Arc sink arg crash

[cooldome]

Compile with --gc:arc -d:danger

Example

proc sinkArg(x: sink seq[string]) =
  discard

proc varArg(lst: var seq[string]) = 
  sinkArg(lst)

var a = @["a", "b"]
varArg(a)
echo a

Should crash

The issue caused by the fact that in generated c code the sinkArg argument is passed by value not by pointer/reference

[cooldome]

Another interesting question.
Do you expect output to be @["a", "b"] or @[] ?

[Clyybber]

I think @["a", "b"], since we didn't explicitly do sinkArg(move lst)

[cooldome]

@Clyybber, your answer is surprise for me.

var param into sink should produce copy. Is this your the statement?

[alaviss]

This is how that should work from what I can tell:

proc sinkArg(x: sink seq[string]) =
  discard

proc varArg(lst: var seq[string]) = 
  # `lst` is `a`
  sinkArg(lst) # analyze if `lst` can be moved
  # last call in scope, switch to analyze `a` lifetime.

var a = @["a", "b"]
varArg(a) # analysis start
echo a # `a` is used after call to varArg() => `a` can't be moved => `lst` can't be moved.

I'm not too familiar with how Nim actually does the analysis, so this might not be the desired result, though IMO it's the least surprising.

[cooldome]

If Nim will pass lst at line sinkArg(lst) it will be destroyed and you should get @[] in the end.
If Nim will pass a copy of lst at line sinkArg(lst) you will get @["a", "b"].

Question: what is the correct behavior according to our spec? to copy or not to copy.

[Clyybber]

var param into sink should produce copy. Is this your the statement?

Yeah, it should copy.

[cooldome]

OK, then the issue is that varArg never went through injectdestructord analysis