Ban foreach(arrayWithStringKeys as k => v)

this is not as good as phpstan/phpstan-src#769 (e.g. array_key_first()/array_key_last() aren't covered by this, nor is array_rand()) but it does eliminate the most infuriating cases where this usually crops up.

tests/phpstan/rules/UnsafeForeachArrayOfStringRule.php

@@ -0,0 +1,90 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+*/
+
+declare(strict_types=1);
+
+namespace pocketmine\phpstan\rules;
+
+use PhpParser\Node;
+use PhpParser\Node\Stmt\Foreach_;
+use PHPStan\Analyser\Scope;
+use PHPStan\Rules\Rule;
+use PHPStan\Rules\RuleErrorBuilder;
+use PHPStan\Type\ClassStringType;
+use PHPStan\Type\IntegerType;
+use PHPStan\Type\StringType;
+use PHPStan\Type\Type;
+use PHPStan\Type\TypeTraverser;
+use PHPStan\Type\VerbosityLevel;
+use function sprintf;
+
+/**
+ * @implements Rule<Foreach_>
+ */
+final class UnsafeForeachArrayOfStringRule implements Rule{
+
+	public function getNodeType() : string{
+		return Foreach_::class;
+	}
+
+	public function processNode(Node $node, Scope $scope) : array{
+		/** @var Foreach_ $node */
+		if($node->keyVar === null){
+			return [];
+		}
+		$iterableType = $scope->getType($node->expr);
+
+		if($iterableType->isArray()->no()){
+			return [];
+		}
+		if($iterableType->isIterableAtLeastOnce()->no()){
+			return [];
+		}
+
+		$hasCastableKeyTypes = false;
+		$expectsIntKeyTypes = false;
+		TypeTraverser::map($iterableType->getIterableKeyType(), function(Type $type, callable $traverse) use (&$hasCastableKeyTypes, &$expectsIntKeyTypes) : Type{
+			if($type instanceof IntegerType){
+				$expectsIntKeyTypes = true;
+				return $type;
+			}
+			if(!$type instanceof StringType){
+				return $traverse($type);
+			}
+			if($type->isNumericString()->no() || $type instanceof ClassStringType){
+				//class-string cannot be numeric, even if PHPStan thinks they can be
+				return $type;
+			}
+			$hasCastableKeyTypes = true;
+			return $type;
+		});
+		if($hasCastableKeyTypes && !$expectsIntKeyTypes){
+			return [
+				RuleErrorBuilder::message(sprintf(
+					"Unsafe foreach on array with key type %s (they might be casted to int).",
+					$iterableType->getIterableKeyType()->describe(VerbosityLevel::value())
+				))->tip("Use Utils::foreachWithStringKeys() for a safe Generator-based iterator.")->build()
+			];
+		}
+		return [];
+	}
+
+}