gen: fix infinite loop when struct's ref field is pointing to self (#8632) (#8641)

vlib/v/gen/c/auto_str_methods.v

@@ -447,7 +447,7 @@ fn (mut g Gen) gen_str_for_struct(info table.Struct, styp string, str_fn_name st
 	// generates all definitions of substructs
 	mut fnames2strfunc := map{
 		'': ''
-	} // map[string]string // TODO vfmt bug
+	}
 	for field in info.fields {
 		sym := g.table.get_type_symbol(field.typ)
 		if !sym.has_method('str') {
@@ -510,7 +510,13 @@ fn (mut g Gen) gen_str_for_struct(info table.Struct, styp string, str_fn_name st
 						g.auto_str_funcs.write('*')
 					}
 				}
+				// handle circular ref type of struct to the struct itself
+				if styp == field_styp {
+					g.auto_str_funcs.write('_SLIT("<circular>")')
+				} else {
 					g.auto_str_funcs.write(func)
+				}
+
 				if i < info.fields.len - 1 {
 					g.auto_str_funcs.write(',\n\t\t')
 				}

vlib/v/tests/string_interpolation_struct_test.v

@@ -50,3 +50,22 @@ fn test_struct_map_field_string_interpolation() {
 	assert s.contains("dict: {'a': 1, 'b': 2}")
 	assert s.ends_with('}')
 }
+
+struct Circular {
+mut:
+	next &Circular
+}
+
+fn test_stack_circular_elem_auto_str() {
+	mut elem := Circular{0}
+	elem.next = &elem
+	s := '$elem'.replace('\n', '|')
+	assert s == 'Circular{|    next: &<circular>|}'
+}
+
+fn test_heap_circular_elem_auto_str() {
+	mut elem := &Circular{0}
+	elem.next = elem
+	s := '$elem'.replace('\n', '|')
+	assert s == '&Circular{|    next: &<circular>|}'
+}