[cDAC] GcScanner stack-ref scanning gaps vs native DAC

[max-charlamb]

Gaps vs native DAC

1. Embedded GC refs in large value types passed by value

• Native: MetaSig::GcScanRoots ELEMENT_TYPE_VALUETYPE case
  (src/coreclr/vm/siginfo.cpp:5437-5457) -> ReportPointersFromValueTypeArg -> walks the
  type's CGCDesc series and reports each embedded GC ref.
• cDAC today: GcTypeKind.Other parameters are silently skipped
  (src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackWalk/GC/GcScanner.cs).
• Where to implement: new helper in GcScanner.cs (or sibling ValueTypeGcWalker.cs)
  consuming a new IRuntimeTypeSystem.GetGCDescSeries(typeHandle) accessor. Requires
  GcSignatureTypeProvider / decoder consumers to surface the TypeHandle alongside
  GcTypeKind.Other so the walker has the type to enumerate. The ArgIterator port already
  provides IsArgPassedByRef, which is the first branch native takes here.

2. String..ctor this-slot handling

• Native: src/coreclr/vm/frames.cpp:1523-1525:

  if (msig.HasThis() && pFunction->GetMethodTable() == g_pStringClass && pFunction->IsCtor())
      msig.ClearHasThis();

  The caller does not pass this for String..ctor; the runtime allocates and overwrites it.
• cDAC today: no special case; the slot is treated as a tracked ref.
• Where to implement: GcScanner.PromoteCallerStack (around the existing hasThis
  capture). Needs IRuntimeTypeSystem accessors for the well-known String MethodTable
  and an IsCtor predicate on MethodDesc.

3. SuppressParamTypeArg

• Native: src/coreclr/vm/frames.cpp:1527-1528:

  if (pFunction->RequiresInstArg() && !SuppressParamTypeArg())
      msig.SetHasParamTypeArg();

  Per-Frame-subclass virtual; some frame kinds suppress the inst-arg slot.
• cDAC today: RequiresInstArg is honored unconditionally in GcScanner.PromoteCallerStack.
• Where to implement: extend the frame-kind table in the StackWalk contract with a
  SuppressParamTypeArg bool per FrameKind, then in GcScanner.PromoteCallerStack:
  requiresInstArg = rts.RequiresInstArg(mdh) && !frameSuppressesParamTypeArg;.

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