New rule: empty ensure

config/ameba.yml

@@ -10,6 +10,10 @@ DebuggerStatement:
   # Disallows calls to debugger.
   Enabled: true
 
+EmptyEnsure:
+  # Disallows empty ensure statement.
+  Enabled: true
+
 EmptyExpression:
   # Disallows empty expressions.
   Enabled: true

spec/ameba/rule/empty_ensure_spec.cr

@@ -0,0 +1,68 @@
+require "../../spec_helper"
+
+module Ameba::Rule
+  describe EmptyEnsure do
+    subject = EmptyEnsure.new
+
+    it "passes if there is no empty ensure blocks" do
+      s = Source.new %(
+        def some_method
+          do_some_stuff
+        ensure
+          do_something_else
+        end
+
+        begin
+          do_some_stuff
+        ensure
+          do_something_else
+        end
+
+        def method_with_rescue
+        rescue
+        ensure
+          nil
+        end
+      )
+      subject.catch(s).should be_valid
+    end
+
+    it "fails if there is an empty ensure in method" do
+      s = Source.new %(
+        def method
+          do_some_stuff
+        ensure
+        end
+      )
+      subject.catch(s).should_not be_valid
+    end
+
+    it "fails if there is an empty ensure in a block" do
+      s = Source.new %(
+        begin
+          do_some_stuff
+        ensure
+          # nothing here
+        end
+      )
+      subject.catch(s).should_not be_valid
+    end
+
+    it "reports rule, pos and message" do
+      s = Source.new %(
+        begin
+          do_some_stuff
+        rescue
+          do_some_other_stuff
+        ensure
+        end
+      ), "source.cr"
+      subject.catch(s).should_not be_valid
+      error = s.errors.first
+
+      error.rule.should_not be_nil
+      error.location.to_s.should eq "source.cr:3:11"
+      error.message.should eq "Empty `ensure` block detected"
+    end
+  end
+end

src/ameba/ast/traverse.cr

@@ -12,6 +12,7 @@ module Ameba::AST
     ClassVar,
     Def,
     EnumDef,
+    ExceptionHandler,
     If,
     InstanceVar,
     LibDef,

src/ameba/rule/empty_ensure.cr

@@ -0,0 +1,47 @@
+module Ameba::Rule
+  # A rule that disallows empty ensure statement.
+  #
+  # For example, this is considered invalid:
+  #
+  # ```
+  # def some_method
+  #   do_some_stuff
+  # ensure
+  # end
+  #
+  # begin
+  #   do_some_stuff
+  # ensure
+  # end
+  # ```
+  #
+  # And it should be written as this:
+  #
+  #
+  # ```
+  # def some_method
+  #   do_some_stuff
+  # ensure
+  #   do_something_else
+  # end
+  #
+  # begin
+  #   do_some_stuff
+  # ensure
+  #   do_something_else
+  # end
+  # ```
+  #
+  struct EmptyEnsure < Base
+    def test(source)
+      AST::Visitor.new self, source
+    end
+
+    def test(source, node : Crystal::ExceptionHandler)
+      node_ensure = node.ensure
+      return if node_ensure.nil? || !node_ensure.nop?
+
+      source.error self, node.location, "Empty `ensure` block detected"
+    end
+  end
+end