If seq is the sink parameter, it can be changed from the function

[alex65536]

If I pass a seq[T] as a sink parameter into the function and change it inside the function, the seq[T] is changed in outer scope. According to https://nim-lang.org/docs/destructors.html, it should be copied to the function, so no change should occur.

Maybe I just don't understand the behavior of sink correctly, but it still remains a vaild bug, because I can change the variables declared with let (see the example below).

Example

func consume(x: sink seq[int]) =
  x[0] += 5

let x = @[1, 2, 3, 4]
consume x
echo x

Current Output

@[6, 2, 3, 4]

Expected Output

@[1, 2, 3, 4]

Additional Information

$ nim -v
Nim Compiler Version 1.0.0 [Linux: amd64]
Compiled at 2019-10-24
Copyright (c) 2006-2019 by Andreas Rumpf

active boot switches: -d:release

The version of Nim I am using is from Debian unstable repos.

[Araq]

According to https://nim-lang.org/docs/destructors.html, it should be copied to the function, so no change should occur

Agreed.

[mratsim]

Quoting myself from #12641

It seems like sink parameter can be mutated.

This is actually very useful to me as for message passing a proc might want to take ownership of a message, update it and sink it into a channel. A var param would be needed otherwise and wouldn't be able to express the sink semantics.

i.e. this compiles

proc foo(x: sink int) =

  x += 1
  echo x

proc main() =
  var z = 3
  foo(z)


main()

However, this mutability of sink parameter is not clearly specified in the destructors spec.
If it's intentional, it should be added, if not it should be removed (and a sink var would be useful).

[Araq]

It's intentional.

[ringabout]

It works with --gc:arc or --gc:destructors.