Add Performance/FlattenAfterMap rule

spec/ameba/rule/performance/flatten_after_map_spec.cr

@@ -0,0 +1,43 @@
+require "../../../spec_helper"
+
+module Ameba::Rule::Performance
+  subject = FlattenAfterMap.new
+
+  describe FlattenAfterMap do
+    it "passes if there is no potential performance improvements" do
+      source = Source.new %(
+        %w[Alice Bob].flat_map(&.chars)
+      )
+      subject.catch(source).should be_valid
+    end
+
+    it "reports if there is map followed by flatten call" do
+      source = Source.new %(
+        %w[Alice Bob].map(&.chars).flatten
+      )
+      subject.catch(source).should_not be_valid
+    end
+
+    context "macro" do
+      it "doesn't report in macro scope" do
+        source = Source.new %(
+          {{ %w[Alice Bob].map(&.chars).flatten }}
+        )
+        subject.catch(source).should be_valid
+      end
+    end
+
+    it "reports rule, pos and message" do
+      s = Source.new %(
+        %w[Alice Bob].map(&.chars).flatten
+      ), "source.cr"
+      subject.catch(s).should_not be_valid
+      issue = s.issues.first
+
+      issue.rule.should_not be_nil
+      issue.location.to_s.should eq "source.cr:1:15"
+      issue.end_location.to_s.should eq "source.cr:1:35"
+      issue.message.should eq "Use `flat_map {...}` instead of `map {...}.flatten`"
+    end
+  end
+end