v/vlib/compiler/tests/repl/chained_fields.repl

/*  1 */ struct A { mut: v int }
/*  2 */ struct B {      a A }
/*  3 */ struct C { mut: b B }
/*  4 */ struct D { mut: c C }

/*  5 */ struct E { mut: v []int }
/*  6 */ struct F {      e []E }

/*  7 */ mut s := 'hello world'
/*( 8)*/ s.len = 0   // Error (field len immutable)

/*  8 */ mut b := B{}
/*( 9)*/ b.a.v = 1   // Error (field a immutable)
/*( 9)*/ b.a = A{}   // Error (field a immutable)
/*  9 */ b = B{A{2}} // Correct

/* 10 */ mut c := C{}
/* 11 */ c.b = B{}   // Correct
/*(12)*/ c.b.a = A{} // Error (field a immutable)
/*(12)*/ c.b.a.v = 1 // Error (field a immutable)

/* 12 */ c2 := C{}
/*(13)*/ c2.b = B{}  // Error (c2 immutable)
/* 13 */ mut d := D{}
/* 14 */ d.c.b = B{} // Correct

/* 15 */ mut f := F{}
/*(16)*/ f.e << E{}      // Error (field e immutable)
/*(16)*/ f.e[0].v << 1   // Error (field e immutable)

/* 16 */ e := E{}
/*(17)*/ e.v << 1    // Error (e immutable)

===output===
cannot modify immutable field `len` (type `string`)
declare the field with `mut:`
struct string {
  mut:
	len int
}
cannot modify immutable field `a` (type `B`)
declare the field with `mut:`
struct B {
  mut:
	a A
}
cannot modify immutable field `a` (type `B`)
declare the field with `mut:`
struct B {
  mut:
	a A
}
cannot modify immutable field `a` (type `B`)
declare the field with `mut:`
struct B {
  mut:
	a A
}
cannot modify immutable field `a` (type `B`)
declare the field with `mut:`
struct B {
  mut:
	a A
}
`c2` is immutable
cannot modify immutable field `e` (type `F`)
declare the field with `mut:`
struct F {
  mut:
	e []E
}
cannot modify immutable field `e` (type `F`)
declare the field with `mut:`
struct F {
  mut:
	e []E
}
`e` is immutable (can't <<)