cgen: circular reference limit to auto_str for interface (#18340)

vlib/v/gen/c/auto_str_methods.v

@@ -1007,7 +1007,7 @@ fn (mut g Gen) gen_str_for_struct(info ast.Struct, lang ast.Language, styp strin
 			if field.typ in ast.charptr_types {
 				fn_body.write_string('tos4((byteptr)${func})')
 			} else {
-				if field.typ.is_ptr() && sym.kind == .struct_ {
+				if field.typ.is_ptr() && sym.kind in [.struct_, .interface_] {
 					funcprefix += '(indent_count > 25)? _SLIT("<probably circular>") : '
 				}
 				// eprintln('>>> caller_should_free: ${caller_should_free:6s} | funcprefix: $funcprefix | func: $func')

vlib/v/tests/option_ptr_iface_test.v

@@ -0,0 +1,18 @@
+interface ITeste {
+mut:
+	teste ?&ITeste
+}
+
+struct Teste {
+pub mut:
+	teste ?&ITeste
+}
+
+fn test_main() {
+	mut t := Teste{}
+	t.teste = &t
+	z := &ITeste(t)
+	dump(z)
+	dump(t)
+	assert voidptr(t.teste?) == voidptr(z.teste?)
+}