fix(iter): add tag to packed table

If pack() is called with a single value, it does not create a table; it
simply returns the value it is passed. When unpack is called with a
table argument, it interprets that table as a list of values that were
packed together into a table.

This causes a problem when the single value being packed is _itself_ a
table. pack() will not place it into another table, but unpack() sees
the table argument and tries to unpack it.

To fix this, we add a simple "tag" to packed table values so that
unpack() only attempts to unpack tables that have this tag. Other tables
are left alone. The tag is simply the length of the table.

runtime/lua/vim/iter.lua

@@ -28,16 +28,17 @@ end
 
 ---@private
 local function unpack(t)
-  if type(t) == 'table' then
-    return _G.unpack(t)
+  if type(t) == 'table' and t.__n ~= nil then
+    return _G.unpack(t, 1, t.__n)
   end
   return t
 end
 
 ---@private
 local function pack(...)
-  if select('#', ...) > 1 then
-    return { ... }
+  local n = select('#', ...)
+  if n > 1 then
+    return { __n = n, ... }
   end
   return ...
 end
@@ -210,6 +211,12 @@ function Iter.totable(self)
     if args == nil then
       break
     end
+
+    if type(args) == 'table' then
+      -- Removed packed table tag if it exists
+      args.__n = nil
+    end
+
     t[#t + 1] = args
   end
   return t
@@ -218,6 +225,14 @@ 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
+    for i = 1, #self._table do
+      local v = self._table[i]
+      if type(v) == 'table' then
+        v.__n = nil
+        self._table[i] = v
+      end
+    end
     return self._table
   end
 
@@ -747,7 +762,7 @@ function ListIter.enumerate(self)
   local inc = self._head < self._tail and 1 or -1
   for i = self._head, self._tail - inc, inc do
     local v = self._table[i]
-    self._table[i] = { i, v }
+    self._table[i] = pack(i, v)
   end
   return self
 end

test/functional/lua/vim_spec.lua

@@ -3381,6 +3381,33 @@ describe('lua stdlib', function()
         end
       end)
       eq({ A = 2, C = 6 }, it:totable())
+
+    it('handles table values mid-pipeline', function()
+      local map = {
+        item = {
+          file = 'test',
+        },
+        item_2 = {
+          file = 'test',
+        },
+        item_3 = {
+          file = 'test',
+        },
+      }
+
+      local output = vim.iter(map):map(function(key, value)
+        return { [key] = value.file }
+      end):totable()
+
+      table.sort(output, function(a, b)
+        return next(a) < next(b)
+      end)
+
+      eq({
+        { item = 'test' },
+        { item_2 = 'test' },
+        { item_3 = 'test' },
+      }, output)
     end)
   end)
 end)