vlib/v/parser/tests/struct_module_section.vv:16:3: error: field `i` of struct `S1` is immutable
   14 | 
   15 | mut s := S1{}
   16 | s.i++
      |   ^
   17 | mut s2 := S2{}
   18 | s2.j++
vlib/v/parser/tests/struct_module_section.vv:18:4: error: field `j` of struct `S2` is immutable
   16 | s.i++
   17 | mut s2 := S2{}
   18 | s2.j++
      |    ^