Opt-in .sln parsing with Microsoft.VisualStudio.SolutionPersistence

eng/BootStrapMsBuild.targets

@@ -35,7 +35,8 @@
         <_NuGetRuntimeDependencies Include="%(RuntimeCopyLocalItems.Identity)" Condition="'@(RuntimeCopyLocalItems->Contains('Newtonsoft.Json'))' == 'true'" />
         <_NuGetRuntimeDependencies Include="%(RuntimeCopyLocalItems.Identity)" Condition="'@(RuntimeCopyLocalItems->Contains('NuGetSdkResolver'))' == 'true'" />
         <_NuGetRuntimeDependencies Include="%(RuntimeCopyLocalItems.Identity)" Condition="'@(RuntimeCopyLocalItems->Contains('Microsoft.Extensions.'))' == 'true'" />
-
+        <_NuGetRuntimeDependencies Include="%(RuntimeCopyLocalItems.Identity)" Condition="'@(RuntimeCopyLocalItems->Contains('Microsoft.VisualStudio.SolutionPersistence'))' == 'true'" />
+
         <!-- NuGet.targets and NuGet.RestoreEx.targets will be in the RuntimeTargetsCopyLocalItems ItemGroup -->
         <_NuGetRuntimeDependencies Include="%(RuntimeTargetsCopyLocalItems.Identity)" Condition="'@(RuntimeTargetsCopyLocalItems->Contains('NuGet.'))' == 'true'" />
 
@@ -48,7 +49,7 @@
 
   <Target Name="RemoveExtraAssemblyReferences" BeforeTargets="ResolveAssemblyReferences">
     <!-- This is really hacky, but these references will cause issues when trying to 'build' this project.
-         To acquire the NuGet binaries we depend on for local run-time ('bootstrap'), we we are using a PackageReference (to
+         To acquire the NuGet binaries we depend on for local run-time ('bootstrap'), we are using a PackageReference (to
          'NuGet.Build.Tasks' and 'Microsoft.Build.NuGetSdkResolver'). This has the advantage of using NuGets compatibility
          check to ensure we choose the right version of those assemblies. But, at 'bootstrap' time these runtime dependencies
          need to be in a specific location that does not mesh with NuGet. To resolve this, we include the default

src/Build.UnitTests/Construction/SolutionFile_NewParser_Tests.cs

@@ -20,13 +20,21 @@
 
 namespace Microsoft.Build.UnitTests.Construction
 {
-    public class SolutionFile_NewParser_Tests
+    public class SolutionFile_NewParser_Tests : IDisposable
     {
         public ITestOutputHelper TestOutputHelper { get; }
 
+        private readonly TestEnvironment _testEnvironment;
+
         public SolutionFile_NewParser_Tests(ITestOutputHelper testOutputHelper)
         {
             TestOutputHelper = testOutputHelper;
+            _testEnvironment = TestEnvironment.Create();
+        }
+
+        public void Dispose()
+        {
+            _testEnvironment.Dispose();
         }
 
         /// <summary>
@@ -80,7 +88,7 @@ public void ProjectWithWebsiteProperties(bool convertToSlnx)
                 EndGlobal
                 """;
 
-            SolutionFile solution = ParseSolutionHelper(solutionFileContents.Replace('`', '"'), convertToSlnx);
+            SolutionFile solution = ParseSolutionHelper(_testEnvironment, solutionFileContents.Replace('`', '"'), convertToSlnx);
 
             solution.ProjectsInOrder.ShouldHaveSingleItem();
 
@@ -129,20 +137,15 @@ public void ProjectWithWebsiteProperties(bool convertToSlnx)
         /// Helper method to create a SolutionFile object, and call it to parse the SLN file
         /// represented by the string contents passed in. Optionally can convert the SLN to SLNX and then parse the solution.
         /// </summary>
-        internal static SolutionFile ParseSolutionHelper(string solutionFileContents, bool convertToSlnx = false)
+        internal static SolutionFile ParseSolutionHelper(TestEnvironment testEnvironment, string solutionFileContents, bool convertToSlnx = false)
         {
             solutionFileContents = solutionFileContents.Replace('\'', '"');
-
-            using (TestEnvironment testEnvironment = TestEnvironment.Create())
-            {
-                TransientTestFile sln = testEnvironment.CreateFile(FileUtilities.GetTemporaryFileName(".sln"), solutionFileContents);
-
-                string solutionPath = convertToSlnx ? ConvertToSlnx(sln.Path) : sln.Path;
-
-                SolutionFile solutionFile = new SolutionFile { FullPath = solutionPath };
-                solutionFile.ParseUsingNewParser();
-                return solutionFile;
-            }
+            testEnvironment.SetEnvironmentVariable("MSBUILD_SLN_PARSING_SOLUTIONPERSISTENCE_OPTIN", "1");
+            TransientTestFile sln = testEnvironment.CreateFile(FileUtilities.GetTemporaryFileName(".sln"), solutionFileContents);
+            string solutionPath = convertToSlnx ? ConvertToSlnx(sln.Path) : sln.Path;
+            SolutionFile solutionFile = new SolutionFile { FullPath = solutionPath };
+            solutionFile.ParseUsingNewParser();
+            return solutionFile;
         }
 
         private static string ConvertToSlnx(string slnPath)