Ensure csc.rsp is always written to disk with dotnet run and dotnet build

src/Cli/dotnet/Commands/Run/VirtualProjectBuildingCommand.cs

@@ -339,6 +339,7 @@ public override int Execute()
                         cache.CurrentEntry.Run = LastRunProperties;
 
                         CacheCscArguments(cache, buildResult);
+                        WriteCscRsp(cache);
                         CollectAdditionalSources(cache, buildRequest.ProjectInstance);
 
                         MarkBuildSuccess(cache);
@@ -489,6 +490,18 @@ void ReuseInfoFromPreviousCacheEntry(CacheInfo cache)
             }
         }
 
+        void WriteCscRsp(CacheInfo cache)
+        {
+            if (cache.CurrentEntry.CscArguments.IsDefaultOrEmpty)
+            {
+                return;
+            }
+
+            string rspPath = Path.Join(Builder.ArtifactsPath, "csc.rsp");
+            File.WriteAllLines(rspPath, cache.CurrentEntry.CscArguments);
+            Reporter.Verbose.WriteLine($"Written '{rspPath}'.");
+        }
+
         bool CanSaveCache(ProjectInstance projectInstance)
         {
             if (!MSBuildUtilities.ConvertStringToBool(projectInstance.GetPropertyValue(FileBasedProgramCanSkipMSBuild), defaultValue: true))

src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers.sarif

@@ -7092,4 +7092,4 @@
       }
     }
   ]
-}
+}

test/dotnet.Tests/CommandTests/Run/RunFileTests.cs

@@ -5401,6 +5401,34 @@ public void CscOnly_AfterMSBuild_AuxiliaryFilesNotReused()
         Build(testInstance, BuildLevel.Csc, expectedOutput: "v3 ");
     }
 
+    /// <summary>
+    /// Verifies that <c>csc.rsp</c> is written to disk after a full MSBuild build,
+    /// so that IDEs can read it to create a virtual project.
+    /// </summary>
+    [Fact]
+    public void MSBuild_WritesCscRsp()
+    {
+        var testInstance = _testAssetsManager.CreateTestDirectory(baseDirectory: OutOfTreeBaseDirectory);
+
+        var programPath = Path.Join(testInstance.Path, "Program.cs");
+        File.WriteAllText(programPath, """
+            #:property Configuration=Release
+            Console.Write("Hello");
+            """);
+
+        // Remove artifacts from possible previous runs of this test.
+        var artifactsDir = VirtualProjectBuilder.GetArtifactsPath(programPath);
+        if (Directory.Exists(artifactsDir)) Directory.Delete(artifactsDir, recursive: true);
+
+        // A build directive forces a full MSBuild build.
+        Build(testInstance, BuildLevel.All, expectedOutput: "Hello");
+
+        // csc.rsp should be written to disk after a full MSBuild build.
+        var rspPath = Path.Join(artifactsDir, "csc.rsp");
+        File.Exists(rspPath).Should().BeTrue("csc.rsp should be written after a full MSBuild build");
+        File.ReadAllLines(rspPath).Should().NotBeEmpty("csc.rsp should contain compiler arguments");
+    }
+
     /// <summary>
     /// Testing <see cref="CscOnly"/> optimization when the NuGet cache is cleared between builds.
     /// See <see href="https://github.com/dotnet/sdk/issues/45169"/>.