fixes #22883; replace default(typeof( with reset; suppress `Unsaf… (

#22895)

fixes #22883

…eDefault` warnings

avoid issues mentioned by https://forum.nim-lang.org namely, it
allocated unnecessary stack objects in the loop

```c
while (1)
{
tyObject_N__8DSNqSGSHBKOhI8CqSgAow T5_;
nimZeroMem((void *)(&T5_), sizeof(tyObject_N__8DSNqSGSHBKOhI8CqSgAow));
eqsink___test4954_u450((&(*t_p0).data.p->data[i].Field1), T5_);
}
```

It might be more efficient in some cases

follow up #21821

(cherry picked from commit 92141e8)

lib/pure/collections/setimpl.nim

@@ -86,7 +86,7 @@ proc exclImpl[A](s: var HashSet[A], key: A): bool {.inline.} =
       var j = i # The correctness of this depends on (h+1) in nextTry,
       var r = j # though may be adaptable to other simple sequences.
       s.data[i].hcode = 0 # mark current EMPTY
-      s.data[i].key = default(typeof(s.data[i].key))
+      reset(s.data[i].key)
       doWhile((i >= r and r > j) or (r > j and j > i) or (j > i and i >= r)):
         i = (i + 1) and msk # increment mod table size
         if isEmpty(s.data[i].hcode): # end of collision cluster; So all done

lib/pure/collections/sets.nim

@@ -379,7 +379,7 @@ proc clear*[A](s: var HashSet[A]) =
   s.counter = 0
   for i in 0 ..< s.data.len:
     s.data[i].hcode = 0
-    s.data[i].key = default(typeof(s.data[i].key))
+    reset(s.data[i].key)
 
 
 proc union*[A](s1, s2: HashSet[A]): HashSet[A] =
@@ -813,7 +813,7 @@ proc clear*[A](s: var OrderedSet[A]) =
   for i in 0 ..< s.data.len:
     s.data[i].hcode = 0
     s.data[i].next = 0
-    s.data[i].key = default(typeof(s.data[i].key))
+    reset(s.data[i].key)
 
 proc len*[A](s: OrderedSet[A]): int {.inline.} =
   ## Returns the number of elements in `s`.

lib/pure/collections/tableimpl.nim

@@ -119,8 +119,8 @@ template delImplIdx(t, i, makeEmpty, cellEmpty, cellHash) =
         var j = i         # The correctness of this depends on (h+1) in nextTry
         var r = j         # though may be adaptable to other simple sequences.
         makeEmpty(i)                     # mark current EMPTY
-        t.data[i].key = default(typeof(t.data[i].key))
-        t.data[i].val = default(typeof(t.data[i].val))
+        reset(t.data[i].key)
+        reset(t.data[i].val)
         while true:
           i = (i + 1) and msk            # increment mod table size
           if cellEmpty(i):               # end of collision cluster; So all done
@@ -151,8 +151,8 @@ template clearImpl() {.dirty.} =
   for i in 0 ..< t.dataLen:
     when compiles(t.data[i].hcode): # CountTable records don't contain a hcode
       t.data[i].hcode = 0
-    t.data[i].key = default(typeof(t.data[i].key))
-    t.data[i].val = default(typeof(t.data[i].val))
+    reset(t.data[i].key)
+    reset(t.data[i].val)
   t.counter = 0
 
 template ctAnd(a, b): bool =

lib/system.nim

@@ -931,14 +931,18 @@ proc default*[T](_: typedesc[T]): T {.magic: "Default", noSideEffect.} =
 proc reset*[T](obj: var T) {.noSideEffect.} =
   ## Resets an object `obj` to its default value.
   when nimvm:
+    {.push warning[UnsafeDefault]:off.}
     obj = default(typeof(obj))
+    {.pop.}
   else:
     when defined(gcDestructors):
       {.cast(noSideEffect), cast(raises: []), cast(tags: []).}:
         `=destroy`(obj)
         `=wasMoved`(obj)
     else:
+      {.push warning[UnsafeDefault]:off.}
       obj = default(typeof(obj))
+      {.pop.}
 
 proc setLen*[T](s: var seq[T], newlen: Natural) {.
   magic: "SetLengthSeq", noSideEffect, nodestroy.}