Raise exception when injecting into modules

To me it happened that I accidentally injected my dependencies into a
namespace module instead of into a class, like this:

  module Operations
    module Articles
      include Import["repositories.articles"]

      class Create
        # ...
      end
    end
  end

When I did that, I got a NoMethodError for NilClass somewhere down the
line. This led me to believe that I've somehow incorrectly passed
parameters to `Import[]`, and it took me a while to realize what was the
error.

The error occurred because the downstream code assumes the #initialize
method will be defined, which is not the case for modules. To improve
the developer experience, we detect that we're attempting to inject into
a module and raise an explicit exception.

lib/dry/auto_inject.rb

@@ -43,4 +43,9 @@ module Dry
   def self.AutoInject(container, options = {})
     AutoInject::Builder.new(container, options)
   end
+
+  module AutoInject
+    class Error < StandardError
+    end
+  end
 end

lib/dry/auto_inject/dependency_map.rb

@@ -1,10 +1,9 @@
 # frozen_string_literal: true
 
+require 'dry/auto_inject/errors'
+
 module Dry
   module AutoInject
-    DuplicateDependencyError = Class.new(StandardError)
-    DependencyNameInvalid = Class.new(StandardError)
-
     VALID_NAME = /([a-z_][a-zA-Z_0-9]*)$/
 
     class DependencyMap

lib/dry/auto_inject/errors.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module Dry
+  module AutoInject
+    Error                    = Class.new(StandardError)
+    DuplicateDependencyError = Class.new(Error)
+    DependencyNameInvalid    = Class.new(Error)
+  end
+end

lib/dry/auto_inject/strategies/constructor.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'dry/auto_inject/dependency_map'
+require 'dry/auto_inject/errors'
 
 module Dry
   module AutoInject
@@ -23,6 +24,8 @@ def initialize(container, *dependency_names)
 
         # @api private
         def included(klass)
+          fail Error, "cannot inject dependencies into a module" unless klass.respond_to?(:new)
+
           define_readers
 
           define_new

spec/dry/auto_inject_spec.rb

@@ -28,6 +28,10 @@ def initialize(*args)
     Class.new(child_class)
   end
 
+  let(:mod) do
+    Module.new
+  end
+
   context 'with positioned args' do
     let(:parent_class) do
       Class.new do
@@ -167,6 +171,14 @@ def initialize(first, *middle, last)
         end
       end
     end
+
+    context 'autoinject in a module' do
+      it 'raises exception' do
+        expect {
+          mod.include Test::AutoInject.args[:one, :two]
+        }.to raise_error(Dry::AutoInject::Error, /cannot inject/)
+      end
+    end
   end
 
   context 'with hash arg' do
@@ -312,6 +324,14 @@ def self.included(klass)
         end
       end
     end
+
+    context 'autoinject in a module' do
+      it 'raises exception' do
+        expect {
+          mod.include Test::AutoInject.hash[:one, :two]
+        }.to raise_error(Dry::AutoInject::Error, /cannot inject/)
+      end
+    end
   end
 
   context 'with keyword args' do
@@ -558,5 +578,13 @@ def initialize(other)
         end
       end
     end
+
+    context 'autoinject in a module' do
+      it 'raises exception' do
+        expect {
+          mod.include Test::AutoInject.kwargs[:one, :two]
+        }.to raise_error(Dry::AutoInject::Error, /cannot inject/)
+      end
+    end
   end
 end