Add Lint/UnusedClassVariableAccess

spec/ameba/rule/lint/unused_class_variable_access_spec.cr

@@ -0,0 +1,46 @@
+require "../../../spec_helper"
+
+module Ameba::Rule::Lint
+  describe UnusedClassVariableAccess do
+    subject = UnusedClassVariableAccess.new
+
+    it "passes if class variables are used for assignment" do
+      expect_no_issues subject, <<-CRYSTAL
+        class MyClass
+          a = @@ivar
+          B = @@ivar
+        end
+        CRYSTAL
+    end
+
+    it "passes if an class variable is used as a target in multi-assignment" do
+      expect_no_issues subject, <<-CRYSTAL
+        class MyClass
+          @@a, @@b = 1, 2
+        end
+        CRYSTAL
+    end
+
+    it "fails if class variables are unused in void context of class" do
+      expect_issue subject, <<-CRYSTAL
+        class Actor
+          @@name : String = "George"
+
+          @@name
+        # ^^^^^^ error: Value from class variable access is unused
+        end
+        CRYSTAL
+    end
+
+    it "fails if class variables are unused in void context of method" do
+      expect_issue subject, <<-CRYSTAL
+        def hello : String
+          @@name
+        # ^^^^^^ error: Value from class variable access is unused
+
+          "Hello, \#{@@name}!"
+        end
+        CRYSTAL
+    end
+  end
+end

src/ameba/rule/lint/unused_class_variable_access.cr

@@ -0,0 +1,58 @@
+module Ameba::Rule::Lint
+  # A rule that disallows unused class variable access.
+  #
+  # For example, this is considered invalid:
+  #
+  # ```
+  # class MyClass
+  #   @@my_var : String = "hello"
+  #
+  #   @@my_var
+  #
+  #   def hello : String
+  #     @@my_var
+  #
+  #     "hello, world!"
+  #   end
+  # end
+  # ```
+  #
+  # And these are considered valid:
+  #
+  # ```
+  # class MyClass
+  #   @@my_var : String = "hello"
+  #
+  #   @@my_other_var = @@my_var
+  #
+  #   def hello : String
+  #     return @@my_var if @@my_var == "hello"
+  #
+  #     "hello, world!"
+  #   end
+  # end
+  # ```
+  #
+  # YAML configuration example:
+  #
+  # ```
+  # Lint/UnusedClassVariableAccess:
+  #   Enabled: true
+  # ```
+  class UnusedClassVariableAccess < Base
+    properties do
+      since_version "1.7.0"
+      description "Disallows unused access to class variables"
+    end
+
+    MSG = "Value from class variable access is unused"
+
+    def test(source : Source)
+      AST::ImplicitReturnVisitor.new(self, source)
+    end
+
+    def test(source, node : Crystal::ClassVar, in_macro : Bool)
+      issue_for node, MSG
+    end
+  end
+end