Lint/UselessAssign: ignore transformed assignemnts by the compiler

closes #127

spec/ameba/ast/variabling/assignment_spec.cr

@@ -87,5 +87,32 @@ module Ameba::AST
         assignment.branch.should be_nil
       end
     end
+
+    describe "#transformed?" do
+      it "returns false if the assignment is not transformed by the compiler" do
+        nodes = as_nodes %(
+          def method(a)
+            a = 2
+          end
+        )
+
+        scope = Scope.new nodes.def_nodes.first
+        variable = Variable.new(nodes.var_nodes.first, scope)
+        assignment = Assignment.new(nodes.assign_nodes.first, variable)
+        assignment.transformed?.should be_false
+      end
+
+      it "returns true if the assignment is transformed by the compiler" do
+        nodes = as_nodes %(
+          array.each do |(a, b)|
+          end
+        )
+
+        scope = Scope.new nodes.block_nodes.first
+        variable = Variable.new(nodes.var_nodes.first, scope)
+        assignment = Assignment.new(nodes.assign_nodes.first, variable)
+        assignment.transformed?.should be_true
+      end
+    end
   end
 end

spec/ameba/rule/lint/useless_assign_spec.cr

@@ -225,6 +225,34 @@ module Ameba::Rule::Lint
       subject.catch(s).should be_valid
     end
 
+    context "when transformed" do
+      it "does not report if the first arg is transformed and not used" do
+        s = Source.new %(
+          collection.each do |(a, b)|
+            puts b
+          end
+        )
+        subject.catch(s).should be_valid
+      end
+
+      it "does not report if the second arg is transformed and not used" do
+        s = Source.new %(
+          collection.each do |(a, b)|
+            puts a
+          end
+        )
+        subject.catch(s).should be_valid
+      end
+
+      it "does not report if all transformed args are not used in a block" do
+        s = Source.new %(
+          collection.each do |(foo, bar), (baz, _qux), index, object|
+          end
+        )
+        subject.catch(s).should be_valid
+      end
+    end
+
     it "does not report if global var" do
       s = Source.new %(
         def method