RUBY-3252 Show user friendly error if no ffi

lib/mongo/crypt.rb

@@ -36,5 +36,16 @@ module Crypt
     autoload(:ExplicitEncrypter, 'mongo/crypt/explicit_encrypter')
     autoload(:AutoEncrypter, 'mongo/crypt/auto_encrypter')
     autoload(:KMS, 'mongo/crypt/kms')
+
+    def validate_ffi!
+      return if defined?(FFI)
+
+      require 'ffi'
+    rescue LoadError => e
+      raise Error::UnmetDependency, 'Cannot enable encryption because the ffi gem ' \
+                                    "has not been installed. Add \"gem 'ffi'\" to your Gemfile and run " \
+                                    "\"bundle install\" to install the gem. (#{e.class}: #{e})"
+    end
+    module_function :validate_ffi!
   end
 end

lib/mongo/crypt/auto_encrypter.rb

@@ -91,6 +91,7 @@ class AutoEncrypter
       # @raise [ ArgumentError ] If required options are missing or incorrectly
       #   formatted.
       def initialize(options)
+        Crypt.validate_ffi!
         # Note that this call may eventually, via other method invocations,
         # create additional clients which have to be cleaned up.
         @options = set_default_options(options).freeze

lib/mongo/crypt/explicit_encrypter.rb

@@ -36,6 +36,7 @@ class ExplicitEncrypter
       #   should be hashes of TLS connection options. The options are equivalent
       #   to TLS connection options of Mongo::Client.
       def initialize(key_vault_client, key_vault_namespace, kms_providers, kms_tls_options)
+        Crypt.validate_ffi!
         @crypt_handle = Handle.new(
           kms_providers,
           kms_tls_options,

spec/mongo/crypt_spec.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Mongo::Crypt do
+  describe '.validate_ffi!' do
+    context 'when ffi is available' do
+      context 'when ffi is loaded' do
+        it 'does not raise' do
+          expect do
+            described_class.validate_ffi!
+          end.not_to raise_error
+        end
+      end
+    end
+    # There is no reasonably simple way to test the path where ffi is not
+    # available. The ffi gem is a part of our standard test dependencies, so
+    # it's always available. So, we would need a dedicated configuration
+    # just to test this feature; it seems to be an overhead.
+  end
+end