distinct seq[byte]

seq32.nim

type
  Seq32 = distinct seq[byte]

proc `[]=`(arr: var Seq32; i: int; x: byte) =
  var s: seq[byte] = cast[seq[byte]](arr)
  s[i] = x

proc firsthalf*(arr: var array[64, byte]): Seq32 =
  Seq32(arr[0..31])

proc mutatehalf*(arr: var Seq32) =
  for i in 0 .. 31:
    arr[i] = byte(i)

seq32user

import seq32

var buf: array[64, byte]
var x = firsthalf(buf)
x.mutatehalf
echo buf # no mutation