Optimize '[Guid]' parsing on Native AOT

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Reflection/Runtime/TypeInfos/NativeFormat/NativeFormatRuntimeNamedTypeInfo.cs

@@ -79,7 +79,11 @@ public sealed override bool IsByRefLike
                         continue;
                     if (guidStringArgumentHandle.HandleType != HandleType.ConstantStringValue)
                         continue;
-                    return new Guid(guidStringArgumentHandle.ToConstantStringValueHandle(_reader).GetString(_reader));
+
+                    ConstantStringValueHandle constantStringValueHandle = guidStringArgumentHandle.ToConstantStringValueHandle(_reader);
+
+                    // Parse a 'Guid' directly from the encoded UTF8 buffer, instead of round-tripping through a 'string'
+                    return Guid.Parse(_reader.ReadStringAsBytes(constantStringValueHandle));
                 }
             }
             return null;

src/coreclr/tools/Common/Internal/Metadata/NativeFormat/NativeMetadataReader.cs

@@ -220,6 +220,16 @@ internal bool StringEquals(ConstantStringValueHandle handle, string value)
         {
             return _streamReader.StringEquals((uint)handle.Offset, value);
         }
+
+        internal ReadOnlySpan<byte> ReadStringAsBytes(ConstantStringValueHandle handle)
+        {
+            if (handle.IsNil)
+            {
+                return ReadOnlySpan<byte>.Empty;
+            }
+
+            return _streamReader.ReadStringAsBytes((uint)handle.Offset);
+        }
     }
 
     internal sealed partial class MetadataHeader

src/coreclr/tools/Common/Internal/NativeFormat/NativeFormatReader.String.cs

@@ -7,6 +7,7 @@
 // UTF8 string reading methods
 // ---------------------------------------------------------------------------
 
+using System;
 using System.Text;
 
 namespace Internal.NativeFormat
@@ -35,6 +36,23 @@ public string ReadString(uint offset)
             return value;
         }
 
+        public unsafe ReadOnlySpan<byte> ReadStringAsBytes(uint offset)
+        {
+            uint numBytes;
+            offset = DecodeUnsigned(offset, out numBytes);
+
+            if (numBytes != 0)
+            {
+                uint endOffset = offset + numBytes;
+                if (endOffset < numBytes || endOffset > _size)
+                    ThrowBadImageFormatException();
+
+                return new(_base + offset, (int)numBytes);
+            }
+
+            return ReadOnlySpan<byte>.Empty;
+        }
+
         public unsafe uint DecodeString(uint offset, out string value)
         {
             uint numBytes;