Add Style/IsAFilter rule

spec/ameba/rule/style/is_a_filter_spec.cr

@@ -0,0 +1,64 @@
+require "../../../spec_helper"
+
+module Ameba::Rule::Style
+  subject = IsAFilter.new
+
+  describe IsAFilter do
+    it "passes if there is no potential performance improvements" do
+      source = Source.new %(
+        [1, 2, nil].select(Int32)
+        [1, 2, nil].reject(Nil)
+      )
+      subject.catch(source).should be_valid
+    end
+
+    it "reports if there is .is_a? call within select" do
+      source = Source.new %(
+        [1, 2, nil].select(&.is_a?(Int32))
+      )
+      subject.catch(source).should_not be_valid
+    end
+
+    it "reports if there is .nil? call within reject" do
+      source = Source.new %(
+        [1, 2, nil].reject(&.nil?)
+      )
+      subject.catch(source).should_not be_valid
+    end
+
+    context "properties" do
+      it "allows to configure filter_names" do
+        source = Source.new %(
+          [1, 2, nil].reject(&.nil?)
+        )
+        rule = IsAFilter.new
+        rule.filter_names = %w(select)
+        rule.catch(source).should be_valid
+      end
+    end
+
+    context "macro" do
+      it "reports in macro scope" do
+        source = Source.new %(
+          {{ [1, 2, nil].reject(&.nil?) }}
+        )
+        subject.catch(source).should_not be_valid
+      end
+    end
+
+    it "reports rule, pos and message" do
+      source = Source.new path: "source.cr", code: %(
+        [1, 2, nil].reject(&.nil?)
+      )
+      subject.catch(source).should_not be_valid
+      source.issues.size.should eq 1
+
+      issue = source.issues.first
+      issue.rule.should_not be_nil
+      issue.location.to_s.should eq "source.cr:1:13"
+      issue.end_location.to_s.should eq "source.cr:1:26"
+
+      issue.message.should eq "Use `reject(Nil)` instead of `reject {...}`"
+    end
+  end
+end