Warn default gems which will be gemified in future

gem_prelude.rb

@@ -4,6 +4,8 @@ rescue LoadError => e
   raise unless e.path == 'rubygems'
 
   warn "`RubyGems' were not loaded."
+else
+  require 'bundled_gems'
 end if defined?(Gem)
 
 begin

lib/bundled_gems.rb

@@ -1,5 +1,5 @@
-module Gem
-  BUNDLED_GEMS = {
+module Gem::BUNDLED_GEMS
+  SINCE = {
     "rexml" => "3.0.0",
     "rss" => "3.0.0",
     "webrick" => "3.0.0",
@@ -20,4 +20,84 @@ module Gem
     "csv" => "3.4.0",
     "base64" => "3.4.0"
   }.freeze
+
+  EXACT = {
+    "abbrev"=>true,
+    "base64"=>true,
+    "csv"=>true,
+    "drb"=>true,
+    "getoptlong"=>true,
+    "mutex_m"=>true,
+    "nkf"=>true, "kconv"=>"nkf",
+    "observer"=>true,
+    "resolv-replace"=>true,
+    "rinda"=>true,
+    "syslog"=>true,
+  }.freeze
+
+  PREFIXED = {
+    "csv" => true,
+    "drb" => true,
+    "rinda" => true,
+    "syslog" => true,
+  }.freeze
+
+  WARNED = {}                   # unfrozen
+
+  def self.[](name)
+    SINCE[name]
+  end
+
+  def self.keys
+    SINCE.keys
+  end
+
+  conf = ::RbConfig::CONFIG
+  LIBDIR = (conf["rubylibdir"] + "/").freeze
+  ARCHDIR = (conf["rubyarchdir"] + "/").freeze
+  DLEXT = /\.#{Regexp.union([conf["DLEXT"], "so"].uniq)}\z/
+
+  def self.find_gem(path)
+    if !path
+      return
+    elsif path.start_with?(ARCHDIR)
+      n = path.delete_prefix(ARCHDIR).sub(DLEXT, "")
+    elsif path.start_with?(LIBDIR)
+      n = path.delete_prefix(LIBDIR).chomp(".rb")
+    else
+      return
+    end
+    EXACT[n] or PREFIXED[n[%r[\A[^/]+(?=/)]]]
+  end
+
+  def self.warning?(name)
+    _t, path = $:.resolve_feature_path(name)
+    return unless gem = find_gem(path)
+    caller, = caller_locations(3, 1)
+    return if find_gem(caller&.absolute_path)
+    return if WARNED[name]
+    WARNED[name] = true
+    if gem == true
+      gem = name
+    elsif gem
+      return if WARNED[gem]
+      WARNED[gem] = true
+      "#{name} is found in #{gem}"
+    else
+      return
+    end + " which is not part of the default gems since Ruby #{SINCE[gem]}"
+  end
+
+  bundled_gems = self
+
+  define_method(:find_unresolved_default_spec) do |name|
+    if msg = bundled_gems.warning?(name)
+      warn msg, uplevel: 1
+    end
+    super(name)
+  end
+
+  freeze
 end
+
+Gem.singleton_class.prepend Gem::BUNDLED_GEMS

lib/rubygems/core_ext/kernel_require.rb

@@ -38,6 +38,7 @@ module Kernel
 
     RUBYGEMS_ACTIVATION_MONITOR.synchronize do
       path = path.to_path if path.respond_to? :to_path
+      path = String.try_convert(path) || path
 
       if spec = Gem.find_unresolved_default_spec(path)
         # Ensure -I beats a default gem