Add support for executing a script directly.

Author: ioquatix

lib/rubygems/ext.rb

@@ -18,3 +18,4 @@ module Gem::Ext; end
 require_relative "ext/rake_builder"
 require_relative "ext/cmake_builder"
 require_relative "ext/cargo_builder"
+require_relative "ext/script_builder"

lib/rubygems/ext/builder.rb

@@ -138,6 +138,7 @@ def initialize(spec, build_args = spec.build_args)
 
   def builder_for(extension) # :nodoc:
     case extension
+    # File that ends with .rb
     when /extconf/ then
       Gem::Ext::ExtConfBuilder
     when /configure/ then
@@ -149,6 +150,8 @@ def builder_for(extension) # :nodoc:
       Gem::Ext::CmakeBuilder
     when /Cargo.toml/ then
       Gem::Ext::CargoBuilder.new
+    when /\.rb/ then
+      Gem::Ext::ScriptBuilder
     else
       build_error("No builder for extension '#{extension}'")
     end

lib/rubygems/ext/script_builder.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+#--
+# Copyright 2023 Samuel Williams.
+# All rights reserved.
+# See LICENSE.txt for permissions.
+#++
+
+class Gem::Ext::ScriptBuilder < Gem::Ext::Builder
+  def self.build(extension, dest_path, results, args=[], lib_dir=nil, extension_dir=Dir.pwd)
+    env = {
+      "PREFIX_PATH" => dest_path,
+      "LIBRARY_PATH" => lib_dir,
+      "EXTENSION_PATH" => extension_dir,
+    }
+
+    run([extension] + args, results, class_name, Dir.pwd, env)
+  end
+end