Delete StructLayoutAttribute.Pack adjustment

This adjustment was trying to minic adjustment performed by type loader, but the logic has been out-of-sync with the type loader evolution for years.
It does not make sense for custom attribute reading via reflection to perform this adjustment.

Fixes dotnet#12480

src/coreclr/System.Private.CoreLib/src/System/Reflection/RuntimeCustomAttributeData.cs

@@ -1873,12 +1873,6 @@ internal static bool IsDefined(RuntimeFieldInfo field, RuntimeType? caType)
  }
  type.GetRuntimeModule().MetadataImport.GetClassLayout(type.MetadataToken, out int pack, out int size);
 
- // Metadata parameter checking should not have allowed 0 for packing size.
- // The runtime later converts a packing size of 0 to 8 so do the same here
- // because it's more useful from a user perspective.
- if (pack == 0)
- pack = 8; // DEFAULT_PACKING_SIZE
-
  StructLayoutAttribute attribute = new StructLayoutAttribute(layoutKind);
 
  attribute.Pack = pack;

src/coreclr/nativeaot/System.Private.Reflection.Core/src/System/Reflection/Runtime/TypeInfos/RuntimeNamedTypeInfo.cs

@@ -129,8 +129,6 @@ public sealed override StructLayoutAttribute StructLayoutAttribute
  {
  get
  {
- const int DefaultPackingSize = 8;
-
  // Note: CoreClr checks HasElementType and IsGenericParameter in addition to IsInterface but those properties cannot be true here as this
  // RuntimeTypeInfo subclass is solely for TypeDef types.)
  if (IsInterface)
@@ -156,15 +154,7 @@ public sealed override StructLayoutAttribute StructLayoutAttribute
  default: charSet = CharSet.None; break;
  }
 
- int pack;
- int size;
- GetPackSizeAndSize(out pack, out size);
-
- // Metadata parameter checking should not have allowed 0 for packing size.
- // The runtime later converts a packing size of 0 to 8 so do the same here
- // because it's more useful from a user perspective.
- if (pack == 0)
- pack = DefaultPackingSize;
+ GetPackSizeAndSize(out int pack, out int size);
 
  return new StructLayoutAttribute(layoutKind)
  {

src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/Types/RoDefinitionType.cs

@@ -146,8 +146,6 @@ public sealed override StructLayoutAttribute? StructLayoutAttribute
  {
  get
  {
- const int DefaultPackingSize = 8;
-
  // Note: CoreClr checks HasElementType and IsGenericParameter in addition to IsInterface but those properties cannot be true here as this
  // RoType subclass is solely for TypeDef types.)
  if (IsInterface)
@@ -170,12 +168,6 @@ public sealed override StructLayoutAttribute? StructLayoutAttribute
  };
  GetPackSizeAndSize(out int pack, out int size);
 
- // Metadata parameter checking should not have allowed 0 for packing size.
- // The runtime later converts a packing size of 0 to 8 so do the same here
- // because it's more useful from a user perspective.
- if (pack == 0)
- pack = DefaultPackingSize;
-
  return new StructLayoutAttribute(layoutKind)
  {
  CharSet = charSet,

src/libraries/System.Reflection.MetadataLoadContext/tests/src/Tests/Type/TypeTests.StructLayoutAttribute.cs

@@ -15,7 +15,7 @@ public static void Test_UndecoratedClass()
  StructLayoutAttribute s = t.StructLayoutAttribute;
  Assert.Equal(LayoutKind.Auto, s.Value);
  Assert.Equal(CharSet.Ansi, s.CharSet);
- Assert.Equal(8, s.Pack);
+ Assert.Equal(0, s.Pack);
  Assert.Equal(0, s.Size);
  }
 
@@ -71,7 +71,7 @@ public static void TestSequentialAutoZeroZero()
  StructLayoutAttribute s = t.StructLayoutAttribute;
  Assert.Equal(LayoutKind.Sequential, s.Value);
  Assert.Equal(CharSet.Auto, s.CharSet);
- Assert.Equal(8, s.Pack); // Not an error: Pack=0 is treated as if it were Pack=8.
+ Assert.Equal(0, s.Pack);
  Assert.Equal(0, s.Size);
  }
 
@@ -149,7 +149,7 @@ public static void Test_Derived()
  StructLayoutAttribute s = t.StructLayoutAttribute;
  Assert.Equal(LayoutKind.Auto, s.Value);
  Assert.Equal(CharSet.Ansi, s.CharSet);
- Assert.Equal(8, s.Pack);
+ Assert.Equal(0, s.Pack);
  Assert.Equal(0, s.Size);
  }
 

src/libraries/System.Reflection/tests/TypeInfoTests.cs

@@ -1490,9 +1490,9 @@ public void IsNestedFamORAssem(Type type, bool expected)
  }
 
  [Theory]
- [InlineData(typeof(StructWithoutExplicitStructLayout), LayoutKind.Sequential, CharSet.Ansi, 8)]
+ [InlineData(typeof(StructWithoutExplicitStructLayout), LayoutKind.Sequential, CharSet.Ansi, 0)]
  [InlineData(typeof(StructWithExplicitStructLayout), LayoutKind.Explicit, CharSet.Ansi, 1)]
- [InlineData(typeof(ClassWithoutExplicitStructLayout), LayoutKind.Auto, CharSet.Ansi, 8)]
+ [InlineData(typeof(ClassWithoutExplicitStructLayout), LayoutKind.Auto, CharSet.Ansi, 0)]
  [InlineData(typeof(ClassWithExplicitStructLayout), LayoutKind.Explicit, CharSet.Unicode, 2)]
  public void StructLayoutAttribute(Type type, LayoutKind kind, CharSet charset, int pack)
  {

src/mono/System.Private.CoreLib/src/System/RuntimeType.Mono.cs

@@ -2317,14 +2317,11 @@ public override bool IsSubclassOf(Type type)
  return RuntimeTypeHandle.IsSubclassOf(this, rtType);
  }
 
- private const int DEFAULT_PACKING_SIZE = 8;
-
  internal StructLayoutAttribute? GetStructLayoutAttribute()
  {
  if (IsInterface || HasElementType || IsGenericParameter)
  return null;
 
- int pack, size;
  LayoutKind layoutKind = LayoutKind.Auto;
  switch (Attributes & TypeAttributes.LayoutMask)
  {
@@ -2343,13 +2340,7 @@ public override bool IsSubclassOf(Type type)
  default: break;
  }
 
- GetPacking(out pack, out size);
-
- // Metadata parameter checking should not have allowed 0 for packing size.
- // The runtime later converts a packing size of 0 to 8 so do the same here
- // because it's more useful from a user perspective.
- if (pack == 0)
- pack = DEFAULT_PACKING_SIZE;
+ GetPacking(out int pack, out int size);
 
  return new StructLayoutAttribute(layoutKind) { Pack = pack, Size = size, CharSet = charSet };
  }