How does Visual Studio decide when and how to rebuild IntelliSense?

I'm trying to write a code generation tool. For this tool it's important that the generated code is available prior to building (i.e., for IntelliSense). I know Visual Studio will at least partially evaluate the project build plan automatically to generate IntelliSense, but I can't find much information on the details.

As a simpler example, let's say I want to take all items with build action None and compile them. I have a project like this:

<Project [...]>
  [...]
  <Compile Include="Foo.cs" />
  <None Include="Bar.cs" />
</Project>

One way to get Bar.cs to compile is to add the following to the project:

<PropertyGroup>
  <CoreCompileDependsOn>
    $(CoreCompileDependsOn);IndirectCompile
  </CoreCompileDependsOn>
</PropertyGroup>
<Target Name="IndirectCompile">
  <CreateItem Include="@(None)">
    <Output ItemName="Compile" TaskParameter="Include" />
  </CreateItem>
</Target>

If I do it this way, Visual Studio acts basically the same as if Bar.cs had the Compile action to begin with. IntelliSense is fully available; if I make a change in Bar.cs it's reflected immediately (well, as immediate as the background operation normally is) in IntelliSense when I'm editing Foo.cs, and so on.

However, say instead of directly compiling the None entry, I want to copy it to the obj directory and then compile it from there. I can do this by changing the IndirectCompile target to this:

<Target Name="IndirectCompile" 
        Inputs="@(None)"
        Outputs="@(None->'$(IntermediateOutputPath)%(FileName).g.cs')"
>
  <Copy SourceFiles="@(None)"
        DestinationFi