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

src/ameba/rule/lint/not_nil.cr

@@ -0,0 +1,57 @@
+module Ameba::Rule::Lint
+  # This rule is used to identify usages of `not_nil!` calls.
+  #
+  # For example, this is considered a code smell:
+  #
+  # ```
+  # names = %w[Alice Bob]
+  # alice = names.find { |name| name == "Alice" }.not_nil!
+  # ```
+  #
+  # And can be written as this:
+  #
+  # ```
+  # names = %w[Alice Bob]
+  # alice = names.find { |name| name == "Alice" }
+  #
+  # if alice
+  #   # ...
+  # end
+  # ```
+  #
+  # YAML configuration example:
+  #
+  # ```
+  # Lint/NotNil:
+  #   Enabled: true
+  # ```
+  class NotNil < Base
+    include AST::Util
+
+    properties do
+      description "Identifies usage of `not_nil!` calls"
+    end
+
+    NOT_NIL_NAME = "not_nil!"
+    MSG          = "Avoid using `not_nil!`"
+
+    def test(source)
+      AST::NodeVisitor.new self, source, skip: [
+        Crystal::Macro,
+        Crystal::MacroExpression,
+        Crystal::MacroIf,
+        Crystal::MacroFor,
+      ]
+    end
+
+    def test(source, node : Crystal::Call)
+      return unless node.name == NOT_NIL_NAME
+      return unless node.obj && node.args.empty?
+
+      return unless name_location = node.name_location
+      return unless end_location = name_end_location(node)
+
+      issue_for name_location, end_location, MSG
+    end
+  end
+end