First implementation for 'const' variables

A variable can be declared const, which means it cannot be assigned to,
with the syntax 'local <const> name = exp'.

testes/constructs.lua

@@ -59,6 +59,41 @@ assert((x>y) and x or y == 2);
 
 assert(1234567890 == tonumber('1234567890') and 1234567890+1 == 1234567891)
 
+do   -- testing operators with diffent kinds of constants
+  -- operands to consider:
+  --  * fit in register
+  --  * constant doesn't fit in register
+  --  * floats with integral values
+  local operand = {3, 100, 5.0, -10, -5.0, 10000, -10000}
+  local operator = {"+", "-", "*", "/", "//", "%", "^",
+                    "&", "|", "^", "<<", ">>",
+                    "==", "~=", "<", ">", "<=", ">=",}
+  for _, op in ipairs(operator) do
+    local f = assert(load(string.format([[return function (x,y)
+                return x %s y
+              end]], op)))();
+    for _, o1 in ipairs(operand) do
+      for _, o2 in ipairs(operand) do
+        local gab = f(o1, o2)
+
+        _ENV.XX = o1
+        code = string.format("return XX %s %s", op, o2)
+        res = assert(load(code))()
+        assert(res == gab)
+
+        _ENV.XX = o2
+        local code = string.format("return (%s) %s XX", o1, op)
+        local res = assert(load(code))()
+        assert(res == gab)
+
+        code = string.format("return (%s) %s %s", o1, op, o2)
+        res = assert(load(code))()
+        assert(res == gab)
+      end
+    end
+  end
+end
+
 
 -- silly loops
 repeat until 1; repeat until true;
@@ -175,6 +210,28 @@ assert(a==1 and b==nil)
 
 print'+';
 
+do   -- testing constants
+  local <const> prog = [[local <XXX> x = 10]]
+  checkload(prog, "unknown attribute 'XXX'")
+
+  checkload([[local <const> xxx = 20; xxx = 10]],
+             ":1: assignment to const variable 'xxx'")
+
+  checkload([[
+    local xx;
+    local <const> xxx = 20;
+    local yyy;
+    local function foo ()
+      local abc = xx + yyy + xxx;
+      return function () return function () xxx = yyy end end
+    end
+  ]], ":6: assignment to const variable 'xxx'")
+
+  checkload([[
+    local <toclose> x = nil
+    x = io.open()
+  ]], ":2: assignment to const variable 'x'")
+end
 
 f = [[
 return function ( a , b , c , d , e )
@@ -245,12 +302,12 @@ print('testing short-circuit optimizations (' .. _ENV.GLOB1 .. ')')
 
 
 -- operators with their respective values
-local binops = {
+local <const> binops = {
   {" and ", function (a,b) if not a then return a else return b end end},
   {" or ", function (a,b) if a then return a else return b end end},
 }
 
-local cases = {}
+local <const> cases = {}
 
 -- creates all combinations of '(cases[i] op cases[n-i])' plus
 -- 'not(cases[i] op cases[n-i])' (syntax + value)