[linker] Trim Export attributes after registration. - #26313
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Remove Export, Action, and Outlet attribute instances after managed registration when static analysis proves runtime metadata fallback is unnecessary. Preserve selector access during post-processing through isolated pre-trim assemblies, and disable or reject removal when safety blockers are found. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 7d63bf91-7585-4d3e-9db4-a17531b71907
Move Export, Action, Outlet, protocol-wrapper, and NSXpc fixtures out of MySimpleApp into a dedicated iOS test application. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 7d63bf91-7585-4d3e-9db4-a17531b71907
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Pull request overview
This PR introduces an optimization to remove Foundation.ExportAttribute, Foundation.ActionAttribute, and Foundation.OutletAttribute instances after managed registration, when static analysis determines runtime metadata fallback isn’t needed. It threads a new TrimExportAttributes decision through assembly preparation/post-processing, emits a feature switch (ObjCRuntime.TrimExportAttributes) to drive ILLink attribute-instance removal, and adds tests + documentation for the new behavior.
Changes:
- Add
TrimExportAttributeseligibility computation (with explicit blockers) in the assembly-preparer pipeline and expose the result back to MSBuild. - Enable ILLink to remove Export/Action/Outlet attribute instances when
ObjCRuntime.TrimExportAttributes=true, while preserving required registrar metadata via pre-trim assemblies during post-processing. - Add unit/integration tests and document the new
TrimExportAttributesMSBuild property.
Reviewed changes
Copilot reviewed 19 out of 20 changed files in this pull request and generated 2 comments.
Show a summary per file
| File | Description |
|---|---|
| tools/mtouch/Errors.resx | Adds new MX4192 localized message for Export-attribute-removal blockers. |
| tools/mtouch/Errors.designer.cs | Adds generated accessor for MX4192. |
| tools/linker/RegistrarRemovalTrackingStep.cs | Detects NSXpcInterface MethodInfo selector-reflection usage as a blocker. |
| tools/dotnet-linker/Steps/ManagedRegistrarStep.cs | Adds blockers when managed registrar must use runtime block wrapper fallback paths. |
| tools/dotnet-linker/LinkerConfiguration.cs | Clarifies comments around DynamicRegistrationSupported and tracking step behavior. |
| tools/common/StaticRegistrar.cs | Extends attribute lookup to consult pre-trim assemblies during post-processing for selected registrar attributes. |
| tools/common/Application.cs | Adds TrimExportAttributes + TrimExportAttributesBlockers state and broadens pre-trim-resolver comments. |
| tools/assembly-preparer/ComputeExportAttributeRemovalStep.cs | New step computing eligibility, setting MSBuild output, and warning/erroring on blockers. |
| tools/assembly-preparer/AssemblyPreparer.cs | Plumbs TrimExportAttributes, adds eligibility step, and enforces registrar compatibility. |
| tests/dotnet/UnitTests/PrepareAssembliesTest.cs | Adds end-to-end build assertions for eligibility + NSXpcInterface blocker behavior. |
| tests/dotnet/ExportAttributeRemovalApp/shared.csproj | New test app shared project to validate attribute removal in assemblies. |
| tests/dotnet/ExportAttributeRemovalApp/iOS/ExportAttributeRemovalApp.csproj | New iOS test app project wrapper. |
| tests/dotnet/ExportAttributeRemovalApp/AppDelegate.cs | New test app code containing Export/Action/Outlet usage + optional NSXpcInterface path. |
| tests/common/DotNet.cs | Extends AssertBuildFailure to support specifying a target. |
| tests/assembly-preparer/ComputeExportAttributeRemovalStepTests.cs | New unit tests covering eligibility computation and blockers. |
| src/ILLink.LinkAttributes.xml.in | Adds feature-gated RemoveAttributeInstances for Export/Action/Outlet. |
| msbuild/Xamarin.Shared/Xamarin.Shared.targets | Passes TrimExportAttributes into assembly-preparer and tracks it in incremental inputs. |
| msbuild/Xamarin.MacDev.Tasks/Tasks/PrepareAssemblies.cs | Adds task plumbing for TrimExportAttributes and MSBuild output file cleanup on failure. |
| dotnet/targets/Xamarin.Shared.Sdk.targets | Enables runtime host config option based on assembly-preparer output; validates TrimExportAttributes usage. |
| docs/building-apps/build-properties.md | Documents the new TrimExportAttributes MSBuild property and behavior. |
Files not reviewed (1)
- tools/mtouch/Errors.designer.cs: Generated file
Comments suppressed due to low confidence (2)
msbuild/Xamarin.Shared/Xamarin.Shared.targets:3694
$(_TrimExportAttributes)isn't set anywhere in this targets file, so the assembly-preparer will always receive an empty value here and won't honor an explicitTrimExportAttributes=true/falseproperty. Pass$(TrimExportAttributes)directly instead.
TrimExportAttributes="$(_TrimExportAttributes)"
msbuild/Xamarin.Shared/Xamarin.Shared.targets:3747
- Same as the preparation path:
TrimExportAttributesis passed as$(_TrimExportAttributes), but that property isn't defined in this targets file, so the post-processing assembly-preparer won't see the user's setting. Pass$(TrimExportAttributes)directly.
TrimExportAttributes="$(_TrimExportAttributes)"
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rolfbjarne
commented
Jul 28, 2026
Use typed localized blockers, centralize Export trimming state, clarify API detection and MSBuild output ownership, and cover automatic registrar fallback behavior. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 7d63bf91-7585-4d3e-9db4-a17531b71907
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✅ API diff for current PR / commitNET (empty diffs)✅ API diff vs stableNET (empty diffs)ℹ️ Generator diffGenerator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes) Pipeline on Agent |
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🔥 [CI Build #616fa5d] Test results 🔥Test results❌ Tests failed on VSTS: test results 0 tests crashed, 34 tests failed, 169 tests passed. Failures❌ dotnettests tests (iOS)1 tests failed, 0 tests passed.Failed tests
Html Report (VSDrops) Download ❌ linker tests (iOS)5 tests failed, 10 tests passed.Failed tests
Html Report (VSDrops) Download ❌ linker tests (MacCatalyst)5 tests failed, 10 tests passed.Failed tests
Html Report (VSDrops) Download ❌ linker tests (macOS)10 tests failed, 11 tests passed.Failed tests
Html Report (VSDrops) Download ❌ linker tests (tvOS)4 tests failed, 11 tests passed.Failed tests
Html Report (VSDrops) Download ❌ monotouch tests (iOS)2 tests failed, 17 tests passed.Failed tests
Html Report (VSDrops) Download ❌ monotouch tests (MacCatalyst)2 tests failed, 16 tests passed.Failed tests
Html Report (VSDrops) Download ❌ monotouch tests (macOS)3 tests failed, 16 tests passed.Failed tests
Html Report (VSDrops) Download ❌ monotouch tests (tvOS)2 tests failed, 17 tests passed.Failed tests
Html Report (VSDrops) Download Successes✅ assembly-processing: All 1 tests passed. Html Report (VSDrops) Download macOS tests✅ Tests on macOS Monterey (12): All 5 tests passed. Html Report (VSDrops) Download Linux Build VerificationPipeline on Agent |
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Summary
Foundation.ExportAttribute,Foundation.ActionAttribute, andFoundation.OutletAttributeinstances through theObjCRuntime.TrimExportAttributesILLink feature after the managed registrar has consumed them.TrimExportAttributesMSBuild property: unset enables automatic removal,truerequires removal, andfalsepreserves the attributes.ExportAttributeRemovalAppintegration-test fixture.Safety
Removal is considered only when
PrepareAssemblies=true,PostProcessAssemblies=true, andRegistrar=trimmable-static. It is enabled only when dynamic registration is unsupported, theblockliteral-setupblockandstatic-block-to-delegate-lookupoptimizations are enabled, the managed registrar emits no reflective block/delegate fallback, and the app does not use theNSXpcInterfaceoverloads that obtain selectors fromMethodInfo.In automatic mode, each blocker emits its corresponding warning (MX4192-MX4197) and preserves the attributes. When removal is explicitly enabled, the same diagnostics are errors. Explicitly disabling removal preserves the existing behavior without diagnostics.
Validation
ComputeExportAttributeRemovalStepTestscases.ExportAttributeRemovalEligibilityandExportAttributeRemovalWithNSXpcInterfaceUsageintegration cases.Xamarin.MacDev.Tasksbuilds fornet10.0andnetstandard2.0.In the test fixture, Export-derived instances drop from 122 to 0 in
Microsoft.iOS.dlland from 3 to 0 in the app assembly. The strippedMicrosoft.iOS.dlldecreases from 130,048 to 81,920 bytes (48,128 bytes, or 37%).Fixes #16678
🤖 Pull request created by Copilot