Move core ProjectReferenceTargets from Managed-only to Common targets

[dfederm]

Summary

Move the core Build/Clean/Rebuild ProjectReferenceTargets protocol from Microsoft.Managed.After.targets into Microsoft.Common.CurrentVersion.targets so it applies to all project types that import the common targets, not just managed languages (C#, VB, F#). This aligns with the static graph spec which prescribes that common protocols be defined in the common targets.

What changed

Targets files

• Microsoft.Common.CurrentVersion.targets: Now contains the core ProjectReferenceTargets for Build, Clean, and Rebuild, including OuterBuild items, SkipNonexistentTargets items, and the extensibility properties (ProjectReferenceTargetsForBuild, etc.)
• Microsoft.Common.CrossTargeting.targets: Now contains the cross-targeting outer build protocol (.default dispatch) that was previously inlined in Managed.After.targets
• Microsoft.Managed.After.targets: Retains only managed-specific extensions: Publish, DeployOnBuild, GetCopyToPublishDirectoryItems, WPF graph isolation, and InnerBuildProperty/InnerBuildPropertyValues

DeployOnBuild refactor

DeployOnBuild handling changed from property-level composition (appending Publish targets to $(ProjectReferenceTargetsForBuild)) to item-level composition (separate ProjectReferenceTargets items). This is functionally equivalent since the static graph engine merges all items with the same Include value, and eliminates a subtle duplication in the old code where ProjectReferenceTargetsForRebuild included publish targets twice.

Documentation

• Updated stale docs.microsoft.com → learn.microsoft.com URL
• Converted absolute GitHub links to relative paths
• Added prose hints for deep-link references into large targets files
• Fixed typo in SkipNonexistentTargets comment

Testing

Unit tests (11 new, all passing)

New file src/Build.UnitTests/Graph/ProjectReferenceTargetsProtocol_Tests.cs covering:

• Non-managed project gets core Build/Clean/Rebuild protocol
• Managed project protocol unchanged after move
• DeployOnBuild property→item migration
• Cross-targeting .default dispatch
• BuildProjectReferences=false and NoBuild=true fallbacks
• Clean/Rebuild propagation through multi-project chains

Manual validation

Tested with old (VS 18 Canary) vs new (bootstrap) MSBuild:

• Key result: Non-managed .proj with /graph /isolate — old MSBuild produces MSB4252 isolation violation ("target not specified in ProjectReferenceTargets"), new MSBuild resolves the graph correctly
• -getItem:ProjectReferenceTargets /p:IsGraphBuild=true on non-managed .proj: old returns empty, new returns full Build/Clean/Rebuild protocol from Microsoft.Common.CurrentVersion.targets
• C++ graph build binlogs produced for manual inspection

[Copilot]

Pull request overview

Moves the static-graph ProjectReferenceTargets protocol for core Build/Clean/Rebuild from managed-only targets into the common MSBuild targets so it applies to all project types that import the common targets, aligning implementation with the static graph spec.

Changes:

• Added core static-graph ProjectReferenceTargets (Build/Clean/Rebuild) to Microsoft.Common.CurrentVersion.targets.
• Added cross-targeting (outer build) static-graph ProjectReferenceTargets dispatch protocol to Microsoft.Common.CrossTargeting.targets.
• Refactored managed DeployOnBuild propagation from property composition to item-based composition and added new unit tests + doc updates.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/Tasks/Microsoft.Managed.After.targets	Removes core Build/Clean/Rebuild protocol and keeps managed-only Publish/DeployOnBuild extensions.
src/Tasks/Microsoft.Common.CurrentVersion.targets	Defines core Build/Clean/Rebuild ProjectReferenceTargets protocol for static graph builds.
src/Tasks/Microsoft.Common.CrossTargeting.targets	Defines cross-targeting (outer build) static graph protocol for Build/Clean/Rebuild.
src/Build.UnitTests/Graph/ProjectReferenceTargetsProtocol_Tests.cs	Adds unit tests validating protocol behavior across managed/non-managed and cross-targeting scenarios.
documentation/ProjectReference-Protocol.md	Updates links and clarifies references to protocol/comment locations in targets files.

[Copilot]

In the file header comment, "managed languaged" is a typo. Please change it to "managed languages" while touching this header so the comment is accurate and professional.

[Copilot]

This test class constructor passes the ITestOutputHelper directly into TestEnvironment.Create(...) but doesn't store it (and the created TestEnvironment) in readonly fields like other graph tests do. Storing the output helper as a field and using it consistently (e.g., for MockLogger diagnostics) makes failures much easier to diagnose and matches the prevailing pattern in this test suite.

Suggested change

	private TestEnvironment _env;
	public ProjectReferenceTargetsProtocolTests(ITestOutputHelper output)
	{
	_env = TestEnvironment.Create(output);
	private readonly TestEnvironment _env;
	private readonly ITestOutputHelper _output;
	public ProjectReferenceTargetsProtocolTests(ITestOutputHelper output)
	{
	_output = output;
	_env = TestEnvironment.Create(_output);

[Copilot]

In the cross-targeting test, outerBuildContent includes CommonCurrentVersionTargetsProtocol. In actual cross-targeting builds, managed projects import Microsoft.Common.CrossTargeting.targets (e.g., via Microsoft.CSharp.CrossTargeting.targets) and do not also import Microsoft.Common.CurrentVersion.targets in the outer build. Including the inner-build protocol in the synthetic outer build can change which ProjectReferenceTargets items exist and may make the test less representative (or accidentally mask protocol issues). Consider modeling the outer build with just the cross-targeting protocol + multitargeting spec, and reserving CommonCurrentVersionTargetsProtocol for inner builds / non-cross-targeting projects.

Suggested change

	// The outer build uses CrossTargeting protocol
	string outerBuildContent = MultitargetingSpec + CrossTargetingProtocol + CommonCurrentVersionTargetsProtocol + AllDummyTargets;
	// The outer build uses CrossTargeting protocol (no current-version protocol in real outer builds)
	string outerBuildContent = MultitargetingSpec + CrossTargetingProtocol + AllDummyTargets;