{.noinit.} ignored in for loop -> bad codegen for non-movable types

[mratsim]

In some situation, in particular in a for loop, the compiler may ignore the {.noInit.} pragma.

This is problematic for non-copyable non-movable type, for example a shared atomic counter.
This can be caught at compile-time by making =sink an error:

import std/atomics

type AtomicCounter* = object
  count: Atomic[int]

proc `=`*(dst: var AtomicCounter, src: AtomicCounter) {.error: "An atomic counter cannot be copied.".}

proc `=sink`*(dst: var AtomicCounter, src: AtomicCounter) {.error: "An atomic counter cannot be moved.".}

proc `=destroy`*(sb: var AtomicCounter) {.inline.}=
  discard

proc initialize*(ac: var AtomicCounter) {.inline.} =
  ac.count.store(0, moRelaxed)

proc inc*(p: ptr AtomicCounter) =
  discard p.count.fetchAdd(1, moRelaxed)

var currentCounter: ptr AtomicCounter

template scopedCounter*(body: untyped): untyped =
  bind currentCounter
  block:
    let suspended = currentCounter
    var a {.noInit.}: AtomicCounter
    a.initialize()
    currentCounter = addr(a)

    body

    currentCounter = suspended

proc foo() =
  scopedCounter:
    echo "Counting ..."

proc bar() =
  for i in 0 .. 2:
    scopedCounter:
      echo "Counting ..."

foo()
bar() # Does not compile, Nim tries to insert `=sink` inside a for loop

Error: '=sink' is not available for type <AtomicCounter>; routine: bar

In the C codegen we can see the eqsink

import std/atomics

type AtomicCounter* = object
  count: Atomic[int]

proc `=`*(dst: var AtomicCounter, src: AtomicCounter) {.error: "An atomic counter cannot be copied.".}

when false:
  proc `=sink`*(dst: var AtomicCounter, src: AtomicCounter) {.error: "An atomic counter cannot be moved.".}
else:
  proc `=sink`*(dst: var AtomicCounter, src: AtomicCounter) {.inline.} =
    system.`=sink`(dst.count, src.count)

proc `=destroy`*(sb: var AtomicCounter) {.inline.}=
  discard

proc initialize*(ac: var AtomicCounter) {.inline.} =
  ac.count.store(0, moRelaxed)

proc inc*(p: ptr AtomicCounter) =
  discard p.count.fetchAdd(1, moRelaxed)

var currentCounter: ptr AtomicCounter

template scopedCounter*(body: untyped): untyped =
  bind currentCounter
  block:
    let suspended = currentCounter
    var a {.noInit.}: AtomicCounter
    a.initialize()
    currentCounter = addr(a)

    body

    currentCounter = suspended

proc foo() =
  scopedCounter:
    echo "Counting ..."

proc bar() =
  for i in 0 .. 2:
    scopedCounter:
      echo "Counting ..."

foo()
bar() # eqsink in C codegen

foo function

N_LIB_PRIVATE N_NIMCALL(void, foo__JzbnjO5RDbS420V3wzhF6Q)(void) {
	tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q aX60gensym5240276_;
	TSafePoint TM__9aQ9cEDplzy5RH1NH07Dxehw_2;
	pushSafePoint(&TM__9aQ9cEDplzy5RH1NH07Dxehw_2);
	TM__9aQ9cEDplzy5RH1NH07Dxehw_2.status = setjmp(TM__9aQ9cEDplzy5RH1NH07Dxehw_2.context);
	if (TM__9aQ9cEDplzy5RH1NH07Dxehw_2.status == 0) {
		{
			tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q* suspendedX60gensym5240275_;
			suspendedX60gensym5240275_ = currentCounter__uTp0tKFcbHqRY13s6w9cfig;
			initialize__5UeDB2gx8kATVzHYGeWFRAnoinit_custom_destructors((&aX60gensym5240276_));
			currentCounter__uTp0tKFcbHqRY13s6w9cfig = (&aX60gensym5240276_);
			echoBinSafe(TM__9aQ9cEDplzy5RH1NH07Dxehw_3, 1);
			currentCounter__uTp0tKFcbHqRY13s6w9cfig = suspendedX60gensym5240275_;
		}
		popSafePoint();
	}
	else {
		popSafePoint();
	}
	{
		eqdestroy___DjicWcWwV2o4NWG9aKCOq3gnoinit_custom_destructors((&aX60gensym5240276_));
		if (TM__9aQ9cEDplzy5RH1NH07Dxehw_2.status != 0) nimLeaveFinally();
	}
	if (TM__9aQ9cEDplzy5RH1NH07Dxehw_2.status != 0) reraiseException();
}

bar function

N_LIB_PRIVATE N_NIMCALL(void, bar__JzbnjO5RDbS420V3wzhF6Q_2)(void) {
	tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q aX60gensym5245035_;
	TSafePoint TM__9aQ9cEDplzy5RH1NH07Dxehw_5;
	pushSafePoint(&TM__9aQ9cEDplzy5RH1NH07Dxehw_5);
	TM__9aQ9cEDplzy5RH1NH07Dxehw_5.status = setjmp(TM__9aQ9cEDplzy5RH1NH07Dxehw_5.context);
	if (TM__9aQ9cEDplzy5RH1NH07Dxehw_5.status == 0) {
		{
			NI i;
			NI res;
			i = (NI)0;
			res = ((NI) 0);
			{
				while (1) {
					NI TM__9aQ9cEDplzy5RH1NH07Dxehw_6;
					if (!(res <= ((NI) 2))) goto LA4;
					i = res;
					{
						tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q* suspendedX60gensym5245034_;
						tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q T6_;
						suspendedX60gensym5245034_ = currentCounter__uTp0tKFcbHqRY13s6w9cfig;
						nimZeroMem((void*)(&T6_), sizeof(tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q));
						eqsink___f9azodprU2vXmRJy7KoSVUAnoinit_custom_destructors((&aX60gensym5245035_), T6_);
						initialize__5UeDB2gx8kATVzHYGeWFRAnoinit_custom_destructors((&aX60gensym5245035_));
						currentCounter__uTp0tKFcbHqRY13s6w9cfig = (&aX60gensym5245035_);
						echoBinSafe(TM__9aQ9cEDplzy5RH1NH07Dxehw_3, 1);
						currentCounter__uTp0tKFcbHqRY13s6w9cfig = suspendedX60gensym5245034_;
					}
					if (nimAddInt(res, ((NI) 1), &TM__9aQ9cEDplzy5RH1NH07Dxehw_6)) { raiseOverflow(); };
					res = (NI)(TM__9aQ9cEDplzy5RH1NH07Dxehw_6);
				} LA4: ;
			}
		}
		popSafePoint();
	}
	else {
		popSafePoint();
	}
	{
		eqdestroy___DjicWcWwV2o4NWG9aKCOq3gnoinit_custom_destructors((&aX60gensym5245035_));
		if (TM__9aQ9cEDplzy5RH1NH07Dxehw_5.status != 0) nimLeaveFinally();
	}
	if (TM__9aQ9cEDplzy5RH1NH07Dxehw_5.status != 0) reraiseException();
}

More importantly, this will avoid bad codegen in C++ that only happen for bar but not for foo

........./nimcache/atcounter/@mnoinit_custom_destructors.nim.cpp: In function ‘void eqsink___f9azodprU2vXmRJy7KoSVUAnoinit_custom_destructors(tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q&, tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q)’:
........./nimcache/atcounter/@mnoinit_custom_destructors.nim.cpp:126:18: error: use of deleted function ‘std::atomic<long int>& std::atomic<long int>::operator=(const std::atomic<long int>&)’
  126 |  dst.count = src.count;
      |                  ^~~~~
In file included from ........./nimcache/atcounter/@mnoinit_custom_destructors.nim.cpp:10:
/usr/include/c++/9.3.0/atomic:802:15: note: declared here
  802 |       atomic& operator=(const atomic&) = delete;
      |               ^~~~~~~~
........./nimcache/atcounter/@mnoinit_custom_destructors.nim.cpp: In function ‘void bar__JzbnjO5RDbS420V3wzhF6Q_2()’:
........./nimcache/atcounter/@mnoinit_custom_destructors.nim.cpp:150:88: error: use of deleted function ‘tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q::tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q(const tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q&)’
  150 |       eqsink___f9azodprU2vXmRJy7KoSVUAnoinit_custom_destructors(aX60gensym5215035_, T8_);
      |                                                                                        ^
........./nimcache/atcounter/@mnoinit_custom_destructors.nim.cpp:38:8: note: ‘tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q::tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q(const tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q&)’ is implicitly deleted because the default definition would be ill-formed:
   38 | struct tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q {
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
........./nimcache/atcounter/@mnoinit_custom_destructors.nim.cpp:38:8: error: use of deleted function ‘std::atomic<long int>::atomic(const std::atomic<long int>&)’
In file included from ........./nimcache/atcounter/@mnoinit_custom_destructors.nim.cpp:10:
/usr/include/c++/9.3.0/atomic:801:7: note: declared here
  801 |       atomic(const atomic&) = delete;
      |       ^~~~~~
........./nimcache/atcounter/@mnoinit_custom_destructors.nim.cpp:125:184: note:   initializing argument 2 of ‘void eqsink___f9azodprU2vXmRJy7KoSVUAnoinit_custom_destructors(tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q&, tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q)’
  125 | static N_INLINE(void, eqsink___f9azodprU2vXmRJy7KoSVUAnoinit_custom_destructors)(tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q& dst, tyObject_AtomicCounter__Yz7FoDTBlGPGeC6wQ9aoO7Q src) {
      |                                                                                                                                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
Error: execution of an external compiler program 'g++ -c  -w -w -fpermissive -pthread -O3 -fno-strict-aliasing -fno-ident -std=gnu++14 -funsigned-char  -I/home/beta/.choosenim/toolchains/nim-1.2.0/lib -I......... -o ........./nimcache/atcounter/@mnoinit_custom_destructors.nim.cpp.o ........./nimcache/atcounter/@mnoinit_custom_destructors.nim.cpp' failed with exit code: 1

This may be related to my C++ codegen woes in #13093 which I have no workaround for at the moment.