Circular references shouldn't mark a variable as being accessed

Closes lunarmodules#33

spec/cli_spec.lua

@@ -191,8 +191,9 @@ Checking spec/samples/bad_code.lua                5 warnings
     spec/samples/bad_code.lua:8:10: variable 'opt' was previously defined as an argument on line 7
     spec/samples/bad_code.lua:9:11: accessing undefined variable 'hepler'
 
-Checking spec/samples/unused_code.lua             9 warnings
+Checking spec/samples/unused_code.lua             10 warnings
 
+    spec/samples/unused_code.lua:1:7: variable 'foo' is mutated but never accessed
     spec/samples/unused_code.lua:3:18: unused argument 'baz'
     spec/samples/unused_code.lua:4:8: unused loop variable 'i'
     spec/samples/unused_code.lua:5:13: unused variable 'q'
@@ -203,7 +204,7 @@ Checking spec/samples/unused_code.lua             9 warnings
     spec/samples/unused_code.lua:14:1: value assigned to variable 'x' is overwritten on line 15 before use
     spec/samples/unused_code.lua:21:7: variable 'z' is never accessed
 
-Total: 14 warnings / 0 errors in 3 files
+Total: 15 warnings / 0 errors in 3 files
 ]], get_output "-q spec/samples/bad_code.lua spec/samples/good_code.lua spec/samples/unused_code.lua --no-config")
       assert.equal([[
 Total: 0 warnings / 0 errors in 1 file
@@ -213,14 +214,14 @@ Total: 0 warnings / 0 errors in 1 file
    it("suppresses warnings output with -qq", function()
       assert.equal([[
 Checking spec/samples/bad_code.lua                5 warnings
-Checking spec/samples/unused_code.lua             9 warnings
+Checking spec/samples/unused_code.lua             10 warnings
 
-Total: 14 warnings / 0 errors in 3 files
+Total: 15 warnings / 0 errors in 3 files
 ]], get_output "-qq spec/samples/bad_code.lua spec/samples/good_code.lua spec/samples/unused_code.lua --no-config")
    end)
 
    it("suppresses file info output with -qqq", function()
-      assert.equal([[Total: 14 warnings / 0 errors in 3 files
+      assert.equal([[Total: 15 warnings / 0 errors in 3 files
 ]], get_output "-qqq spec/samples/bad_code.lua spec/samples/good_code.lua spec/samples/unused_code.lua --no-config")
    end)
 
@@ -318,8 +319,9 @@ Total: 1 warning / 0 errors in 1 file
 
    it("recognizes different types of variables", function()
       assert.equal([[
-Checking spec/samples/unused_code.lua             9 warnings
+Checking spec/samples/unused_code.lua             10 warnings
 
+    spec/samples/unused_code.lua:1:7: variable 'foo' is mutated but never accessed
     spec/samples/unused_code.lua:3:18: unused argument 'baz'
     spec/samples/unused_code.lua:4:8: unused loop variable 'i'
     spec/samples/unused_code.lua:5:13: unused variable 'q'
@@ -330,20 +332,21 @@ Checking spec/samples/unused_code.lua             9 warnings
     spec/samples/unused_code.lua:14:1: value assigned to variable 'x' is overwritten on line 15 before use
     spec/samples/unused_code.lua:21:7: variable 'z' is never accessed
 
-Total: 9 warnings / 0 errors in 1 file
+Total: 10 warnings / 0 errors in 1 file
 ]], get_output "spec/samples/unused_code.lua --no-config")
    end)
 
    it("allows to ignore unused arguments", function()
       assert.equal([[
-Checking spec/samples/unused_code.lua             4 warnings
+Checking spec/samples/unused_code.lua             5 warnings
 
+    spec/samples/unused_code.lua:1:7: variable 'foo' is mutated but never accessed
     spec/samples/unused_code.lua:5:13: unused variable 'q'
     spec/samples/unused_code.lua:13:7: value assigned to variable 'x' is overwritten on line 14 before use
     spec/samples/unused_code.lua:14:1: value assigned to variable 'x' is overwritten on line 15 before use
     spec/samples/unused_code.lua:21:7: variable 'z' is never accessed
 
-Total: 4 warnings / 0 errors in 1 file
+Total: 5 warnings / 0 errors in 1 file
 ]], get_output "spec/samples/unused_code.lua --no-unused-args --no-config")
    end)
 
@@ -1190,7 +1193,7 @@ Total: 5 warnings / 0 errors in 2 files
 
          it("allows using cli-specific options in top level config", function()
             assert.equal([[Files: 2
-Warnings: 14
+Warnings: 15
 Errors: 0
 Quiet: 0
 Color: false
@@ -1213,12 +1216,12 @@ Checking spec/samples/read_globals.lua            5 warnings
 Checking spec/samples/read_globals_inline_options.lua 3 warnings
 Checking spec/samples/redefined.lua               7 warnings
 Checking spec/samples/reversed_fornum.lua         1 warning
-Checking spec/samples/unused_code.lua             9 warnings
+Checking spec/samples/unused_code.lua             10 warnings
 Checking spec/samples/unused_secondaries.lua      4 warnings
 Checking spec/samples/utf8.lua                    4 warnings
 Checking spec/samples/utf8_error.lua              1 error
 
-Total: 72 warnings / 5 errors in 19 files
+Total: 73 warnings / 5 errors in 19 files
 ]]):gsub("(spec/samples)/", "%1"..package.config:sub(1, 1)),
             get_output "spec/samples --config=spec/configs/exclude_files_config.luacheckrc -qq --exclude-files spec/samples/global_fields.lua")
          end)
@@ -1238,12 +1241,12 @@ Checking read_globals.lua                         5 warnings
 Checking read_globals_inline_options.lua          3 warnings
 Checking redefined.lua                            7 warnings
 Checking reversed_fornum.lua                      1 warning
-Checking unused_code.lua                          9 warnings
+Checking unused_code.lua                          10 warnings
 Checking unused_secondaries.lua                   4 warnings
 Checking utf8.lua                                 4 warnings
 Checking utf8_error.lua                           1 error
 
-Total: 72 warnings / 5 errors in 19 files
+Total: 73 warnings / 5 errors in 19 files
 ]], get_output(". --config=spec/configs/exclude_files_config.luacheckrc -qq --exclude-files global_fields.lua", "spec/samples/"))
          end)
 
@@ -1260,12 +1263,12 @@ Checking line_length.lua                          8 warnings
 Checking python_code.lua                          1 error
 Checking redefined.lua                            7 warnings
 Checking reversed_fornum.lua                      1 warning
-Checking unused_code.lua                          9 warnings
+Checking unused_code.lua                          10 warnings
 Checking unused_secondaries.lua                   4 warnings
 Checking utf8.lua                                 4 warnings
 Checking utf8_error.lua                           1 error
 
-Total: 64 warnings / 5 errors in 17 files
+Total: 65 warnings / 5 errors in 17 files
 ]], get_output(". --config=spec/configs/exclude_files_config.luacheckrc -qq --exclude-files global_fields.lua --exclude-files " .. quote("./read*"), "spec/samples/"))
          end)
 

spec/samples/bad_code.lua

@@ -8,4 +8,3 @@ function embrace(opt)
    local opt = opt or "default"
    return hepler(opt.."?")
 end
-

spec/samples/unused_code.lua

@@ -21,3 +21,18 @@ y = 6
 local z = 5;
 (function() z = 4 end)()
 z = 6
+
+foo[foo] = 1
+foo[1] = foo
+foo[foo] = foo
+function foo.meta()
+   return function() print(foo) end
+end
+
+local t = {}
+function t.func() print(t) return {val = 1} end
+t[t] = t.func().val + 1
+
+local s = {}
+function s:func() print(self) return {val = 1} end
+s[s] = s:func().val + 1

src/luacheck/stages/resolve_locals.lua

@@ -78,6 +78,46 @@ local function in_scope(var, index)
    return (var.scope_start <= index) and (index <= var.scope_end)
 end
 
+local function contains_call(node)
+   if node.tag == "Call" or node.tag == "Invoke" then
+      return true
+   end
+
+   if node.tag ~= "Function" then
+      for _, sub_node in ipairs(node) do
+         if type(sub_node) == 'table' and contains_call(sub_node) then
+            return true
+         end
+      end
+   end
+
+   return false
+end
+
+local function is_circular_reference(item, var)
+   -- No support for matching multiple assignment to the specific assignment
+   if not item.lhs or #item.lhs ~= 1 or not item.rhs or #item.rhs ~= 1 then
+      return false
+   end
+
+   -- Case t[t.function()] = t.func()
+   -- Functions can have side-effects, so this isn't purely circular
+   local right_assignment = item.rhs[1]
+   if contains_call(right_assignment) then
+      return false
+   end
+
+   local left_assignment = item.lhs[1]
+   if contains_call(left_assignment) then
+      return false
+   end
+   local node = left_assignment[1]
+   if node.var == var then
+      return true
+   end
+   return false
+end
+
 -- Called when main assignment propagation reaches a line item.
 local function main_assignment_propagation_callback(line, index, item, var, value)
    -- Check entrance condition.
@@ -91,7 +131,9 @@ local function main_assignment_propagation_callback(line, index, item, var, valu
 
    -- Accesses (and mutations) of the variable can resolve to reaching assignment.
    if item.accesses and item.accesses[var] then
-      add_resolution(line, item, var, value)
+      if not is_circular_reference(item, var) then
+         add_resolution(line, item, var, value)
+      end
    end
 
    if item.mutations and item.mutations[var] then
@@ -102,7 +144,9 @@ local function main_assignment_propagation_callback(line, index, item, var, valu
    -- can resolve to reaching assignment.
    if item.lines then
       for _, created_line in ipairs(item.lines) do
-         add_resolutions(created_line, created_line.accessed_upvalues[var], var, value)
+         if not is_circular_reference(item, var) then
+            add_resolutions(created_line, created_line.accessed_upvalues[var], var, value)
+         end
          add_resolutions(created_line, created_line.mutated_upvalues[var], var, value, true)
       end
    end