New rule: a literal in the condition

spec/ameba/rules/literal_in_condition_spec.cr

@@ -0,0 +1,72 @@
+require "../../spec_helper"
+
+module Ameba::Rules
+  subject = LiteralInCondition.new
+
+  describe LiteralInCondition do
+    it "passes if there is not literals in conditional" do
+      s = Source.new %(
+        if a == 2
+          :ok
+        end
+
+        :ok unless b
+
+        case string
+        when "a"
+          :ok
+        when "b"
+          :ok
+        end
+
+        unless a.nil?
+          :ok
+        end
+      )
+      subject.catch(s).valid?.should be_true
+    end
+
+    it "fails if there is a predicate in if conditional" do
+      s = Source.new %(
+        if "string"
+          :ok
+        end
+      )
+      subject.catch(s).valid?.should be_false
+    end
+
+    it "fails if there is a predicate in unless conditional" do
+      s = Source.new %(
+        unless true
+          :ok
+        end
+      )
+      subject.catch(s).valid?.should be_false
+    end
+
+    it "fails if there is a predicate in case conditional" do
+      s = Source.new %(
+        case [1, 2, 3]
+        when :array
+          :ok
+        when :not_array
+          :also_ok
+        end
+      )
+      subject.catch(s).valid?.should be_false
+    end
+
+    it "reports rule, pos and message" do
+      s = Source.new %(
+        puts "hello" if true
+      )
+      subject.catch(s)
+
+      s.errors.size.should eq 1
+      error = s.errors.first
+      error.rule.should_not be_nil
+      error.pos.should eq 2
+      error.message.should eq "Literal value found in conditional"
+    end
+  end
+end

src/ameba/ast_traverse.cr

@@ -2,8 +2,10 @@ require "compiler/crystal/syntax/*"
 
 module Ameba
   NODE_VISITORS = [
-    Unless,
     Call,
+    Case,
+    If,
+    Unless,
   ]
 
   {% for name in NODE_VISITORS %}

src/ameba/rules/literal_in_condition.cr

@@ -0,0 +1,44 @@
+module Ameba::Rules
+  # A rule that disallows useless conditional statements that contain a literal
+  # in place of a variable or predicate function.
+  #
+  # This is because a conditional construct with a literal predicate will
+  # always result in the same behaviour at run time, meaning it can be
+  # replaced with either the body of the construct, or deleted entirely.
+  #
+  # This is considered invalid:
+  # ```
+  # if "something"
+  #   :ok
+  # end
+  # ```
+  struct LiteralInCondition < Rule
+    def test(source)
+      IfVisitor.new self, source
+      UnlessVisitor.new self, source
+      CaseVisitor.new self, source
+    end
+
+    def check_node(source, node)
+      return unless literal?(node.cond)
+      source.error self, node.location.try &.line_number,
+        "Literal value found in conditional"
+    end
+
+    def test(source, node : Crystal::If)
+      check_node source, node
+    end
+
+    def test(source, node : Crystal::Unless)
+      check_node source, node
+    end
+
+    def test(source, node : Crystal::Case)
+      check_node source, node
+    end
+
+    private def literal?(node)
+      node.try &.class.name.ends_with? "Literal"
+    end
+  end
+end