refactor(iter): use metatable as packed table tag (#23254)

This is a more robust method for tagging a packed table as it completely
eliminates the possibility of mistaking an actual table key as the
packed table tag.

runtime/lua/vim/iter.lua

@@ -18,10 +18,13 @@ ListIter.__call = function(self)
   return self:next()
 end
 
+--- Packed tables use this as their metatable
+local packedmt = {}
+
 ---@private
 local function unpack(t)
-  if type(t) == 'table' and t.__n ~= nil then
-    return _G.unpack(t, 1, t.__n)
+  if getmetatable(t) == packedmt then
+    return _G.unpack(t, 1, t.n)
   end
   return t
 end
@@ -30,11 +33,20 @@ end
 local function pack(...)
   local n = select('#', ...)
   if n > 1 then
-    return { __n = n, ... }
+    return setmetatable({ n = n, ... }, packedmt)
   end
   return ...
 end
 
+---@private
+local function sanitize(t)
+  if getmetatable(t) == packedmt then
+    -- Remove length tag
+    t.n = nil
+  end
+  return t
+end
+
 --- Add a filter step to the iterator pipeline.
 ---
 --- Example:
@@ -208,12 +220,7 @@ function Iter.totable(self)
       break
     end
 
-    if type(args) == 'table' then
-      -- Removed packed table tag if it exists
-      args.__n = nil
-    end
-
-    t[#t + 1] = args
+    t[#t + 1] = sanitize(args)
   end
   return t
 end
@@ -221,12 +228,10 @@ end
 ---@private
 function ListIter.totable(self)
   if self._head == 1 and self._tail == #self._table + 1 and self.next == ListIter.next then
-    -- Remove any packed table tags
+    -- Sanitize packed table values
+    if getmetatable(self._table[1]) == packedmt then
       for i = 1, #self._table do
-      local v = self._table[i]
-      if type(v) == 'table' then
-        v.__n = nil
-        self._table[i] = v
+        self._table[i] = sanitize(self._table[i])
       end
     end
     return self._table

test/functional/lua/vim_spec.lua

@@ -3416,6 +3416,41 @@ describe('lua stdlib', function()
         { item_3 = 'test' },
       }, output)
     end)
+
+    it('handles nil values', function()
+      local t = {1, 2, 3, 4, 5}
+      do
+        local it = vim.iter(t):enumerate():map(function(i, v)
+          if i % 2 == 0 then
+            return nil, v*v
+          end
+          return v, nil
+        end)
+        eq({
+          { [1] = 1 },
+          { [2] = 4 },
+          { [1] = 3 },
+          { [2] = 16 },
+          { [1] = 5 },
+        }, it:totable())
+      end
+
+      do
+        local it = vim.iter(ipairs(t)):map(function(i, v)
+          if i % 2 == 0 then
+            return nil, v*v
+          end
+          return v, nil
+        end)
+        eq({
+          { [1] = 1 },
+          { [2] = 4 },
+          { [1] = 3 },
+          { [2] = 16 },
+          { [1] = 5 },
+        }, it:totable())
+      end
+    end)
   end)
 end)