Reject callback with :string return type

Callbacks returning a :string type were not supported so far.
It was possible to define such a callback, but the value returned was NULL in any case.
This implementation rejects :string return type of callbacks.

The reason for the reject is the following:
In contrast to :string parameters to called functions there is no well defined ownership of the memory of the string returned by callbacks.
Instead an explicit FFI::MemoryPointer or similar should be used, which allows to track the validity of underlying memory instead of relying on some Ruby implementation details.

Fixes #751

ext/ffi_c/FunctionInfo.c

@@ -172,14 +172,17 @@ fntype_initialize(int argc, VALUE* argv, VALUE self)
         VALUE typeName = rb_funcall2(rbReturnType, rb_intern("inspect"), 0, NULL);
         rb_raise(rb_eTypeError, "Invalid return type (%s)", RSTRING_PTR(typeName));
     }
-    
+
     if (rb_obj_is_kind_of(fnInfo->rbReturnType, rbffi_StructByValueClass)) {
         fnInfo->hasStruct = true;
     }
 
     Data_Get_Struct(fnInfo->rbReturnType, Type, fnInfo->returnType);
     fnInfo->ffiReturnType = fnInfo->returnType->ffiType;
 
+    if (fnInfo->returnType->nativeType == NATIVE_STRING) {
+        rb_raise(rb_eTypeError, "return type :string is not allowed as return type of callbacks");
+    }
 
 #if defined(X86_WIN32)
     rbConventionStr = (rbConvention != Qnil) ? rb_funcall2(rbConvention, rb_intern("to_s"), 0, NULL) : Qnil;

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" 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 }