Treat mb_str_pad() as side-effect-free like other mbstring string functions

bin/functionMetadata_original.php

@@ -136,6 +136,11 @@
 	'lchgrp' => ['hasSideEffects' => true],
 	'lchown' => ['hasSideEffects' => true],
 	'link' => ['hasSideEffects' => true],
+	// Pragmatic: stub is #[Pure(true)], but treat like other mbstring string functions.
+	// Result can change if the analysed program changes internal encoding
+	// (mb_internal_encoding() or default_charset); PHPStan does not track that.
+	// https://github.com/phpstan/phpstan/issues/15224
+	'mb_str_pad' => ['hasSideEffects' => false],
 	'mkdir' => ['hasSideEffects' => true],
 	'move_uploaded_file' => ['hasSideEffects' => true],
 	'mysqli_affected_rows' => ['hasSideEffects' => true],

bin/generate-function-metadata.php

@@ -75,6 +75,16 @@ public function enterNode(Node $node)
 							break 2;
 						}
 
+						// phpstorm-stubs marks mb_str_pad #[Pure(true)], unlike other mbstring string
+						// functions (#[Pure]). The result can change if the analysed program changes
+						// internal encoding (mb_internal_encoding() or default_charset); PHPStan does
+						// not track that. Treat as pure to match mb_strlen et al.
+						// https://github.com/phpstan/phpstan/issues/15224
+						if ($functionName === 'mb_str_pad') {
+							$this->functions[] = $functionName;
+							break 2;
+						}
+
 						// PhpStorm stub's #[Pure(true)] means the function has side effects but its return value is important.
 						// In PHPStan's criteria, these functions are simply considered as ['hasSideEffect' => true].
 						if (isset($attr->args[0]->value->name->name) && $attr->args[0]->value->name->name === 'true') {

resources/functionMetadata.php

@@ -1378,7 +1378,7 @@
 	'mb_rtrim' => ['hasSideEffects' => false],
 	'mb_scrub' => ['hasSideEffects' => false],
 	'mb_split' => ['hasSideEffects' => false],
-	'mb_str_pad' => ['hasSideEffects' => true],
+	'mb_str_pad' => ['hasSideEffects' => false],
 	'mb_str_split' => ['hasSideEffects' => false],
 	'mb_strcut' => ['hasSideEffects' => false],
 	'mb_strimwidth' => ['hasSideEffects' => false],

tests/PHPStan/Analyser/nsrt/bug-15224.php

@@ -0,0 +1,16 @@
+<?php // lint >= 8.3
+
+namespace Bug15224;
+
+use function PHPStan\Testing\assertType;
+
+function rememberMbStrPad(string $s): void
+{
+	if (mb_str_pad($s, 5) === 'x') {
+		assertType("'x'", mb_str_pad($s, 5));
+	} else {
+		assertType('string', mb_str_pad($s, 5));
+	}
+
+	assertType('string', mb_str_pad($s, 5));
+}

tests/PHPStan/Rules/Functions/CallToFunctionStatementWithoutSideEffectsRuleTest.php

@@ -167,6 +167,25 @@ public function testFirstClassCallables(): void
 		]);
 	}
 
+	#[RequiresPhp('>= 8.3.0')]
+	public function testBug15224(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-15224-no-side-effects.php'], [
+			[
+				'Call to function mb_str_pad() on a separate line has no effect.',
+				7,
+			],
+			[
+				'Call to function mb_str_pad() on a separate line has no effect.',
+				8,
+			],
+			[
+				'Call to function mb_str_pad() on a separate line has no effect.',
+				11,
+			],
+		]);
+	}
+
 	#[RequiresPhp('>= 8.5.0')]
 	public function testPipeOperator(): void
 	{

tests/PHPStan/Rules/Functions/data/bug-15224-no-side-effects.php

@@ -0,0 +1,12 @@
+<?php // lint >= 8.3
+
+namespace Bug15224NoSideEffects;
+
+function doFoo(string $s): void
+{
+	mb_str_pad($s, 5);
+	mb_str_pad($s, 5, encoding: 'UTF-8');
+
+	$pad = mb_str_pad(...);
+	mb_str_pad(...);
+}

tests/PHPStan/Rules/Pure/Bug15224Php83PureFunctionRuleTest.php

@@ -0,0 +1,31 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Pure;
+
+use PHPStan\Rules\Rule;
+use PHPStan\Testing\RuleTestCase;
+
+/**
+ * @extends RuleTestCase<PureFunctionRule>
+ */
+class Bug15224Php83PureFunctionRuleTest extends RuleTestCase
+{
+
+	protected function getRule(): Rule
+	{
+		return new PureFunctionRule(new FunctionPurityCheck());
+	}
+
+	public function testBug15224(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-15224-php83.php'], []);
+	}
+
+	public static function getAdditionalConfigFiles(): array
+	{
+		return [
+			__DIR__ . '/data/bug-15224-php83.neon',
+		];
+	}
+
+}

tests/PHPStan/Rules/Pure/PureFunctionRuleTest.php

@@ -230,6 +230,12 @@ public function testBug6574(): void
 		$this->analyse([__DIR__ . '/data/bug-6574.php'], []);
 	}
 
+	#[RequiresPhp('>= 8.4.0')]
+	public function testBug15224(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-15224.php'], []);
+	}
+
 	public function testPureUnlessCallableIsImpure(): void
 	{
 		$this->analyse([__DIR__ . '/data/pure-unless-callable-is-impure.php'], [

tests/PHPStan/Rules/Pure/data/bug-15224-php83.neon

@@ -0,0 +1,2 @@
+parameters:
+	phpVersion: 80300

tests/PHPStan/Rules/Pure/data/bug-15224-php83.php

@@ -0,0 +1,50 @@
+<?php // lint >= 8.3
+declare(strict_types = 1);
+
+namespace Bug15224Php83;
+
+/**
+ * @phpstan-pure
+ */
+function test1(): string
+{
+	return mb_strcut('foo', 1) .
+		   mb_strcut('foo', 1, encoding: 'UTF-8') .
+		   mb_str_pad('foo', 123, 'x') .
+		   mb_str_pad('foo', 123, 'x', encoding: 'UTF-8')
+		;
+}
+
+/**
+ * @phpstan-pure
+ */
+function test2a(): int|false
+{
+	return mb_strpos('foo', 'x', 0);
+}
+
+/**
+ * @phpstan-pure
+ */
+function test2b(): int|false
+{
+	return mb_strpos('foo', 'x', 0, encoding: 'UTF-8');
+}
+
+/**
+ * @phpstan-pure
+ * @return array<string>
+ */
+function test3a(): array
+{
+	return mb_str_split('foo');
+}
+
+/**
+ * @phpstan-pure
+ * @return array<string>
+ */
+function test3b(): array
+{
+	return mb_str_split('foo', encoding: 'UTF-8');
+}

tests/PHPStan/Rules/Pure/data/bug-15224.php

@@ -0,0 +1,67 @@
+<?php // lint >= 8.4
+declare(strict_types = 1);
+
+namespace Bug15224;
+
+/**
+ * @phpstan-pure
+ */
+function test1(): string
+{
+	return mb_trim('foo', 'x') .
+		   mb_trim('foo', 'x', encoding: 'UTF-8') .
+		   mb_strcut('foo', 1) .
+		   mb_strcut('foo', 1, encoding: 'UTF-8') .
+		   mb_ucfirst('foo') .
+		   mb_ucfirst('foo', encoding: 'UTF-8') .
+		   mb_str_pad('foo', 123, 'x') .
+		   mb_str_pad('foo', 123, 'x', encoding: 'UTF-8')
+		;
+}
+
+/**
+ * @phpstan-pure
+ */
+function test2a(): int|false
+{
+	return mb_strpos('foo', 'x', 0);
+}
+
+/**
+ * @phpstan-pure
+ */
+function test2b(): int|false
+{
+	return mb_strpos('foo', 'x', 0, encoding: 'UTF-8');
+}
+
+/**
+ * @phpstan-pure
+ * @return array<string>
+ */
+function test3a(): array
+{
+	return mb_str_split('foo');
+}
+
+/**
+ * @phpstan-pure
+ * @return array<string>
+ */
+function test3b(): array
+{
+	return mb_str_split('foo', encoding: 'UTF-8');
+}
+
+/**
+ * @phpstan-pure
+ */
+function test4(): string
+{
+	return mb_ltrim('foo') .
+		mb_ltrim('foo', encoding: 'UTF-8') .
+		mb_rtrim('foo') .
+		mb_rtrim('foo', encoding: 'UTF-8') .
+		mb_lcfirst('foo') .
+		mb_lcfirst('foo', encoding: 'UTF-8');
+}