Lazy load error_highlight, did_you_mean, and syntax_suggest

Replace eager require with autoload + a one-shot Exception#detailed_message
hook. At boot, gem_prelude.rb replaces the empty marker modules (from
rb_define_module in ruby.c) with autoload entries. Direct constant access
triggers autoload transparently; error display uses the hook.

During gem loading, Kernel#require is temporarily restored to the
original (pre-monkeypatch) version so that nested requires inside the
gems are not intercepted by user monkeypatching.

[Feature #21951]

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

gem_prelude.rb

@@ -8,20 +8,45 @@
   require 'bundled_gems'
 end if defined?(Gem)
 
-begin
-  require 'error_highlight'
-rescue LoadError
-  warn "`error_highlight' was not loaded."
-end if defined?(ErrorHighlight)
+if defined?(ErrorHighlight) || defined?(DidYouMean) || defined?(SyntaxSuggest)
+  # Replace empty modules with autoload entries for lazy loading.
+  # Direct constant access (e.g. DidYouMean::SpellChecker) triggers
+  # autoload transparently; error display uses the hook below.
+  if defined?(ErrorHighlight)
+    Object.send(:remove_const, :ErrorHighlight)
+    autoload :ErrorHighlight, 'error_highlight'
+  end
+  if defined?(DidYouMean)
+    Object.send(:remove_const, :DidYouMean)
+    autoload :DidYouMean, 'did_you_mean'
+  end
+  if defined?(SyntaxSuggest)
+    Object.send(:remove_const, :SyntaxSuggest)
+    autoload :SyntaxSuggest, 'syntax_suggest'
+  end
 
-begin
-  require 'did_you_mean'
-rescue LoadError
-  warn "`did_you_mean' was not loaded."
-end if defined?(DidYouMean)
+  module Exception::DetailedMessage # :nodoc:
+    @require = Kernel.instance_method(:require)
 
-begin
-  require 'syntax_suggest/core_ext'
-rescue LoadError
-  warn "`syntax_suggest' was not loaded."
-end if defined?(SyntaxSuggest)
+    def detailed_message(...)
+      gem_above = self.class.instance_method(:detailed_message).owner != Exception::DetailedMessage
+
+      # Temporarily restore the original require to bypass any user
+      # monkeypatching during gem loading (including nested requires).
+      orig = Exception::DetailedMessage.instance_variable_get(:@require)
+      patched = Kernel.instance_method(:require)
+      Kernel.define_method(:require, orig)
+      begin
+        require 'error_highlight' rescue LoadError
+        require 'did_you_mean' rescue LoadError
+        require 'syntax_suggest' rescue LoadError
+      ensure
+        Kernel.define_method(:require, patched)
+      end
+
+      Exception::DetailedMessage.remove_method(:detailed_message)
+      gem_above ? super : detailed_message(...)
+    end
+  end
+  Exception.prepend(Exception::DetailedMessage)
+end