inline_cext_rb_define_dummy_encoding.rb

require 'tmpdir'
require 'rbconfig'

def inline_c_extension(c_code)
  Dir.mktmpdir('inline_c_extension') do |dir|
    File.write("#{dir}/cext.c", c_code)
    File.write("#{dir}/extconf.rb", <<~RUBY)
    require 'mkmf'
    create_makefile('cext')
    RUBY

    out = IO.popen([RbConfig.ruby, 'extconf.rb'], chdir: dir, &:read)
    raise "ruby extconf.rb failed: #{$?.inspect}\n#{out}" unless $?.success?

    out = IO.popen(['make'], chdir: dir, &:read)
    raise "make failed: #{$?.inspect}\n#{out}" unless $?.success?

    require "#{dir}/cext.#{RbConfig::CONFIG['DLEXT']}"
  end
end

inline_c_extension <<~C
#include "ruby.h"
#include "ruby/encoding.h"

static VALUE foo(VALUE self) {
  return INT2FIX(rb_define_dummy_encoding("foo"));
}

void Init_cext(void) {
  VALUE c = rb_define_class("Foo", rb_cObject);
  rb_define_singleton_method(c, "foo", foo, 0);
}
C

p Foo.foo
p enc=Encoding.find("foo")
p Encoding.find("foo").ascii_compatible?
p "abc".force_encoding(enc).encode("UTF-8")