Fix UnsafeAccessor signature for byref struct this

[hez2010]

When calling an instance method on a struct via UnsafeAccessor, the implicit this argument is represented as a byref. The signature emitter was using that byref signature as the target type signature.

This is incorrect as using a byref target type can cause the runtime to resolve the generic context with a null MethodTable which can lead subsequent crashes.

Fix the emitter by stripping byref from the target type signature so that the target type is the underlying element type T rather than T&.

Resolves #122678

[Copilot]

Pull request overview

This PR fixes a bug in handling byref types for struct instances in UnsafeAccessors when dealing with shared generic contexts. When performing instance method calls on structs through UnsafeAccessors, the this parameter can be a byref type, but the code was calling GetMethodTable() directly without first checking if the type handle was a byref and extracting the type parameter.

Key changes:

• Added byref checks in Dictionary::PopulateEntry to properly handle byref types before calling GetMethodTable()
• Added comprehensive test coverage for both generic and non-generic structs with UnsafeAccessors

Reviewed changes

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

File	Description
src/coreclr/vm/genericdict.cpp	Added byref type checks at lines 851-852 and 996 to extract the type parameter before calling GetMethodTable() in the generic dictionary population logic
src/tests/baseservices/compilerservices/UnsafeAccessors/UnsafeAccessorsTests.Generics.cs	Added Struct and GenericStruct types with SetType methods, and new Accessors classes to test byref struct handling with both generic and non-generic scenarios

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[hez2010]

@jkotas PTAL

[jkotas]

/azp run runtime-nativeaot-outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[hez2010]

Oops I found that I just pushed a commit right after the outerloop run being triggered...
@jkotas I think you may want to trigger it again

[jkotas]

/azp run runtime-nativeaot-outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jkotas]

LGTM. Thank you!

[AaronRobinsonMSFT]

Thanks.