HashSet misses elements when excl'ing while iterating over it

[liquidev]

Example

import std/sets
import std/hashes

type
  Vec2i = tuple[x, y: int32]

proc hash(vec: Vec2i): Hash =
  result = Hash(0)
  result = result !& hash(vec.x)
  result = result !& hash(vec.y)
  result = !$result

var set: HashSet[Vec2i]

for y in 1..4:
  for x in 0..<32:
    let vec: Vec2i = (x.int32, y.int32)
    set.incl(vec)

let i = set.len

var j = 0
for vec in set:
  inc(j)
  set.excl(vec)
echo (i, j)

Current Output

(128, 96)

32 elements are magically gone from the set while we're iterating over it.

Expected Output

(128, 128)

Additional Information

If we change the HashSet to an OrderedSet, the program gets stuck at an infinite loop. Adding echo vec to the loop at the end of the program yields that it gets stuck at (x: 22, y: 1). ^C'ing out of the program, I could trace that it gets stuck in this while loop:

Nim/lib/pure/collections/sets.nim

Lines 620 to 629 in a156484

	template forAllOrderedPairs(yieldStmt: untyped) {.dirty.} =
	if s.data.len > 0:
	var h = s.first
	var idx = 0
	while h >= 0:
	var nxt = s.data[h].next
	if isFilled(s.data[h].hcode):
	yieldStmt
	inc(idx)
	h = nxt

If sets cannot handle excluding elements while iterating over them, then that should be clearly stated in the documentation and ideally also trigger an assertion.

$ nim -v
Nim Compiler Version 1.5.1 [Linux: amd64]
Compiled at 2020-10-22
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: 2cb484cefb13e4663afd8e0eb2ad81d43f097803
active boot switches: -d:release

[cooldome]

Yes, it is not safe to iterate over container and modify it at the same time. Nim should raise an error in debug build.