Don't trim xunit.core from deps.json

Fixes #49248

Author: dsplaisted

src/Tasks/Microsoft.NET.Build.Tasks/DependencyContextBuilder.cs

@@ -362,6 +362,14 @@ public DependencyContext Build(string[] userRuntimeAssemblies = null)
                 var lib = unprocessedLibraries.First();
                 unprocessedLibraries.Remove(lib);
 
+                if (lib.Library.Name.Equals("xunit.core", StringComparison.OrdinalIgnoreCase))
+                {
+                    // Special case xunit.core, it should not be removed because the xUnit v2 runner looks for this library in the deps.json to
+                    // identify test projects.
+                    // See https://github.com/dotnet/sdk/issues/49248
+                    continue;
+                }
+
                 if (lib.Library.RuntimeAssemblyGroups.Count == 0 && lib.Library.NativeLibraryGroups.Count == 0 && lib.Library.ResourceAssemblies.Count == 0)
                 {
                     if (lib.Library.Dependencies.All(d => !libraries.TryGetValue(d.Name, out var dependency) || dependency.Dependents.Count > 1))

test/Microsoft.NET.Build.Tests/GivenThatWeWantToBuildADesktopLibrary.cs

@@ -150,13 +150,35 @@ public void PackageWithoutAssets_ShouldNotShowUpInDepsJson()
             var buildCommand = new BuildCommand(testAsset);
             buildCommand.Execute().Should().Pass();
 
-            using (var depsJsonFileStream = File.OpenRead(Path.Combine(buildCommand.GetOutputDirectory(ToolsetInfo.CurrentTargetFramework).FullName, "PackageWithoutAssets_ShouldNotShowUpInDepsJson.deps.json")))
+            using (var depsJsonFileStream = File.OpenRead(Path.Combine(buildCommand.GetOutputDirectory(ToolsetInfo.CurrentTargetFramework).FullName, $"{testProject.Name}.deps.json")))
             {
                 var dependencyContext = new DependencyContextJsonReader().Read(depsJsonFileStream);
                 dependencyContext.RuntimeLibraries.Any(l => l.Name.Equals("Nerdbank.GitVersioning")).Should().BeFalse();
             }
         }
 
+        [Fact]
+        public void XUnitCoreIsNotTrimmed()
+        {
+            //  Regression test for https://github.com/dotnet/sdk/issues/49248
+
+            var testProject = new TestProject();
+            testProject.PackageReferences.Add(new TestPackageReference("xunit.core", "2.9.3"));
+            testProject.PackageReferences.Add(new TestPackageReference("xunit.extensibility.core", "2.9.3"));
+            testProject.PackageReferences.Add(new TestPackageReference("xunit.extensibility.execution", "2.9.3"));
+
+            var testAsset = _testAssetsManager.CreateTestProject(testProject);
+
+            var buildCommand = new BuildCommand(testAsset);
+            buildCommand.Execute().Should().Pass();
+
+            using (var depsJsonFileStream = File.OpenRead(Path.Combine(buildCommand.GetOutputDirectory(ToolsetInfo.CurrentTargetFramework).FullName, $"{testProject.Name}.deps.json")))
+            {
+                var dependencyContext = new DependencyContextJsonReader().Read(depsJsonFileStream);
+                dependencyContext.RuntimeLibraries.Any(l => l.Name.Equals("xunit.core")).Should().BeTrue();
+            }
+        }
+
         [Fact]
         public void ProjectNameCanMatchPackageReferenceName()
         {