State in generic 'for' acts as a to-be-closed variable

The implicit variable 'state' in a generic 'for' is marked as a
to-be-closed variable, so that the state will be closed as soon
as the loop ends, no matter how.

Taking advantage of this new facility, the call 'io.lines(filename)'
now returns the open file as a second result. Therefore,
an iteraction like 'for l in io.lines(name)...' will close the
file even when the loop ends with a break or an error.

testes/files.lua

@@ -462,13 +462,13 @@ X
 -                                   y;
 ]]:close()
 _G.X = 1
-assert(not load(io.lines(file)))
+assert(not load((io.lines(file))))
 collectgarbage()   -- to close file in previous iteration
-load(io.lines(file, "L"))()
+load((io.lines(file, "L")))()
 assert(_G.X == 2)
-load(io.lines(file, 1))()
+load((io.lines(file, 1)))()
 assert(_G.X == 4)
-load(io.lines(file, 3))()
+load((io.lines(file, 3)))()
 assert(_G.X == 8)
 
 print('+')

testes/locals.lua

@@ -108,7 +108,7 @@ print'+'
 if rawget(_G, "T") then
   -- testing clearing of dead elements from tables
   collectgarbage("stop")   -- stop GC
-  local a = {[{}] = 4, [3] = 0, alo = 1, 
+  local a = {[{}] = 4, [3] = 0, alo = 1,
              a1234567890123456789012345678901234567890 = 10}
 
   local t = T.querytab(a)
@@ -360,6 +360,54 @@ end)
 co()                 -- start coroutine
 assert(co == nil)    -- eventually it will be collected
 
+
+-- to-be-closed variables in generic for loops
+do
+  local numopen = 0
+  local function open (x)
+    numopen = numopen + 1
+    return
+      function ()   -- iteraction function
+        x = x - 1
+        if x > 0 then return x end
+      end,
+      function ()   -- closing function
+        numopen = numopen - 1
+      end
+  end
+
+  local s = 0
+  for i in open(10) do
+     s = s + i
+  end
+  assert(s == 45 and numopen == 0)
+
+  local s = 0
+  for i in open(10) do
+     if i < 5 then break end
+     s = s + i
+  end
+  assert(s == 35 and numopen == 0)
+
+  -- repeat test with '__open' metamethod instead of a function
+  local function open (x)
+    numopen = numopen + 1
+    return
+      function (t)   -- iteraction function
+        t[1] = t[1] - 1
+        if t[1] > 0 then return t[1] end
+      end,
+      setmetatable({x}, {__close = function () numopen = numopen - 1 end})
+  end
+
+  local s = 0
+  for i in open(10) do
+     if (i < 5) then break end
+     s = s + i
+  end
+  assert(s == 35 and numopen == 0)
+end
+
 print('OK')
 
 return 5,f