WIP: RuntimeSignatureDecoder, centralized SignatureDecoder contract

- Move RuntimeSignatureDecoder to Contracts/Signature/
- Move GcSignatureTypeProvider to Contracts/Signature/ with module-scoped caching
- Add DecodeMethodSignatureForGC(BlobHandle, ModuleHandle) to ISignatureDecoder
- Add DecodeFieldSignature to RuntimeSignatureDecoder
- Add BlobHandleSignatureReader for lazy blob reading
- SignatureTypeProvider implements IRuntimeSignatureTypeProvider
- Switch DecodeFieldSignature to use RuntimeSignatureDecoder
- GcScanner uses contract API instead of direct decoder construction

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Max Charlamb

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Abstractions/Contracts/ISignatureDecoder.cs

@@ -6,10 +6,33 @@
 
 namespace Microsoft.Diagnostics.DataContractReader.Contracts;
 
+/// <summary>
+/// Classification of a signature type for GC scanning purposes.
+/// </summary>
+public enum GcTypeKind
+{
+    /// <summary>Not a GC reference (primitives, pointers).</summary>
+    None,
+    /// <summary>Object reference (class, string, array).</summary>
+    Ref,
+    /// <summary>Interior pointer (byref).</summary>
+    Interior,
+    /// <summary>Value type that may contain embedded GC references.</summary>
+    Other,
+}
+
 public interface ISignatureDecoder : IContract
 {
     static string IContract.Name { get; } = nameof(SignatureDecoder);
+
     TypeHandle DecodeFieldSignature(BlobHandle blobHandle, ModuleHandle moduleHandle, TypeHandle ctx) => throw new NotImplementedException();
+
+    /// <summary>
+    /// Decodes a method's signature for GC scanning purposes, classifying each parameter
+    /// as a GC reference, interior pointer, value type, or non-GC type.
+    /// Handles ELEMENT_TYPE_INTERNAL via the runtime type system.
+    /// </summary>
+    MethodSignature<GcTypeKind> DecodeMethodSignatureForGC(BlobHandle blobHandle, ModuleHandle moduleHandle) => throw new NotImplementedException();
 }
 
 public readonly struct SignatureDecoder : ISignatureDecoder

…/StackWalk/GC/GcSignatureTypeProvider.cs …cts/Signature/GcSignatureTypeProvider.cssrc/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackWalk/GC/GcSignatureTypeProvider.cs renamed to src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/Signature/GcSignatureTypeProvider.cs src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/StackWalk/GC/GcSignatureTypeProvider.cs renamed to src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/Signature/GcSignatureTypeProvider.cs

@@ -3,33 +3,27 @@
 
 using System.Collections.Immutable;
 using System.Reflection.Metadata;
+using Microsoft.Diagnostics.DataContractReader.Contracts;
 
-namespace Microsoft.Diagnostics.DataContractReader.Contracts.StackWalkHelpers;
-
-/// <summary>
-/// Classification of a signature type for GC scanning purposes.
-/// </summary>
-internal enum GcTypeKind
-{
-    /// <summary>Not a GC reference (primitives, pointers).</summary>
-    None,
-    /// <summary>Object reference (class, string, array).</summary>
-    Ref,
-    /// <summary>Interior pointer (byref).</summary>
-    Interior,
-    /// <summary>Value type that may contain embedded GC references.</summary>
-    Other,
-}
+namespace Microsoft.Diagnostics.DataContractReader.SignatureHelpers;
 
 /// <summary>
 /// Classifies signature types for GC scanning purposes.
-/// Implements <see cref="ISignatureTypeProvider{TType, TGenericContext}"/> for use
-/// with SRM's <see cref="SignatureDecoder{TType, TGenericContext}"/>.
+/// Implements <see cref="IRuntimeSignatureTypeProvider{TType, TGenericContext}"/> which
+/// is a superset of SRM's <see cref="ISignatureTypeProvider{TType, TGenericContext}"/>,
+/// adding support for <c>ELEMENT_TYPE_INTERNAL</c>.
 /// </summary>
 internal sealed class GcSignatureTypeProvider
-    : ISignatureTypeProvider<GcTypeKind, object?>
+    : IRuntimeSignatureTypeProvider<GcTypeKind, object?>
 {
-    public static readonly GcSignatureTypeProvider Instance = new();
+    private readonly Target _target;
+    private readonly ModuleHandle _moduleHandle;
+
+    public GcSignatureTypeProvider(Target target, ModuleHandle moduleHandle)
+    {
+        _target = target;
+        _moduleHandle = moduleHandle;
+    }
 
     public GcTypeKind GetPrimitiveType(PrimitiveTypeCode typeCode)
         => typeCode switch
@@ -60,5 +54,41 @@ public GcTypeKind GetGenericInstantiation(GcTypeKind genericType, ImmutableArray
     public GcTypeKind GetGenericTypeParameter(object? genericContext, int index) => GcTypeKind.Ref;
     public GcTypeKind GetFunctionPointerType(MethodSignature<GcTypeKind> signature) => GcTypeKind.None;
     public GcTypeKind GetModifiedType(GcTypeKind modifier, GcTypeKind unmodifiedType, bool isRequired) => unmodifiedType;
+    public GcTypeKind GetInternalModifiedType(Target target, TargetPointer typeHandlePointer, GcTypeKind unmodifiedType, bool isRequired) => unmodifiedType;
     public GcTypeKind GetPinnedType(GcTypeKind elementType) => elementType;
+
+    public GcTypeKind GetInternalType(Target target, TargetPointer typeHandlePointer)
+    {
+        if (typeHandlePointer == TargetPointer.Null)
+            return GcTypeKind.None;
+
+        try
+        {
+            IRuntimeTypeSystem rts = target.Contracts.RuntimeTypeSystem;
+            TypeHandle th = rts.GetTypeHandle(typeHandlePointer);
+            CorElementType corType = rts.GetSignatureCorElementType(th);
+
+            return corType switch
+            {
+                CorElementType.Void or CorElementType.Boolean or CorElementType.Char
+                    or CorElementType.I1 or CorElementType.U1
+                    or CorElementType.I2 or CorElementType.U2
+                    or CorElementType.I4 or CorElementType.U4
+                    or CorElementType.I8 or CorElementType.U8
+                    or CorElementType.R4 or CorElementType.R8
+                    or CorElementType.I or CorElementType.U
+                    or CorElementType.FnPtr or CorElementType.Ptr
+                    => GcTypeKind.None,
+
+                CorElementType.Byref => GcTypeKind.Interior,
+                CorElementType.ValueType => GcTypeKind.Other,
+
+                _ => GcTypeKind.Ref,
+            };
+        }
+        catch
+        {
+            return GcTypeKind.Ref;
+        }
+    }
 }