Automatically implement Stringable interface

UPGRADING

@@ -410,6 +410,8 @@ PHP 8.0 UPGRADE NOTES
   . Some consistency fixes to variable syntax have been applied, for example
     writing `Foo::BAR::$baz` is now allowed.
     RFC: https://wiki.php.net/rfc/variable_syntax_tweaks
+  . Added Stringable.
+    RFC: https://wiki.php.net/rfc/stringable
 
 - Date:
   . Added DateTime::createFromInterface() and

Zend/tests/stringable_automatic_implementation.phpt

@@ -0,0 +1,35 @@
+--TEST--
+Stringable is automatically implemented
+--FILE--
+<?php
+
+class Test {
+    public function __toString() {
+        return "foo";
+    }
+}
+
+var_dump(new Test instanceof Stringable);
+var_dump((new ReflectionClass(Test::class))->getInterfaceNames());
+
+class Test2 extends Test {
+    public function __toString() {
+        return "bar";
+    }
+}
+
+var_dump(new Test2 instanceof Stringable);
+var_dump((new ReflectionClass(Test2::class))->getInterfaceNames());
+
+?>
+--EXPECT--
+bool(true)
+array(1) {
+  [0]=>
+  string(10) "Stringable"
+}
+bool(true)
+array(1) {
+  [0]=>
+  string(10) "Stringable"
+}

Zend/tests/type_declarations/variance/stringable.phpt

@@ -0,0 +1,17 @@
+--TEST--
+Automatic Stringable implementation participates in variance
+--FILE--
+<?php
+
+class Foo {
+    public function test(): Stringable {}
+}
+class Bar extends Foo {
+    public function test(): Bar {}
+    public function __toString() {}
+}
+
+?>
+===DONE===
+--EXPECT--
+===DONE===

Zend/zend_compile.c

@@ -6066,6 +6066,24 @@ static void zend_check_magic_method_attr(uint32_t attr, const char* method, zend
 }
 /* }}} */
 
+static void add_stringable_interface(zend_class_entry *ce) {
+	for (uint32_t i = 0; i < ce->num_interfaces; i++) {
+		if (zend_string_equals_literal(ce->interface_names[i].lc_name, "stringable")) {
+			/* Interface already explicitly implemented */
+			return;
+		}
+	}
+
+	ce->num_interfaces++;
+	ce->interface_names =
+		erealloc(ce->interface_names, sizeof(zend_class_name) * ce->num_interfaces);
+	// TODO: Add known interned strings instead?
+	ce->interface_names[ce->num_interfaces - 1].name =
+		zend_string_init("Stringable", sizeof("Stringable") - 1, 0);
+	ce->interface_names[ce->num_interfaces - 1].lc_name =
+		zend_string_init("stringable", sizeof("stringable") - 1, 0);
+}
+
 void zend_begin_method_decl(zend_op_array *op_array, zend_string *name, zend_bool has_body) /* {{{ */
 {
 	zend_class_entry *ce = CG(active_class_entry);
@@ -6147,6 +6165,7 @@ void zend_begin_method_decl(zend_op_array *op_array, zend_string *name, zend_boo
 	} else if (zend_string_equals_literal(lcname, ZEND_TOSTRING_FUNC_NAME)) {
 		zend_check_magic_method_attr(fn_flags, "__toString", 0);
 		ce->__tostring = (zend_function *) op_array;
+		add_stringable_interface(ce);
 	} else if (zend_string_equals_literal(lcname, ZEND_INVOKE_FUNC_NAME)) {
 		zend_check_magic_method_attr(fn_flags, "__invoke", 0);
 	} else if (zend_string_equals_literal(lcname, ZEND_DEBUGINFO_FUNC_NAME)) {
@@ -6680,6 +6699,10 @@ zend_op *zend_compile_class_decl(zend_ast *ast, zend_bool toplevel) /* {{{ */
 
 	CG(active_class_entry) = ce;
 
+	if (implements_ast) {
+		zend_compile_implements(implements_ast);
+	}
+
 	zend_compile_stmt(stmt_ast);
 
 	/* Reset lineno for final opcodes and errors */
@@ -6719,10 +6742,6 @@ zend_op *zend_compile_class_decl(zend_ast *ast, zend_bool toplevel) /* {{{ */
 		}
 	}
 
-	if (implements_ast) {
-		zend_compile_implements(implements_ast);
-	}
-
 	if ((ce->ce_flags & (ZEND_ACC_IMPLICIT_ABSTRACT_CLASS|ZEND_ACC_INTERFACE|ZEND_ACC_TRAIT|ZEND_ACC_EXPLICIT_ABSTRACT_CLASS)) == ZEND_ACC_IMPLICIT_ABSTRACT_CLASS) {
 		zend_verify_abstract_class(ce);
 	}

ext/spl/tests/class_implements_variation1.phpt

@@ -184,7 +184,9 @@ Error: 2 - class_implements(): Class  does not exist and could not be loaded, %s
 bool(false)
 
 --instance of classWithToString--
-array(0) {
+array(1) {
+  ["Stringable"]=>
+  string(10) "Stringable"
 }
 
 --instance of classWithoutToString--