v/vlib/compiler/tests/repl/immutable_len_fields.repl

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

mut a := []string
a.len = 0   // Error (field len immutable)

mut ints := []int
ints.len = 0   // Error (field len immutable)

println('BYE')
===output===
cannot modify immutable field `len` (type `string`)
declare the field with `mut:`
struct string {
mut:
	len int
}
cannot modify immutable field `len` (type `array`)
declare the field with `mut:`
struct array {
mut:
	len int
}
cannot modify immutable field `len` (type `array`)
declare the field with `mut:`
struct array {
mut:
	len int
}
BYE