Shadowing via short instance variable syntax

closes #122

spec/ameba/rule/lint/shadowing_local_outer_var_spec.cr

@@ -151,6 +151,16 @@ module Ameba::Rule::Lint
       subject.catch(source).should be_valid
     end
 
+    it "does not report if argument shadows an ivar assignment" do
+      s = Source.new %(
+        def bar(@foo)
+          @foo.try do |foo|
+          end
+        end
+      )
+      subject.catch(s).should be_valid
+    end
+
     it "reports rule, location and message" do
       source = Source.new %(
         foo = 1

src/ameba/ast/scope.cr

@@ -10,6 +10,9 @@ module Ameba::AST
     # Link to the arguments in current scope
     getter arguments = [] of Argument
 
+    # Link to the instance variables used in current scope
+    getter ivariables = [] of InstanceVariable
+
     # Link to the outer scope
     getter outer_scope : Scope?
 
@@ -44,11 +47,27 @@ module Ameba::AST
       variables << Variable.new(node, self)
     end
 
+    # Creates a new argument in the current scope.
+    #
+    # ```
+    # scope = Scope.new(class_node, nil)
+    # scope.add_argument(arg_node)
+    # ```
     def add_argument(node)
       add_variable Crystal::Var.new(node.name).at(node)
       arguments << Argument.new(node, variables.last)
     end
 
+    # Adds a new instance variable to the current scope.
+    #
+    # ```
+    # scope = Scope.new(class_node, nil)
+    # scope.add_ivariable(ivar_node)
+    # ```
+    def add_ivariable(node)
+      ivariables << InstanceVariable.new(node)
+    end
+
     # Returns variable by its name or nil if it does not exist.
     #
     # ```
@@ -75,6 +94,12 @@ module Ameba::AST
       node.is_a?(Crystal::Block) || node.is_a?(Crystal::ProcLiteral)
     end
 
+    # Returns true instance variable assinged in this scope.
+    def assigns_ivar?(name)
+      arguments.find { |arg| arg.name == name } &&
+        ivariables.find { |var| var.name == "@#{name}" }
+    end
+
     # Returns true if and only if current scope represents some
     # type definition, for example a class.
     def type_definition?

src/ameba/ast/variabling/ivariable.cr

@@ -0,0 +1,13 @@
+module Ameba::AST
+  class InstanceVariable
+    getter node : Crystal::InstanceVar
+
+    delegate location, to: @node
+    delegate end_location, to: @node
+    delegate name, to: @node
+    delegate to_s, to: @node
+
+    def initialize(@node)
+    end
+  end
+end

src/ameba/ast/visitors/scope_visitor.cr

@@ -135,6 +135,11 @@ module Ameba::AST
       @current_scope.add_argument node
     end
 
+    # :nodoc:
+    def visit(node : Crystal::InstanceVar)
+      @current_scope.add_ivariable(node)
+    end
+
     # :nodoc:
     def visit(node : Crystal::Var)
       variable = @current_scope.find_variable node.name

src/ameba/rule/lint/shadowing_local_outer_var.cr

@@ -54,7 +54,8 @@ module Ameba::Rule::Lint
     private def find_shadowing(source, scope)
       scope.arguments.each do |arg|
         next if arg.ignored?
-        if scope.outer_scope.try &.find_variable(arg.name)
+        outer_scope = scope.outer_scope
+        if outer_scope && outer_scope.find_variable(arg.name) && !outer_scope.assigns_ivar?(arg.name)
           issue_for arg.node, MSG % arg.name
         end
       end