feat: add check for modified constant variables (#116)

docsrc/warnings.rst

@@ -44,6 +44,7 @@ Code Description
 431  Shadowing an upvalue.
 432  Shadowing an upvalue argument.
 433  Shadowing an upvalue loop variable.
+441  Constant local variable is modified.
 511  Unreachable code.
 512  Loop can be executed at most once.
 521  Unused label.

spec/linearize_spec.lua

@@ -404,3 +404,78 @@ g, a = f()]]))
       end)
    end)
 end)
+
+describe("constant modification detection", function()
+   it("detects modified constants", function()
+      assert.same({
+         {code = "441", line = 4, column = 1, end_column = 17, defined_line = 1, name = 'b'}
+      }, helper.get_stage_warnings("linearize", [[
+local a, b <const>, c = 1, 1, 1
+print(a, b, c)
+
+a, b, c = 2, 2, 2
+print(a, b, c)
+]]))
+   end)
+
+   it("detects a constant overwritten by a function", function()
+      assert.same({
+         {code = "441", line = 4, column = 1, end_column = 16, defined_line = 1, name = 'a'}
+      }, helper.get_stage_warnings("linearize", [[
+local a <const> = 1
+print(a)
+
+function a() end
+a()
+]]))
+   end)
+
+   it("doesn't trigger on a constant redefinition", function()
+      assert.same({
+         {code = "411", line = 4, column = 7, end_column = 7, name = 'a',
+            prev_column = 7, prev_end_column = 7, prev_line = 1}
+      }, helper.get_stage_warnings("linearize", [[
+local a <const> = 1
+print(a)
+
+local a = 1
+print(a)
+]]))
+   end)
+
+   it("doesn't trigger on a loop overriding a constant", function()
+      assert.same({
+         {code = "421", line = 4, column = 5, end_column = 5, name = 'a',
+            prev_column = 7, prev_end_column = 7, prev_line = 1}
+      }, helper.get_stage_warnings("linearize", [[
+local a <const> = 1
+print(a)
+
+for a = 1,10 do
+   print(a)
+end
+]]))
+   end)
+
+   it("handles variable scoping correctly", function()
+      assert.same({
+         {code = "421", line = 5, column = 10, end_column = 10, name = 'a',
+            prev_column = 7, prev_end_column = 7, prev_line = 1},
+         {code = "421", line = 5, column = 13, end_column = 13, name = 'b',
+            prev_column = 18, prev_end_column = 18, prev_line = 1},
+         {code = "441", line = 9, column = 1, end_column = 11, name = 'a',
+            defined_line = 1}
+      }, helper.get_stage_warnings("linearize", [[
+local a <const>, b = 1, 1
+print(a, b)
+
+do
+   local a, b <const> = 2, 2
+   print(a, b)
+end
+
+a, b = 3, 3
+print(a, b)
+]]))
+   end)
+end)

spec/parser_spec.lua

@@ -492,14 +492,14 @@ describe("parser", function()
          )
       end)
 
-      it("accepts (and ignores for now) Lua 5.4 attributes", function()
+      it("accepts Lua 5.4 attributes, ignoring close and parsing const", function()
          assert.same({tag = "Local", {
                            {tag = "Id", "a"}
                         }
                      }, get_node("local a <close>"))
          assert.same({tag = "Local", {
                            {tag = "Id", "a"},
-                           {tag = "Id", "b"}
+                           {tag = "Id", const = true, "b"}
                         }
                      }, get_node("local a <close>, b <const>"))
          assert.same({
@@ -512,7 +512,7 @@ describe("parser", function()
          assert.same({
             tag = "Local", {
                {tag = "Id", "a"},
-               {tag = "Id", "b"}
+               {tag = "Id", const = true, "b"}
          }, {
                {tag = "Id", "c"},
                {tag = "Id", "d"}
@@ -522,6 +522,14 @@ describe("parser", function()
             {line = 1, offset = 16, end_offset = 16, msg = "expected '>' near '='"},
             get_error("local a <close = ")
          )
+         assert.same(
+            {line = 1, offset = 10, end_offset = 13, msg = "Unknown attribute 'cons'"},
+            get_error("local a <cons = ")
+         )
+         assert.same(
+            {line = 1, offset = 18, end_offset = 18, msg = "expected '(' near '<'"},
+            get_error("local function y <const> () end")
+         )
       end)
 
       it("parses local declaration with assignment correctly", function()

src/luacheck/parser.lua

@@ -821,10 +821,16 @@ statements["local"] = function(state)
       lhs[#lhs + 1] = parse_id(state)
 
       -- Check if a Lua 5.4 attribute is present
+      -- TODO: Warn on different syntax between lua versions?
       if state.token == "<" then
-         -- For now, just consume and ignore it.
          skip_token(state)
-         check_name(state)
+         local attribute = check_name(state)
+         if attribute == "const" then
+            lhs[#lhs].const = true
+         -- Accept but ignore close
+         elseif attribute ~= "close" then
+            parser.syntax_error("Unknown attribute '" .. attribute .. "'", state)
+         end
          skip_token(state)
          check_and_skip_token(state, ">")
       end

src/luacheck/stages/linearize.lua

@@ -20,6 +20,10 @@ stage.warnings = {
    ["431"] = redefined_warning("shadowing upvalue {name!} on line {prev_line}"),
    ["432"] = redefined_warning("shadowing upvalue argument {name!} on line {prev_line}"),
    ["433"] = redefined_warning("shadowing upvalue loop variable {name!} on line {prev_line}"),
+   ["441"] = {
+      message_format = "variable {name!} was defined as const on line {defined_line}",
+      fields = {"name", "defined_line"}
+   },
    ["521"] = {message_format = "unused label {label!}", fields = {"label"}}
 }
 
@@ -42,6 +46,13 @@ local function warn_redefined(chstate, var, prev_var, is_same_scope)
    })
 end
 
+local function warn_modified_const_label(chstate, node, var)
+   chstate:warn_range("441", node, {
+      defined_line = var.node.line,
+      name = var.name
+   })
+end
+
 local function warn_unused_label(chstate, label)
    chstate:warn_range("521", label.range, {
       label = label.name
@@ -518,6 +529,10 @@ function LinState:emit_stmt_Set(node)
 
          if var then
             self:register_upvalue_action(item, var, "set_upvalues")
+
+            if var.node.const then
+               warn_modified_const_label(self.chstate, node, var)
+            end
          end
       else
          assert(expr.tag == "Index")