Variable can be moved while it's being passed as var

[alex65536]

Description

Repro:

type
  O = object
    l: proc(s: string)
    a: string
    b: string

proc dovar(a: var O, b: sink O) =
  echo "a: ", a.a, " ", a.b
  echo "b: ", b.a, " ", b.b
  
proc doit(o: sink O) =
  o.l(o.a)
  dovar(o, o)

let l = proc(s: string) = echo "l: ", s
doit(O(l: l, a: "1", b: "2"))

Nim Version

2.0.6. Also reproducible on current devel as of 2024-06-28.

Current Output

l: 1
a:  
b: 1 2

Expected Output

l: 1
a: 1 2
b: 1 2

Possible Solution

The compiler should not move the variable if it's being passed as var into the same function.

Currently, the possibility of move vs copy is checked with a simple DFA in compiler/dfa.nim, which handles only sequential variable use + branches. We should probably add something like borrow and endborrow in DFA, to indicate that the variable is already borrowed into a var argument and cannot be moved while the borrow is active.

Additional Information

expandArc shows that move occurs:

try:
  o.l(o.a)
  dovar(o):
    let blitTmp = o
    `=wasMoved`(o)
    blitTmp
finally:
  `=destroy`(o)

Various changes to the code can actually fix it. For example, adding discard o to the end leads to correct result, because it prevents moving. Also, if doit() is rewritten as

proc do2(s: string) = echo s
proc doit(o: sink O) =
  do2(o.a)
  dovar(o, o)

then =wasMoved is elided for come reason, and the program works fine. expandArc in this case works as follows:

do2(o.a)
dovar(o):
  let blitTmp = o
  blitTmp

[alex65536]

Kinda related to #23748. Though, it seems to be a more general bug, and #23748 can be fixed separately without fixing the general one.

[Araq]

dovar(#[byvar]# o, o) is UB in general. Simple cases should be forbidden at compile-time and the complex cases need a better borrow checker. Doing the borrow checking via the DFA might be a good idea.

[alex65536]

dovar(#[byvar]# o, o) is UB in general.

Is it documented anywhere that it's UB?

[Araq]

Sure: https://nim-lang.org/docs/manual_experimental.html#aliasing-restrictions-in-parameter-passing