ARC introduces copies that can cause object slicing

[narimiran]

To reproduce this, you need to bootstrap using ARC: koch boot --gc:arc.

Example

Save the example as .rst file, and run: nim rst2html thisfile.rst

==========
Main title
==========

Some text.

.. code-block:: nim
    :test: "nim c --gc:arc $1"
  var a: array[0..1, char]
  let i = 5

More text.

Current Output

/home/miran/code/nim-lang/Nim/compiler/nim.nim(125) nim
/home/miran/code/nim-lang/Nim/compiler/nim.nim(84) handleCmdLine
/home/miran/code/nim-lang/Nim/compiler/main.nim(273) mainCommand
/home/miran/code/nim-lang/Nim/compiler/docgen.nim(1406) commandRst2Html
/home/miran/code/nim-lang/Nim/compiler/docgen.nim(1400) commandRstAux
/home/miran/code/nim-lang/Nim/lib/packages/docutils/rstgen.nim(1156) renderRstToOut
/home/miran/code/nim-lang/Nim/lib/packages/docutils/rstgen.nim(287) renderAux
/home/miran/code/nim-lang/Nim/lib/packages/docutils/rstgen.nim(1313) renderRstToOut
/home/miran/code/nim-lang/Nim/lib/packages/docutils/rstgen.nim(1010) renderCode
/home/miran/code/nim-lang/Nim/lib/packages/docutils/rstgen.nim(1520) :anonymous
/home/miran/code/nim-lang/Nim/lib/system/fatal.nim(53) sysFatal
Error: unhandled exception: invalid object conversion [ObjectConversionDefect]

Expected Output

The same .html as when there is no :test: "nim c --gc:arc $1" line. (Without that line, the example is successful)

Additional Information

Devel version of Nim bootstrapped using ARC (koch boot --gc:arc).

cc @Clyybber

[Araq]

Reduced example program:

type
  RstGenerator* = object of RootObj
    splitAfter*: int          # split too long entries in the TOC
    listingCounter*: int
    hasToc*: bool
    theIndex: string # Contents of the index file to be dumped at the end.
    outDir*: string      ## output directory, initialized by docgen.nim
    destFile*: string    ## output (HTML) file, initialized by docgen.nim
    filename*: string         ## source Nim or Rst file
    currentSection: string
    id*: int               ## A counter useful for generating IDs.
    onTestSnippet*: proc (d: var RstGenerator; filename, cmd: string; status: int;
                          content: string)

  TDocumentor = object of RstGenerator
    modDesc: string       # module description
    modDeprecationMsg: string

  PDoc = ref TDocumentor

proc newDocumentor*(): PDoc =
  new(result)
  result.onTestSnippet =
    proc (gen: var RstGenerator; filename, cmd: string; status: int; content: string) =
      inc(gen.id)
      var d = TDocumentor(gen)

proc call(result: PDoc) =
  result.onTestSnippet(result[], "filename", "cmd", 990, "content")

var d = newDocumentor()
d.call

[Araq]

Well ... ARC is correct, the docgen does some invalid object slicing here as TDocumentor is a value type. We need to fix our docgen.

[Clyybber]

I think the issue is that we don't copy over the RTTI in =copy:

type
  RstGenerator = object of RootObj
    filename: string

  TDocumentor = object of RstGenerator

proc test(gen: var RstGenerator) =
  gen.filename.add "he"
  echo gen of TDocumentor
  var d = TDocumentor(gen)

when false:
  # Raises ObjectConversionDefect both on arc and refc
  var r = RstGenerator()
  test(r)
else:
  # # Raises ObjectConversionDefect only on arc
  var r = TDocumentor()
  test(r)

This one shows the RTTI getting lost:

type
  RstGenerator = object of RootObj
    filename: string

  TDocumentor = object of RstGenerator

proc test2(r: var RstGenerator) =
  # With cursorinference:off shows that assignment did not copy the RTTI
  echo r of TDocumentor
  var a = r
  var b = r
  echo a of TDocumentor
var r = TDocumentor()
test2(r)

(needs to be compiled with --gc:arc --cursorinference:off)

EDIT: Hmm, nevermind, I don't think this can work with value types :D

[a-mr]

reduced "reduced example":

type
  A* = object of RootObj
    x*: int
    f*: proc (a: var A)

  B = object of A

proc newB*(): ref B =
  new(result)
  result.f =
    proc (a: var A) =
      inc(a.x)
      var b = B(a)

var b = newB()
b.f(b[])

I did not understand what's wrong with object slicing here. B is allocated on heap and var is passed by reference. So why is it incorrect to do var b = B(a)?

[Araq]

@a-mr your analysis is correct and mine was incorrect. But ARC currently constructs B(a) from a copy of a (which is of type A) and fixing this looked very hard esp considering the fringe case -- inheritance is almost always done with ref object instead. But ok, re-opening.