cgen: use heuristic to detect circular reference in auto str (#11090)

vlib/v/gen/c/auto_str_methods.v

@@ -834,6 +834,9 @@ fn (mut g Gen) gen_str_for_struct(info ast.Struct, styp string, str_fn_name stri
 			if field.typ in ast.charptr_types {
 				fn_builder.write_string('tos2((byteptr)$func)')
 			} else {
+				if field.typ.is_ptr() && sym.kind == .struct_ {
+					fn_builder.write_string('(indent_count > 25) ? _SLIT("<probably circular>") : ')
+				}
 				fn_builder.write_string(func)
 			}
 		}

vlib/v/tests/str_circular_test.v

@@ -0,0 +1,21 @@
+[heap]
+struct Aa {
+mut:
+	bs []Bb
+}
+
+struct Bb {
+mut:
+	a &Aa
+}
+
+fn test_circular() {
+	mut b := Bb{
+		a: &Aa{
+			bs: []Bb{cap: 1}
+		}
+	}
+	b.a.bs << b
+	s := b.str()
+	assert s.len < 3500
+}