shallowcopy string doesn't work with arc/orc

[ringabout]

type RotatedString* = object
  underlying: string
  shift: int

proc rotate*(s: var string, i: int): RotatedString =
  result = RotatedString(shift: i)
  shallowCopy(result.underlying, s)
  # prepareMutation(result.underlying)

proc `[]=`*(r: var RotatedString, i: int, c: char) {.inline.} =
  # when defined(gcArc) or defined(gcOrc):
  #   prepareMutation(r.underlying)
  let L = r.underlying.len
  assert 0 <= i and i < L
  let s = i + r.shift
  if s < L:
    r.underlying[s] = c
  else:
    r.underlying[s - L] = c


var
  x = "Hello, world"
  y = x.rotate(5)

y[0] = 'f'
doAssert x[5] == 'f'

ref #19972

[Araq]

shallowCopy does a real copy with ARC/ORC. This needs to be documented.

[ringabout]

Is there some alternatives or workarounds? It makes https://github.com/andreaferretti/cello fail.

[Araq]

You can move the strings around in order to avoid copies. But in general the point of v2 is to allow for these breaking changes.