sink var/var sink both accepted; sink var leads to bad codegen and strange behavior

[jrfondren]

Example

import os

when defined(good):
  proc ex(s1: var sink TaintedString) =
    var
      a1 = s1[0].addr
      s2 = move(s1).string
    echo a1 == s2[0].addr

when defined(bad):
  proc ex(s1: sink var TaintedString) =
    var
      a1 = s1[0].addr
      s2 = move(s1).string
    echo a1 == s2[0].addr

var x = paramStr(0)
ex(x)

Current Output -d:good

true

Current Output -d:bad --gc:refc

false

Current Output -d:bad --gc:arc

/home/jfondren/nim/shadow/nc/conv3.nim(14, 20) Hint: conversion from string to itself is pointless [ConvFromXtoItselfNotNeeded]                                       
CC: conv3.nim                                                                      
/home/jfondren/.cache/nim/conv3_d/@mconv3.nim.c: In function ‘ex__NQwMCZiZiLaucv2cJhGPYw’:
/home/jfondren/.cache/nim/conv3_d/@mconv3.nim.c:124:7: error: incompatible types when assigning to type ‘NimStringV2’ from type ‘NimStringV2 *’
  124 |  s2 = s1;
      |       ^~
/home/jfondren/.cache/nim/conv3_d/@mconv3.nim.c:125:4: error: ‘s1’ is a pointer; did you mean to use ‘->’?
  125 |  s1.len = 0; s1.p = NIM_NIL;
      |    ^
      |    ->
/home/jfondren/.cache/nim/conv3_d/@mconv3.nim.c:125:16: error: ‘s1’ is a pointer; did you mean to use ‘->’?
  125 |  s1.len = 0; s1.p = NIM_NIL;
      |                ^
      |                ->
Error: execution of an external compiler program 'gcc -c  -w -fmax-errors=3   -I/home/jfondren/.choosenim/toolchains/nim-1.4.0/lib -I/home/jfondren/nim/shadow/nc -o /home/jfondren/.cache/nim/conv3_d/@mconv3.nim.c.o /home/jfondren/.cache/nim/conv3_d/@mconv3.nim.c' failed with exit code: 1

Additional Information

Nim Compiler Version 1.4.0 [Linux: amd64]
Compiled at 2020-10-16
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: bdcd87afca238a0a7b2c70971827cf9172817b12
active boot switches: -d:release