add(a, b) buggy in RT js or in VM when a or b is a nil cstring

[timotheecour]

Example 1

proc main()=
  block:
    var a: cstring = nil
    a.add "ba"
    echo (a,)
  block:
    var b: cstring = "ba"
    var b2: cstring
    b.add b2
    echo (b,)
  block:
    var a: cstring = nil
    a.add a
    echo (a,)
  block:
    var a: cstring = "ab"
    a.add a
    echo (a,)
  block:
    var a: cstring = nil
    echo (a,)

static: main()
main()

Current Output

vm js:
("ba",)
("ba",)
("",)
("abab",)
("",) # this is a separate issue, fixed in #16026

rt js:
("nullba",)
("banull",)
("",)
("abab",)
(nil,)

Expected Output

vm js:
("ba",)
("ba",)
("",)
("abab",)
(nil,)

rt js:
("ba",)
("ba",)
("",)
("abab",)
(nil,)

Example 2

when true:
  proc main()=
    block:
      var a: cstring = nil
      a.add "ba".cstring
      echo (a,)
    block:
      var b = "ba"
      var b2: cstring
      b.add b2
      echo (b,)
  # static: main()
  main()

prints:
("nullba",)
/Users/timothee/git_clone/nim/timn/build/nimcache/t11680.js:376
x_33556413[x_33556413_Idx].length += y_33556414.length;
(2 bugs here)

Example 3: js vm

uncomment static: main() in Example 2
gives CT error:

system.nim(1987, 5) Error: cannot generate VM code for asm "      if ("x

Example 4: vm (eg c vm)

when true:
  proc main=
    var a = "abc"
    a.add nil
    echo (a,)
  static: main()
  main()

VM:
Error: index out of bounds, the container is empty
while y[i] != '\0':

Additional Information

1.5.1 be6e891

[metagn]

This overload add(var cstring, cstring) is not in the system docs. The block that declares it does not check for defined(nimdoc). The overload here uses AppendStrStr, for which I, without thinking of the possibility that cstrings might be nil, deleted the null checks for and simply turned into a += b for some reason in #14158. Dead easy fix, just revert this line and add back the deleted temp to the cstring branch (#16674 is also introduced by this PR though so you might need to look for more problems), but also add or defined(nimdoc) to the when statement in the system module and maybe look for other declarations that might not have docs in the standard library.

As for VM, maybe when nimvm: add(x, string(y)) else: asm ... in the same proc.