destructor for seq[T] elements not called with gc:refc upon setLen 0

[timotheecour]

destructor not called for elements in a seq after a shrink via for example setLen 0

Example

when true:
  type Foo = object
    f0: int

  proc `=destroy`(a: var Foo) =
    echo ("dtor", a.f0)

  proc main() =
    block:
      var b: seq[Foo]
      b.add Foo(f0: 1)
      b.add Foo(f0: 2)
    echo "ok2"
    block:
      var b: seq[Foo]
      b.add Foo(f0: 11)
      b.add Foo(f0: 12)
      echo "ok3"
      b.setLen 0
      echo "ok4"
    echo "ok5"
    # GC_fullCollect() # makes no difference
  main()

Current Output

("dtor", 1)
("dtor", 2)
ok2
ok3
ok4
ok5

Expected Output

should output same as with gc:arc
("dtor", 1)
("dtor", 2)
ok2
ok3
("dtor", 12)
("dtor", 11)
ok4
ok5

Additional Information

• 1.5.1 578d1ee
• EDIT: similar to destructor not called (leak) for seq elements; destructor called for default-initialized object #14604 but different and hopefully this isse is easier to fix, as it concerns setLen directly

[timotheecour]

note

destructors should work, even for gc:refc, and even for vm and js, otherwise you end up with a language split, where code needs to be rewritten for specific backends.

example use case: auto-closing FILE:

type CFileWrap = object
  cfile: File

proc open2(path: string): CFileWrap =
  result.cfile = open(path)

proc `=destroy`(a: var CFileWrap) =
  if a.cfile != nil: close(a.cfile)

[ringabout]

refc shouldn't accept dedicated destructor support anymore.