Fix ReferenceTrackerHost fixed-address storage

[Copilot]

ReferenceTrackerHost stored s_globalHostServices as an IntPtr annotated with [FixedAddressValueType], but that attribute does not apply to primitive value types. This change avoids relying on that unsupported combination while keeping the COM host object layout unchanged.

• What changed

  • Replaced s_globalHostServices from IntPtr with a small wrapper value type that contains the vtable pointer.
  • Kept [FixedAddressValueType] on s_globalHostServices, but now applied to a non-primitive value type where the runtime honors it.

• Why this is safer

  • Removes the dependency on FixedAddressValueType behavior for IntPtr.
  • Preserves the existing native shape expected by SetReferenceTrackerHost: a pointer to a single-field object whose first field is the vtable pointer.

• Shape

  [FixedAddressValueType]
  private static readonly unsafe ReferenceTrackerHostObject s_globalHostServices =
      new((IReferenceTrackerHostVftbl*)Unsafe.AsPointer(in HostServices.Vftbl));
  
  private readonly unsafe struct ReferenceTrackerHostObject
  {
      public readonly IReferenceTrackerHostVftbl* Vftbl;
  }

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[jkotas]

@copilot

[Copilot]

@copilot

Addressed in 721b060.

Note

This comment was created by GitHub Copilot.

[Copilot]

Pull request overview

This PR adjusts how ReferenceTrackerHost stores the global COM host “this” pointer so that the fixed-address storage is applied to a non-primitive value type while preserving the native layout expected by IReferenceTrackerManager.SetReferenceTrackerHost.

Changes:

• Replaced the fixed-address static IntPtr used as the host object with a single-field value-type instance holding the vtable pointer.
• Updated SetReferenceTrackerHost to pass the address of that fixed instance to the native SetReferenceTrackerHost call.

[hamarb123]

(Btw, you can also just use ValueTuple<IntPtr> - would give a smaller diff of +2/-2)

[jkotas]

ValueTuple has large set of dependencies. It comes with code bloat risk. Avoiding it saves you from having to worry about it in code that's likely included in minimal apps that we track naot binary sizes for.