Emit a performance warning when OpenStruct is used (#56)

The OpenStruct documentation clearly state that it shouldn't
be used when performance is expected.

Ruby 3.3 introduce a new category of warnings that is silenced
by default: performance.

The expected use case is to enable this warning when looking
for potential performance issues within an application.

As such I think it would make sense to emit a performance warning
when OpenStruct is used, as it may help pinpoint that a dependency
rely on it, etc.

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

lib/ostruct.rb

@@ -109,6 +109,14 @@
 class OpenStruct
   VERSION = "0.5.5"
 
+  HAS_PERFORMANCE_WARNINGS = begin
+    Warning[:performance]
+    true
+  rescue NoMethodError, ArgumentError
+    false
+  end
+  private_constant :HAS_PERFORMANCE_WARNINGS
+
   #
   # Creates a new OpenStruct object.  By default, the resulting OpenStruct
   # object will have no attributes.
@@ -124,6 +132,10 @@ class OpenStruct
   #   data   # => #<OpenStruct country="Australia", capital="Canberra">
   #
   def initialize(hash=nil)
+    if HAS_PERFORMANCE_WARNINGS && Warning[:performance]
+       warn "OpenStruct use is discouraged for performance reasons", uplevel: 1, category: :performance
+    end
+
     if hash
       update_to_values!(hash)
     else

ostruct.gemspec

@@ -24,4 +24,5 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"
+  spec.add_development_dependency "test-unit"
 end

test/ostruct/test_ostruct.rb

@@ -412,4 +412,23 @@ def test_class
     assert_equal('my-class', os.class)
     assert_equal(OpenStruct, os.class!)
   end
+
+  has_performance_warnings = begin
+    Warning[:performance]
+    true
+  rescue NoMethodError, ArgumentError
+    false
+  end
+
+  if has_performance_warnings
+    def test_performance_warning
+      assert_in_out_err(
+        %w(-Ilib -rostruct -w -W:performance -e) + ['OpenStruct.new(a: 1)'],
+        "",
+        [],
+        ["-e:1: warning: OpenStruct use is discouraged for performance reasons"],
+        success: true,
+      )
+    end
+  end
 end