cgen,js: support [autostr: allowrecurse] attribute, for allowing deep printing of data structures with pointer fields (trees, instead of graphs with cycles) (#16973)

vlib/v/gen/c/auto_str_methods.v

@@ -860,6 +860,8 @@ fn (mut g Gen) gen_str_for_struct(info ast.Struct, styp string, typ_str string,
 	}
 	fn_body.writeln('\tstring res = str_intp( ${(info.fields.len - field_skips.len) * 4 + 3}, _MOV((StrIntpData[]){')
 	fn_body.writeln('\t\t{_SLIT("${clean_struct_v_type_name}{\\n"), 0, {.d_c=0}},')
+
+	allow_circular := info.attrs.any(it.name == 'autostr' && it.arg == 'allowrecurse')
 	mut is_first := true
 	for i, field in info.fields {
 		// Skip `str:skip` fields
@@ -937,7 +939,7 @@ fn (mut g Gen) gen_str_for_struct(info ast.Struct, styp string, typ_str string,
 			}
 		}
 		// handle circular ref type of struct to the struct itself
-		if styp == field_styp {
+		if styp == field_styp && !allow_circular {
 			fn_body.write_string('${funcprefix}_SLIT("<circular>")')
 		} else {
 			// manage C charptr

vlib/v/gen/js/auto_str_methods.v

@@ -712,6 +712,7 @@ fn (mut g JsGen) gen_str_for_struct(info ast.Struct, styp string, str_fn_name st
 
 	fn_builder.writeln('\tlet res = /*struct name*/new string("${clean_struct_v_type_name}{\\n")')
 
+	allow_circular := info.attrs.any(it.name == 'autostr' && it.arg == 'allowrecurse')
 	for i, field in info.fields {
 		mut ptr_amp := if field.typ.is_ptr() { '&' } else { '' }
 		mut prefix := ''
@@ -758,7 +759,7 @@ fn (mut g JsGen) gen_str_for_struct(info ast.Struct, styp string, str_fn_name st
 			}
 		}
 		// handle circular ref type of struct to the struct itself
-		if styp == field_styp {
+		if styp == field_styp && !allow_circular {
 			fn_builder.write_string('res.str += new string("<circular>")')
 		} else {
 			// manage C charptr

vlib/v/tests/tag_autostr_allowrecurse_test.v

@@ -0,0 +1,23 @@
+[autostr: allowrecurse]
+struct Node {
+	a &Node = unsafe { nil }
+	b &Node = unsafe { nil }
+}
+
+fn test_stringifying_a_value_of_a_struct_tagged_with_autostr_allowrecurse() {
+	abc := &Node{
+		a: &Node{
+			b: &Node{}
+		}
+		b: &Node{
+			a: &Node{}
+		}
+	}
+	// println(abc)
+	s := abc.str()
+	assert !s.contains('&<circular>')
+	assert s.contains('a: &Node{')
+	assert s.contains('a: &nil')
+	assert s.contains('b: &Node{')
+	assert s.contains('b: &nil')
+}