Refresh copy marker when implementation output changes

[AlesProkop]

Fixes #13478.

Context

CopyUpToDateMarker is used by Visual Studio's fast up-to-date check to decide whether referencing projects may need to observe updated implementation outputs when reference assemblies are unchanged.

Today the marker only advances from _CopyFilesMarkedCopyLocal when a copy-local reference is actually written. That preserves no-op incremental builds, but misses implementation-only changes where the project's main assembly is updated and no copy-local references were copied.

Changes

• Capture WroteAtLeastOneFile from the main assembly copy in CopyFilesToOutputDirectory.
• Touch @(CopyUpToDateMarker) when ProduceReferenceAssembly=true and the main implementation assembly was actually copied to OutDir.
• Keep no-op builds from advancing the marker.
• Add regression coverage for marker advancement, no-op behavior, and ProduceReferenceAssembly=false.

Validation

• dotnet test D:\msbuild\src\Tasks.UnitTests\Microsoft.Build.Tasks.UnitTests.csproj --framework net472 --filter "FullyQualifiedName~Microsoft.Build.Tasks.UnitTests.RegressionTests.CopyUpToDateMarker"
• D:\msbuild\build.cmd -v quiet
• Manual end-to-end MSBuild check with the built artifacts\bin\MSBuild\Debug\net472\MSBuild.exe: marker unchanged on no-op build and advanced after implementation-output change.

[Copilot]

Pull request overview

This PR updates MSBuild’s up-to-date marker behavior to better support Visual Studio’s fast up-to-date check when reference assemblies are produced, ensuring the marker advances when the implementation output is actually copied to OutDir (even if no copy-local references changed).

Changes:

• Capture whether the main assembly copy in CopyFilesToOutputDirectory wrote at least one file, and use that to decide when to touch @(CopyUpToDateMarker).
• Touch @(CopyUpToDateMarker) on implementation assembly writes when $(ProduceReferenceAssembly) is true, covering implementation-only changes.
• Add regression tests validating marker advancement, no-op behavior, and the ProduceReferenceAssembly=false case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 8 comments.

File	Description
src/Tasks/Microsoft.Common.CurrentVersion.targets	Tracks main assembly copy writes and touches CopyUpToDateMarker when reference assemblies are produced and implementation output changes.
src/Tasks.UnitTests/RegressionTests.cs	Adds regression tests for marker advancement/no-op behavior and for ProduceReferenceAssembly=false.

Comments suppressed due to low confidence (1)

src/Tasks.UnitTests/RegressionTests.cs:10

• This file adds new test assertions, but the repo’s testing guidance prefers Shouldly assertions in modified test code. Add the Shouldly using so the new assertions can be expressed with ShouldBe* helpers (and update the asserts below accordingly).

using System;
using System.IO;
using System.Threading;
using Microsoft.Build.Evaluation;
using Microsoft.Build.Shared;
using Microsoft.Build.UnitTests;
using Xunit;