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[browser][CoreCLR][trimmed R2R] Reflection metadata tests lose pseudo-attribute and forwarder data #134263
Description
Description
Trimmed browser-Wasm CoreCLR ReadyToRun exposes reflection metadata differences in two test assemblies:
System.Reflection.MetadataLoadContext.Tests: threeMarshalAsAttributepseudo-custom-attribute cases lose expected metadata values.System.Reflection.Tests:AssemblyGetForwardedTypesLoadFailureno longer observes the expected forwarded-type load failure shape.
Found while bringing up #133193.
Reproduction Steps
Build and run the affected library tests for browser-Wasm CoreCLR with:
TestWasmReadyToRun=true
EnableAggressiveTrimming=true
RuntimeFlavor=CoreCLR
Configuration=Release
The failures reproduced in PR #133656, Azure DevOps build 1603190, Helix job 3158c862-61f5-4cf9-bb24-6ecdf2cfe895, work items WasmTestOnChrome-CLR-ST-Batch-10 and Batch-13.
Expected behavior
MetadataLoadContext preserves the expected MarshalAsAttribute pseudo-attribute fields, and GetForwardedTypes reports the expected loader exception behavior.
Actual behavior
Examples:
CustomAttributeTests.TestMarshalAsPseudoCustomAttribute("F4")
Expected IidParameterIndex: 42
Actual: 0
F11 and F12 also report differing pseudo-attribute values.
AssemblyTests.AssemblyGetForwardedTypesLoadFailure
Assert.True() Failure
Expected: True
Actual: False
Regression?
Unknown. Broad trimmed ReadyToRun library coverage is newly enabled. Related historical Mono/EAT reflection trimming issue: #50715, but these failures are different tests and runtime configuration.
Known Workarounds
Keep System.Reflection.MetadataLoadContext.Tests.dll interpreted and temporarily exclude only AssemblyGetForwardedTypesLoadFailure under aggressively trimmed browser tests. Product reflection assemblies remain ReadyToRun compiled.
Configuration
Browser-Wasm CoreCLR Release, aggressively trimmed per-app ReadyToRun, Linux Helix host, Chrome 153.
Other information
Re-enable when the required pseudo-attribute and forwarded-type metadata survives this publish configuration.
Note
This issue was generated with GitHub Copilot assistance and reviewed as part of the #133193 bring-up.
Activity
dotnet-policy-service commentedon Sep 19, 2026
Tagging subscribers to 'arch-wasm': @lewing, @pavelsavara
See info in area-owners.md if you want to be subscribed.
dotnet-policy-service commentedon Sep 19, 2026
Tagging subscribers to this area: @steveisok, @dotnet/area-system-reflection
See info in area-owners.md if you want to be subscribed.
lewing commentedon Sep 27, 2026
Root causes, both reproduced locally on trimmed browser-wasm CoreCLR:
AssemblyGetForwardedTypesLoadFailure: ILLink dropsUnloadableAssembly.dll, because a type forwarder is its only reference. Resolving the forwarded type then fails with a missing-file error instead ofBadImageFormatException. Fixed in [browser][CoreCLR] Root UnloadableAssembly so trimmed builds keep it for AssemblyGetForwardedTypesLoadFailure #134759.MarshalAscases F4/F11/F12: a Cecil bug. When ILLink rewrites the test assembly, Cecil drops theIidParameterIndexandSafeArrayUserDefinedSubTypebytes from the FieldMarshal blobs (confirmed byte-for-byte). Tracked in ILLink/Cecil drops MarshalAs IidParameterIndex and SafeArrayUserDefinedSubType when rewriting assemblies #134761. The existingTrimMode=copyworkaround forSystem.Reflection.MetadataLoadContext.Tests.dllstays until that's fixed.Note
This comment was generated with GitHub Copilot assistance.
[browser][CoreCLR] Root UnloadableAssembly so trimmed builds keep it …