Add ReloadableRequire for hot-reloading files in IRB

Introduce ReloadableRequire module that enables reloading of files
loaded via require/require_relative in IRB sessions. This feature
requires Ruby::Box (Ruby 4.0+) and can be enabled with:

  IRB.conf[:RELOADABLE_REQUIRE] = true

The module tracks loaded Ruby files and allows them to be reloaded
while keeping native extensions loaded normally.

Author: ahogappa

lib/irb/init.rb

@@ -196,6 +196,10 @@ def IRB.init_config(ap_path)
     }
 
     @CONF[:COPY_COMMAND] = ENV.fetch("IRB_COPY_COMMAND", nil)
+
+    @CONF[:RELOADABLE_REQUIRE] = false
+    @CONF[:__RELOADABLE_FILES__] = Set.new
+    @CONF[:__AUTOLOAD_FILES__] = Set.new
   end
 
   def IRB.set_measure_callback(type = nil, arg = nil, &block)

lib/irb/reloadable_require.rb

@@ -0,0 +1,136 @@
+# frozen_string_literal: true
+
+if !defined?(Ruby::Box) || !Ruby::Box.enabled?
+  raise "ReloadableRequire requires Ruby::Box to be enabled"
+end
+
+module IRB
+  # Provides reload-aware require functionality for IRB.
+  #
+  # Limitations:
+  # - Native extensions cannot be reloaded (load doesn't support them)
+  # - Files loaded via box.require are not tracked
+  # - Constant redefinition warnings will appear on reload (uses load internally)
+  # - Context mode 5 (running IRB inside a Ruby::Box) is not supported
+  #
+  # This feature requires Ruby::Box (Ruby 4.0+).
+
+  class << self
+    def track_and_load_files(source, current_box)
+      before = source.dup
+      result = yield
+      new_files = source - before
+
+      return result if new_files.empty?
+
+      ruby_files, native_extensions = new_files.partition { |path| path.end_with?('.rb') }
+
+      native_extensions.each { |path| current_box.require(path) }
+
+      IRB.conf[:__RELOADABLE_FILES__].merge(ruby_files)
+
+      main_loaded_features = current_box.eval('$LOADED_FEATURES')
+      main_loaded_features.concat(ruby_files - main_loaded_features)
+      ruby_files.each { |path| current_box.load(path) }
+
+      result
+    end
+  end
+
+  Ruby::Box.class_eval do
+    alias_method :__irb_original_require__, :require
+    alias_method :__irb_original_require_relative__, :require_relative
+
+    def __irb_reloadable_require__(feature)
+      unless IRB.conf[:__AUTOLOAD_FILES__].include?(feature)
+        return __irb_original_require__(feature)
+      end
+
+      IRB.conf[:__AUTOLOAD_FILES__].delete(feature)
+      IRB.track_and_load_files($LOADED_FEATURES, Ruby::Box.main) { __irb_original_require__(feature) }
+    end
+
+    def __irb_reloadable_require_relative__(feature)
+      __irb_original_require_relative__(feature)
+    end
+  end
+
+  module ReloadableRequire
+    class << self
+      def extended(base)
+        apply_autoload_hook
+      end
+
+      def apply_autoload_hook
+        Ruby::Box.class_eval do
+          alias_method :require, :__irb_reloadable_require__
+          alias_method :require_relative, :__irb_reloadable_require_relative__
+        end
+      end
+    end
+
+    private
+
+    def reloadable_require_internal(absolute_path, caller_box)
+      return false if caller_box.eval('$LOADED_FEATURES').include?(absolute_path)
+
+      box = Ruby::Box.new
+      load_path = caller_box.eval('$LOAD_PATH')
+      # Copy $LOAD_PATH to the box so it can resolve dependencies.
+      box.eval("$LOAD_PATH.concat(#{load_path})")
+
+      IRB.track_and_load_files(box.eval('$LOADED_FEATURES'), caller_box) { box.__irb_original_require__(absolute_path) }
+    end
+
+    def require(feature)
+      caller_loc = caller_locations(1, 1).first
+      current_box = Ruby::Box.main
+      resolved = current_box.eval("$LOAD_PATH.resolve_feature_path(#{feature.dump})")
+
+      # Fallback for calls outside IRB prompt
+      if caller_loc.path != "(irb)" || !resolved || resolved[0] != :rb
+        return current_box.require(feature)
+      end
+
+      reloadable_require_internal(resolved[1], current_box)
+    end
+
+    def require_relative(feature)
+      caller_loc = caller_locations(1, 1).first
+      current_box = Ruby::Box.main
+
+      # Fallback for calls outside IRB prompt
+      if caller_loc.path != "(irb)"
+        file_path = caller_loc.absolute_path || caller_loc.path
+        return current_box.eval("eval('Kernel.require_relative(#{feature.dump})', nil, #{file_path.dump}, #{caller_loc.lineno})")
+      end
+
+      absolute_path = resolve_require_relative_path(feature)
+
+      if !absolute_path.end_with?('.rb') || !File.exist?(absolute_path)
+        file_path = File.join(Dir.pwd, "(irb)")
+        return current_box.eval("eval('Kernel.require_relative(#{feature.dump})', nil, #{file_path.dump}, 1)")
+      end
+
+      reloadable_require_internal(absolute_path, current_box)
+    end
+
+    def resolve_require_relative_path(feature)
+      absolute_path = File.expand_path(feature, Dir.pwd)
+      return absolute_path unless File.extname(absolute_path).empty?
+
+      dlext = RbConfig::CONFIG['DLEXT']
+      ['.rb', ".#{dlext}"].each do |ext|
+        candidate = absolute_path + ext
+        return candidate if File.exist?(candidate)
+      end
+
+      absolute_path
+    end
+
+    def autoload(const, feature)
+      IRB.conf[:__AUTOLOAD_FILES__] << feature
+      Ruby::Box.main.eval("Kernel.autoload(:#{const}, #{feature.dump})")
+    end
+  end
+end

lib/irb/workspace.rb

@@ -5,6 +5,7 @@
 #
 
 require_relative "helper_method"
+require_relative "reloadable_require" if defined?(Ruby::Box) && Ruby::Box.enabled?
 
 IRB::TOPLEVEL_BINDING = binding
 module IRB # :nodoc:
@@ -103,6 +104,10 @@ def load_helper_methods_to_main
       ancestors = class<<main;ancestors;end
       main.extend ExtendCommandBundle if !ancestors.include?(ExtendCommandBundle)
       main.extend HelpersContainer if !ancestors.include?(HelpersContainer)
+
+      if IRB.conf[:RELOADABLE_REQUIRE] && defined?(ReloadableRequire) && !ancestors.include?(ReloadableRequire)
+        main.extend ReloadableRequire
+      end
     end
 
     # Evaluate the given +statements+ within the  context of this workspace.