Add Lint/NotNil rule

spec/ameba/rule/lint/not_nil_spec.cr

@@ -0,0 +1,42 @@
+require "../../../spec_helper"
+
+module Ameba::Rule::Lint
+  subject = NotNil.new
+
+  describe NotNil do
+    it "passes for valid cases" do
+      expect_no_issues subject, <<-CRYSTAL
+        (1..3).first?.not_nil!(:foo)
+        not_nil!
+        CRYSTAL
+    end
+
+    it "reports if there is a `not_nil!` call" do
+      expect_issue subject, <<-CRYSTAL
+        (1..3).first?.not_nil!
+                    # ^^^^^^^^ error: Avoid using `not_nil!`
+        CRYSTAL
+    end
+
+    context "macro" do
+      it "doesn't report in macro scope" do
+        expect_no_issues subject, <<-CRYSTAL
+          {{ [1, 2, 3].first.not_nil! }}
+          CRYSTAL
+      end
+    end
+
+    it "reports rule, pos and message" do
+      s = Source.new %(
+        (1..3).first?.not_nil!
+      ), "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:1:22"
+      issue.message.should eq "Avoid using `not_nil!`"
+    end
+  end
+end