Traversal projects don't work with the Visual Studio Fast Up-to-date Check

VS's fast up-to-date check (FUTDC) performs a quick file timestamp check on a projects inputs and outputs to determine whether it needs to call MSBuild. When it can avoid doing so, builds are much faster.

However, this requires having both inputs and outputs to compare against.

Normally, building a project twice should make the second build a no-op. For a traversal project, the project will eternally be considered out-of-date.

Logging shows:

Output 'D:\repo\MyTraversalProject\Debug\net472\MyTraversalProject.dll' does not exist, not up-to-date.

This problem might be addressed by having the build for this project produce a dummy file in the intermediate (obj) folder, and referencing that file as a UpToDateCheckOutput item. On every build, touch that file.

It'll also have to stop listing the non-existent output file as an expected output.

For more info on this, see https://github.com/dotnet/project-system/blob/main/docs/up-to-date-check.md.

Start with the traversal project build behavior and the Visual Studio Fast Up-to-date Check guidance in docs/up-to-date-check.md. Investigate how the project currently declares expected outputs, then verify that a build produces and touches an intermediate UpToDateCheckOutput while no longer reporting the nonexistent DLL; done means the second build is treated as up to date.