Useless Assign: ignore variables with '_' prefix

closes #164

spec/ameba/ast/variabling/variable_spec.cr

@@ -140,6 +140,23 @@ module Ameba::AST
       end
     end
 
+    describe "#ignored?" do
+      it "is true if variable is ignored" do
+        variable = Variable.new(Crystal::Var.new("_var"), scope)
+        variable.ignored?.should be_true
+      end
+
+      it "is false if variable is not ignored" do
+        variable = Variable.new(Crystal::Var.new("v_ar"), scope)
+        variable.ignored?.should be_false
+      end
+
+      it "is true if variable is a black hole" do
+        variable = Variable.new(Crystal::Var.new("_"), scope)
+        variable.ignored?.should be_true
+      end
+    end
+
     describe "#eql?" do
       var = Crystal::Var.new("foo").at(Crystal::Location.new(nil, 1, 2))
       variable = Variable.new var, scope

spec/ameba/rule/lint/useless_assign_spec.cr

@@ -54,6 +54,14 @@ module Ameba::Rule::Lint
       subject.catch(s).should_not be_valid
     end
 
+    it "does not report ignored assigments" do
+      s = Source.new %(
+        payload, _header = decode
+        puts payload
+      )
+      subject.catch(s).should be_valid
+    end
+
     it "reports a useless assignment in a proc inside a block" do
       s = Source.new %(
         def method

src/ameba/ast/variabling/variable.cr

@@ -143,6 +143,11 @@ module Ameba::AST
       end
     end
 
+    # Returns true if the name starts with '_', false if not.
+    def ignored?
+      name.starts_with? '_'
+    end
+
     # Returns true if the `node` represents exactly
     # the same Crystal node as `@node`.
     def eql?(node)

src/ameba/rule/lint/useless_assign.cr

@@ -39,7 +39,7 @@ module Ameba::Rule::Lint
 
     def test(source, node, scope : AST::Scope)
       scope.variables.each do |var|
-        next if var.captured_by_block? || var.used_in_macro?
+        next if var.captured_by_block? || var.used_in_macro? || var.ignored?
 
         var.assignments.each do |assign|
           next if assign.referenced? || assign.transformed?