Skip full Build for AOT publish, use Compile instead

[marcpopMSFT]

When publishing with PublishAot=true, the implicit Build step was producing a full self-contained managed deployment (apphost, DLLs, deps.json, runtimeconfig, runtime pack files) in the output directory. This output is completely unused by the AOT pipeline, which reads from @(IntermediateAssembly) in obj\, not from bin\ output.

This change introduces _PublishAotBuildAlternative that runs Compile (plus resource and satellite assembly targets) instead of full Build for AOT publish. This eliminates the confusing managed artifacts from the output directory while preserving all AOT publish functionality.

The target list for AOT publish is:

• BuildOnlySettings: configures build-time settings
• PrepareForBuild: creates output directories
• PrepareResources: compiles .resx to .resources files
• Compile: produces IL assembly in obj\ (includes ResolveReferences)
• CreateSatelliteAssemblies: generates culture-specific resource DLLs

Tactics

Summary

When publishing with PublishAot=true, the full MSBuild Build target was executing as part of the publish pipeline, producing a complete self-contained managed deployment (apphost, DLLs, deps.json, runtimeconfig.json, and runtime pack files) in the output (bin\\) directory. These artifacts are entirely unused by the AOT pipeline, which reads from @(IntermediateAssembly) in obj\\, not from bin\\. This change introduces a new _PublishOptimizedBuild target that runs only Compile (plus resource and satellite assembly targets) instead of the full Build for AOT publish, eliminating the confusing managed artifacts from the output directory while preserving all AOT publish functionality. A public opt-out property UseOptimizedPublish is provided for projects that rely on Build extensibility points (e.g., BeforeBuild, AfterBuild, BeforeTargets/AfterTargets="Build") or packages like Microsoft.Extensions.ApiDescription.Server. This fix is scoped to AOT-only in .NET 11 to minimize risk; expansion to other publish modes is planned for .NET 12 early previews (tracked in #55911).

Customer Impact

Customers performing dotnet publish /p:PublishAot=true received a cluttered output directory containing the full managed self-contained deployment alongside the expected publish\\ and native\\ subdirectories. This caused confusion when developers accidentally deployed the build-output DLLs/apphost instead of the AOT-compiled binaries. The issue affects all users on .NET 11 using Native AOT publish. There is no workaround other than manually deleting the spurious files. After this fix, the output directory contains only subdirectories (native\\, publish\\), with no loose managed artifacts. Projects with custom Build targets or packages with BeforeTargets="Build" hooks must opt out via <UseOptimizedPublish>false</UseOptimizedPublish>.

Regression?

Unknown — this is not a regression but rather a long-standing behavior of the publish pipeline where Build was always invoked unconditionally. The PR improves the behavior for .NET 11 Native AOT publish. The feature is new in .NET 11 and targets release/11.0.1xx.

Testing

• Unit/integration tests added: 175 lines of new tests added in test/Microsoft.NET.Publish.Tests/GivenThatWeWantToPublishAnAotApp.cs, including assertions that the parent output directory contains no loose files after AOT publish, and a focused test that records the evaluated CoreBuildDependsOn target sequence so any unexpected additions require explicit review. 5 lines added in test/Microsoft.NET.Sdk.Web.Tests/PublishTests.cs.
• Ecosystem compatibility: NuGet top-100 package scan performed; only Microsoft.Extensions.ApiDescription.Server was found with a direct Build hook, documented in the opt-out guidance.
• MAUI/mobile concern: iOS and Android workload impact was flagged; the PR is intentionally held from rc1 and targeting rc2 to allow MAUI team testing. The extension point $(_AdditionalOptimizedBuildTargets) and UseOptimizedPublish=false opt-out are provided for workloads.
• CI: PR CI runs on the release/11.0.1xx branch.

Risk

Medium. The change alters the publish pipeline for all PublishAot=true projects by skipping the full Build target in favor of Compile-only. While Build extensibility points (BeforeBuild, AfterBuild, custom BeforeTargets/AfterTargets="Build") are bypassed by default, an opt-out (UseOptimizedPublish=false) is provided and documented. The MAUI/iOS/Android workloads have not yet completed testing, which is why the PR is targeted for rc2. The NuGet ecosystem scan found only one affected package. The change is limited to PublishAot=true scenarios, reducing blast radius.

[Copilot]

Pull request overview

This PR changes Native AOT publish so it compiles the intermediate IL needed by the AOT pipeline without running the full Build target, avoiding unused managed self-contained artifacts in bin output.

Changes:

• Adds an AOT-specific publish build alternative in Microsoft.NET.Publish.targets.
• Routes PublishAot=true publishes through Compile/resource targets instead of full Build.
• Updates and adds Native AOT publish tests for clean output and project-reference publishing.

Reviewed changes

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

File	Description
src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Publish.targets	Adds _PublishAotBuildAlternative and excludes AOT publish from the normal full-build path.
test/Microsoft.NET.Publish.Tests/GivenThatWeWantToPublishAnAotApp.cs	Updates property-recording hooks for skipped Build and adds Native AOT publish regression tests.

[dsplaisted]

Looks good. There is some risk here but after reviewing it is less than I was initially thinking.

The PR is targeting main, I think it would need to be re-targeted for .NET 11 and CI should be re-run.

It would be nice to scan popular NuGet packages and see if they include any MSBuild logic that would be broken by this. Copilot flagged Microsoft.XmlSerializer.Generator as having logic that would be broken by this, but it seems like it may not work with NativeAOT anyway, regardless of this change.

[marcpopMSFT]

NuGet compatibility scan: I checked the top 100 stable packages returned by NuGet search for build/buildTransitive/buildMultiTargeting hooks. The only package with a direct Build hook was Microsoft.Extensions.ApiDescription.Server 10.0.11 (_GenerateOpenApiDocuments, BeforeTargets="Build"). That target also consumes $(TargetPath) from the build output, so projects that need OpenAPI document generation during AOT publish need the documented <UseAotOptimizedPublish>false</UseAotOptimizedPublish> opt-out. No other direct Build hooks were found in the scanned packages.

The base branch was changed.

[marcpopMSFT]

/backport to release/11.0.1xx-rc1

[github-actions]

Started backporting to release/11.0.1xx-rc1 (link to workflow run)

[github-actions]

@marcpopMSFT backporting to release/11.0.1xx-rc1 was not run because the source pull request has not been merged. Please merge this pull request before requesting a backport.

[jonathanpeppers]

@marcpopMSFT have we checked this doesn't break iOS and Android?

[marcpopMSFT]

Per request of maui who expects to be impacted by this, we'll old it from rc1 but get it merged for rc2 early so testing can be done.

[marcpopMSFT]

/tactics

[marcpopMSFT]

/ba-g all the checks are green so ba appears stuck