msbuild: Source Generators fail in WinUI 3 projects because Csc is invoked without AnalyzerConfigFiles

Describe the bug

A project reunion project fails to build when it contains code that relies on a Source Generator in any of the XAML code-behind files.

Steps to reproduce the bug

(based on @dotMorten’s report https://github.com/microsoft/CsWin32/issues/219#issuecomment-811272354)

Minimal repro solution

Or manual steps:

1. Install Project Reunion Project Templates: https://marketplace.visualstudio.com/items?itemName=ProjectReunion.MicrosoftProjectReunion
2. Create a new Blank App, Packaged (WinUI 3 in Desktop) project.
3. Install the Microsoft.Windows.CsWin32 nuget package (0.1.422-beta)
4. Add NativeMethods.txt and some interop methods to it.
5. Open MainWindow.xaml.cs, and in the constructor add a pinvoke method call.
6. Compile
7. Observe errors in the error list

Using VS16.10.p1

Expected behavior

The build is successful.

Analysis

Examining the build log I’ve found the problem: The XamlPreCompile target as defined in MSBuild\Current\Bin\Microsoft.CSharp.CurrentVersion.targets invokes the Csc task but omits AnalyzerConfigFiles that is normally passed in. As a result, the CsWin32 source generator is not getting the info it needs to generate the APIs referenced in the XAML code-behind.

As the faulty target is defined in Microsoft.CSharp.CurrentVersion.targets, this repo seemed the most appropriate place to file the issue.

Screenshots

The CoreCompile target’s invocation of Csc:

XamlPreCompile target’s invocation of Csc: