Handle hardlinks with file-based app csc optimization

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

@@ -110,9 +110,18 @@ public int Execute(out bool fallbackToNormalBuild)
         if (BuildResultFile != null &&
             CSharpCommandLineParser.Default.Parse(CscArguments, BaseDirectory, sdkDirectory: null) is { OutputFileName: { } outputFileName } parsedArgs)
         {
-            var objFile = parsedArgs.GetOutputFilePath(outputFileName);
-            Reporter.Verbose.WriteLine($"Copying '{objFile}' to '{BuildResultFile}'.");
-            File.Copy(objFile, BuildResultFile, overwrite: true);
+            var objFile = new FileInfo(parsedArgs.GetOutputFilePath(outputFileName));
+            var binFile = new FileInfo(BuildResultFile);
+
+            if (HaveMatchingSizeAndTimeStamp(objFile, binFile))
+            {
+                Reporter.Verbose.WriteLine($"Skipping copy of '{objFile}' to '{BuildResultFile}' because the files have matching size and timestamp.");
+            }
+            else
+            {
+                Reporter.Verbose.WriteLine($"Copying '{objFile}' to '{BuildResultFile}'.");
+                File.Copy(objFile.FullName, binFile.FullName, overwrite: true);
+            }
         }
 
         return exitCode;
@@ -153,6 +162,27 @@ static int ProcessBuildResponse(BuildResponse response, out bool fallbackToNorma
                     return 1;
             }
         }
+
+        // Inspired by MSBuild: https://github.com/dotnet/msbuild/blob/a7a4d5af02be5aa6dc93a492d6d03056dc811388/src/Tasks/Copy.cs#L208
+        static bool HaveMatchingSizeAndTimeStamp(FileInfo sourceFile, FileInfo destinationFile)
+        {
+            if (!destinationFile.Exists)
+            {
+                return false;
+            }
+
+            if (sourceFile.LastWriteTimeUtc != destinationFile.LastWriteTimeUtc)
+            {
+                return false;
+            }
+
+            if (sourceFile.Length != destinationFile.Length)
+            {
+                return false;
+            }
+
+            return true;
+        }
     }
 
     private void PrepareAuxiliaryFiles(out string rspPath)

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

@@ -3258,6 +3258,37 @@ Release config
             """);
     }
 
+    /// <summary>
+    /// See <see cref="CscOnly_AfterMSBuild"/>.
+    /// If hard links are enabled, the <c>bin/app.dll</c> and <c>obj/app.dll</c> files are going to be the same,
+    /// so our "copy obj to bin" logic must account for that.
+    /// </summary>
+    [Fact]
+    public void CscOnly_AfterMSBuild_HardLinks()
+    {
+        var testInstance = _testAssetsManager.CreateTestDirectory(baseDirectory: OutOfTreeBaseDirectory);
+        var programPath = Path.Join(testInstance.Path, "Program.cs");
+
+        var code = $"""
+            #:property CreateHardLinksForCopyFilesToOutputDirectoryIfPossible=true
+            #:property CreateSymbolicLinksForCopyFilesToOutputDirectoryIfPossible=true
+            {s_program}
+            """;
+
+        File.WriteAllText(programPath, code);
+
+        // Remove artifacts from possible previous runs of this test.
+        var artifactsDir = VirtualProjectBuildingCommand.GetArtifactsPath(programPath);
+        if (Directory.Exists(artifactsDir)) Directory.Delete(artifactsDir, recursive: true);
+
+        Build(testInstance, BuildLevel.All);
+
+        code = code.Replace("Hello", "Hi");
+        File.WriteAllText(programPath, code);
+
+        Build(testInstance, BuildLevel.Csc, expectedOutput: "Hi from Program");
+    }
+
     /// <summary>
     /// See <see cref="CscOnly_AfterMSBuild"/>.
     /// This optimization currently does not support <c>#:project</c> references and hence is disabled if those are present.