Reject FFI callback with :string return type

* Applies ffi/ffi#782
* So the semantics of a callback returning :string is well defined.

lib/truffle/ffi/library.rb

@@ -394,7 +394,11 @@ def callback(*args)
       options = Hash.new
       options[:convention] = ffi_convention
       options[:enums] = @ffi_enums if defined?(@ffi_enums)
-      cb = FFI::CallbackInfo.new(find_type(ret), native_params, options)
+      ret_type = find_type(ret)
+      if ret_type == Type::STRING
+        raise TypeError, ":string is not allowed as return type of callbacks"
+      end
+      cb = FFI::CallbackInfo.new(ret_type, native_params, options)
 
       # Add to the symbol -> type map (unless there was no name)
       unless name.nil?

lib/truffle/ffi/struct.rb

@@ -228,7 +228,11 @@ def layout(*spec)
 
       def callback(params, ret)
         mod = enclosing_module
-        FFI::CallbackInfo.new(find_type(ret, mod), params.map { |e| find_type(e, mod) })
+        ret_type = find_type(ret, mod)
+        if ret_type == Type::STRING
+          raise TypeError, ":string is not allowed as return type of callbacks"
+        end
+        FFI::CallbackInfo.new(ret_type, params.map { |e| find_type(e, mod) })
       end
 
       def packed(packed = 1)

spec/ffi/callback_spec.rb

@@ -309,6 +309,15 @@ class S8F32S32 < FFI::Struct
       expect(s2[:f32]).to be_within(0.0000001).of 1.234567
     end
 
+    it "returning :string is rejected as typedef" do
+      expect {
+        Module.new do
+          extend FFI::Library
+          ffi_lib TestLibrary::PATH
+          callback :cbVrA, [], :string
+        end
+      }.to raise_error(TypeError)
+    end
 
     it "global variable" do
       proc = Proc.new { 0x1e }

spec/ffi/struct_callback_spec.rb

@@ -66,4 +66,12 @@ class TestStruct < FFI::Struct
     expect(fn.respond_to?(:call)).to be true
     expect(fn.call(1, 2)).to eq(3)
   end
+
+  it "callback returning :string is rejected in struct" do
+    expect {
+      Class.new(FFI::Struct) do
+        layout :function1, callback([:int, :int], :string)
+      end
+    }.to raise_error(TypeError)
+  end
 end