)
+    public function testStringDecode($source, $decoded): void
     {
         $this->assertSame($decoded, PoLoader::decode($source));
     }
diff --git a/tests/ReferencesTest.php b/tests/ReferencesTest.php
index 885e209..c704a71 100644
--- a/tests/ReferencesTest.php
+++ b/tests/ReferencesTest.php
@@ -8,7 +8,7 @@
 
 class ReferencesTest extends TestCase
 {
-    public function testReferences()
+    public function testReferences(): void
     {
         $references = new References();
 
@@ -36,7 +36,7 @@ public function testReferences()
         }
     }
 
-    public function testMergeReferences()
+    public function testMergeReferences(): void
     {
         $references1 = new References();
         $references2 = new References();
@@ -72,7 +72,7 @@ public function testMergeReferences()
         $this->assertNotSame($merged, $references2);
     }
 
-    public function testCreateFromState()
+    public function testCreateFromState(): void
     {
         $state = [
             'references' => [
diff --git a/tests/TranslationTest.php b/tests/TranslationTest.php
index 9ad6eac..4a7cfaa 100644
--- a/tests/TranslationTest.php
+++ b/tests/TranslationTest.php
@@ -11,7 +11,7 @@
 
 class TranslationTest extends TestCase
 {
-    public function testTranslation()
+    public function testTranslation(): void
     {
         $translation = Translation::create('foo', 'bar');
 
@@ -56,7 +56,7 @@ public function testTranslation()
         $this->assertNotSame($translation->getReferences(), $clone->getReferences());
     }
 
-    public function testMergeTranslation()
+    public function testMergeTranslation(): void
     {
         $translation1 = Translation::create('context', 'Original');
         $translation1->translate('Orixinal');