Assignment overload does not distinguish between var, let, return values

[mratsim]

To implement efficient seq-like value semantics, overloading the assignment operator = is needed. However, every use of = will then create a copy even object construction

type
  Baz = object
    data: int
  Foobar = object
    a: int
    data_ref: ref Baz


proc newFoobar(a: int, data: int): Foobar =
  result.a = a
  new result.data_ref
  result.data_ref[].data = data

proc `=`(dest: var Foobar, src: Foobar) =
  dest.a = src.a
  new dest.data_ref
  dest.data_ref[] = src.data_ref[]

  echo "Value copied"

let a = newFoobar(10, 50)
echo a.repr

Output:

Value copied
[a = 10,
data_ref = ref 0x10eec8060 --> [data = 50]]

Trying to implement move optimization doesn't work, I get Error: cannot bind another '=' to: Foobar
with the following code:

type
  Baz = object
    data: int
  Foobar = object
    a: int
    data_ref: ref Baz


proc newFoobar(a: int, data: int): Foobar =
  result.a = a
  new result.data_ref
  result.data_ref[].data = data

proc `=`(dest: var Foobar, src: Foobar) =
  dest.a = src.a
  new dest.data_ref
  dest.data_ref[] = src.data_ref[]

  echo "Value copied"

proc `=`(dest: var Foobar, src: Foobar{call}) =
  system.`=`(dest, src)

  echo "Value moved"

let a = newFoobar(10, 50)
echo a.repr

[edubart]

This makes assignment overloading unusable for huge objects in performance critical apps.

[Araq]

Assignment overloading is unusable for everything right now.

[Araq]

I'm closing this since =sink is now a thing and any bugs with it should be reported separately.