[WIP] fix {.incompleteStruct.} (implied wrong typeinfo)

[timotheecour]

EDIT: DO NOT REVIEW YET...

the meaning of {.incompleteStruct.} is for types like DIR (eg http://manpages.ubuntu.com/manpages/bionic/man7/dirent.h.7posix.html) which are forward declared, and hence for which sizeof (or accessing members or creating an instance on the stack etc) would result in cgen CT error on some platforms. Treating it like we used to (via void*) is clearly wrong though. Investigating how to best handle this... stay tuned.

THE DESCRIPTION BELOW NEEDS TO BE UPDATED

• followup on CT sizeof(+friends) for {.importc, completeStruct.} types, enable ABI static checks #13926 and addresses this: CT sizeof(+friends) for {.importc, completeStruct.} types, enable ABI static checks #13926 (comment)

needs a changelog entry and needs to be documented well

• implements proposal 2 from importc types: CT sizeof, $, {.sizeof.}, const CVAR {.importc.}: int RFCs#205 by turning {.incompleteStruct.} into a deprecated noop

incompleteStruct (only used on 2 types, DIR and FILE) is problematic because when that pragma was set but the type was in fact not forward decared, it resulted in incorrect typeinfo

when true: # D20200504T161748
  type CFile {.importc: "FILE", header: "<stdio.h>", incompleteStruct.} = object
  let a = newSeq[CFile](1000)
  dumpNumberOfInstances()

(with -d:nimTypeNames)
before PR:
[Heap] seq[CFile]: #1; bytes: 8032
[Heap] total number of bytes: 8032
[Heap] allocs/deallocs: 6/0

after PR:
[Heap] seq[CFile]: #1; bytes: 152032
[Heap] total number of bytes: 152032
[Heap] allocs/deallocs: 6/0

note that FILE is a struct in C, not a struct pointer, so treating it as void* was incorrect as can be seen with this:

#include <stdio.h>
int main (int argc, char *argv[]) {
  printf("%d\n", sizeof(FILE));
  printf("%d\n", sizeof(FILE*));
  return 0;
}

links

When to use IncompleteStruct pragma? - Nim forum