Analyze implicit indexer reference operations (#2839)

Adds support for `IImplicitIndexerReferenceOperation`, which
represents an implicit access to an indexer that uses
`System.Index`. Implicit means that there is no `System.Index`
accessor in IL, but the compiler supports `System.Index` access
via an existing indexer (for example one that takes int) for
types that satisfy certain criteria. See
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-8.0/ranges#implicit-index-support
for details.

This operation only showed up in the CFG with a more recent
version of the Roslyn APIs, so this includes an update to match
the one used in dotnet/runtime. This resulted in a few changes to
the generated code that required tweaking some of the test
validation:

- Delegate cache fields are emitted. This reuses an existing
attribute originally designed for mcs, but it has been updated to
check for these compiler-generated fields.

- `<PrivateImplementationDetails>.ThrowSwitchExpressionException`
is emitted for an implicit unhandled case in switch
expressions. This change includes a new attribute to check that
this is kept in a few of the tests.

15 files changed, 99 insertions, 11 deletions

diff --git a/test/Mono.Linker.Tests.Cases/Attributes/Csc/OnlyTypeUsedInAssemblyIsTypeOnAttributeCtorOnEvent.cs b/test/Mono.Linker.Tests.Cases/Attributes/Csc/OnlyTypeUsedInAssemblyIsTypeOnAttributeCtorOnEvent.cs
index 6b9c5ff8e..eaf9a4866 100644
--- a/test/Mono.Linker.Tests.Cases/Attributes/Csc/OnlyTypeUsedInAssemblyIsTypeOnAttributeCtorOnEvent.cs
+++ b/test/Mono.Linker.Tests.Cases/Attributes/Csc/OnlyTypeUsedInAssemblyIsTypeOnAttributeCtorOnEvent.cs
@@ -14,7 +14,7 @@ namespace Mono.Linker.Tests.Cases.Attributes.Csc
 	[KeptTypeInAssembly ("LibraryWithType.dll", typeof (TypeDefinedInReference))]
 	[RemovedMemberInAssembly ("LibraryWithType.dll", typeof (TypeDefinedInReference), "Unused()")]
 	[KeptMemberInAssembly ("LibraryWithAttribute.dll", typeof (AttributeDefinedInReference), ".ctor(System.Type)")]
-	[KeptDelegateCacheField ("0")]
+	[KeptDelegateCacheField ("0", nameof (FooOnMyEvent))]
 	public class OnlyTypeUsedInAssemblyIsTypeOnAttributeCtorOnEvent
 	{
 		public static void Main ()
diff --git a/test/Mono.Linker.Tests.Cases/Attributes/Csc/OnlyTypeUsedInAssemblyIsTypeOnAttributeFieldOnEvent.cs b/test/Mono.Linker.Tests.Cases/Attributes/Csc/OnlyTypeUsedInAssemblyIsTypeOnAttributeFieldOnEvent.cs
index 9026e6048..d069585c0 100644
--- a/test/Mono.Linker.Tests.Cases/Attributes/Csc/OnlyTypeUsedInAssemblyIsTypeOnAttributeFieldOnEvent.cs
+++ b/test/Mono.Linker.Tests.Cases/Attributes/Csc/OnlyTypeUsedInAssemblyIsTypeOnAttributeFieldOnEvent.cs
@@ -15,7 +15,7 @@ namespace Mono.Linker.Tests.Cases.Attributes.Csc
 	[RemovedMemberInAssembly ("LibraryWithType.dll", typeof (TypeDefinedInReference), "Unused()")]
 	[KeptMemberInAssembly ("LibraryWithAttribute.dll", typeof (AttributeDefinedInReference), ".ctor()")]
 	[KeptMemberInAssembly ("LibraryWithAttribute.dll", typeof (AttributeDefinedInReference), "FieldType")]
-	[KeptDelegateCacheField ("0")]
+	[KeptDelegateCacheField ("0", nameof (FooOnMyEvent))]
 	public class OnlyTypeUsedInAssemblyIsTypeOnAttributeFieldOnEvent
 	{
 		public static void Main ()
diff --git a/test/Mono.Linker.Tests.Cases/Attributes/Csc/OnlyTypeUsedInAssemblyIsTypeOnAttributePropertyOnEvent.cs b/test/Mono.Linker.Tests.Cases/Attributes/Csc/OnlyTypeUsedInAssemblyIsTypeOnAttributePropertyOnEvent.cs
index 5f1791948..83466e847 100644
--- a/test/Mono.Linker.Tests.Cases/Attributes/Csc/OnlyTypeUsedInAssemblyIsTypeOnAttributePropertyOnEvent.cs
+++ b/test/Mono.Linker.Tests.Cases/Attributes/Csc/OnlyTypeUsedInAssemblyIsTypeOnAttributePropertyOnEvent.cs
@@ -15,7 +15,7 @@ namespace Mono.Linker.Tests.Cases.Attributes.Csc
 	[RemovedMemberInAssembly ("LibraryWithType.dll", typeof (TypeDefinedInReference), "Unused()")]
 	[KeptMemberInAssembly ("LibraryWithAttribute.dll", typeof (AttributeDefinedInReference), ".ctor()")]
 	[KeptMemberInAssembly ("LibraryWithAttribute.dll", typeof (AttributeDefinedInReference), "set_PropertyType(System.Type)")]
-	[KeptDelegateCacheField ("0")]
+	[KeptDelegateCacheField ("0", nameof (FooOnMyEvent))]
 	public class OnlyTypeUsedInAssemblyIsTypeOnAttributePropertyOnEvent
 	{
 		public static void Main ()
diff --git a/test/Mono.Linker.Tests.Cases/Attributes/OnlyKeepUsed/UnusedAttributeTypeOnEventIsRemoved.cs b/test/Mono.Linker.Tests.Cases/Attributes/OnlyKeepUsed/UnusedAttributeTypeOnEventIsRemoved.cs
index ff8775974..c05e9ad30 100644
--- a/test/Mono.Linker.Tests.Cases/Attributes/OnlyKeepUsed/UnusedAttributeTypeOnEventIsRemoved.cs
+++ b/test/Mono.Linker.Tests.Cases/Attributes/OnlyKeepUsed/UnusedAttributeTypeOnEventIsRemoved.cs
@@ -5,7 +5,7 @@ using Mono.Linker.Tests.Cases.Expectations.Metadata;
 namespace Mono.Linker.Tests.Cases.Attributes.OnlyKeepUsed
 {
 	[SetupLinkerArgument ("--used-attrs-only", "true")]
-	[KeptDelegateCacheField ("0")]
+	[KeptDelegateCacheField ("0", nameof (Tmp_Something))]
 	class UnusedAttributeTypeOnEventIsRemoved
 	{
 		static void Main ()
diff --git a/test/Mono.Linker.Tests.Cases/Attributes/OnlyKeepUsed/UsedAttributeTypeOnEventIsKept.cs b/test/Mono.Linker.Tests.Cases/Attributes/OnlyKeepUsed/UsedAttributeTypeOnEventIsKept.cs
index 27d50d3cd..0a5e3cfe3 100644
--- a/test/Mono.Linker.Tests.Cases/Attributes/OnlyKeepUsed/UsedAttributeTypeOnEventIsKept.cs
+++ b/test/Mono.Linker.Tests.Cases/Attributes/OnlyKeepUsed/UsedAttributeTypeOnEventIsKept.cs
@@ -5,7 +5,7 @@ using Mono.Linker.Tests.Cases.Expectations.Metadata;
 namespace Mono.Linker.Tests.Cases.Attributes.OnlyKeepUsed
 {
 	[SetupLinkerArgument ("--used-attrs-only", "true")]
-	[KeptDelegateCacheField ("0")]
+	[KeptDelegateCacheField ("0", nameof (Tmp_Something))]
 	class UsedAttributeTypeOnEventIsKept
 	{
 		static void Main ()
diff --git a/test/Mono.Linker.Tests.Cases/Basic/DelegateBeginInvokeEndInvokePair.cs b/test/Mono.Linker.Tests.Cases/Basic/DelegateBeginInvokeEndInvokePair.cs
index d39ab89d9..99e8e90ae 100644
--- a/test/Mono.Linker.Tests.Cases/Basic/DelegateBeginInvokeEndInvokePair.cs
+++ b/test/Mono.Linker.Tests.Cases/Basic/DelegateBeginInvokeEndInvokePair.cs
@@ -5,6 +5,9 @@ using Mono.Linker.Tests.Cases.Expectations.Assertions;
 
 namespace Mono.Linker.Tests.Cases.Basic
 {
+	[KeptDelegateCacheField ("0", nameof (Method))]
+	[KeptDelegateCacheField ("1", nameof (Method))]
+	[KeptDelegateCacheField ("2", nameof (Method))]
 	class DelegateBeginInvokeEndInvokePair
 	{
 		public static void Main ()
diff --git a/test/Mono.Linker.Tests.Cases/Basic/UsedEventIsKept.cs b/test/Mono.Linker.Tests.Cases/Basic/UsedEventIsKept.cs
index 8453f1624..490ffa348 100644
--- a/test/Mono.Linker.Tests.Cases/Basic/UsedEventIsKept.cs
+++ b/test/Mono.Linker.Tests.Cases/Basic/UsedEventIsKept.cs
@@ -3,8 +3,7 @@ using Mono.Linker.Tests.Cases.Expectations.Assertions;
 
 namespace Mono.Linker.Tests.Cases.Basic
 {
-	[KeptDelegateCacheField ("0")]
-	[KeptDelegateCacheField ("1")]
+	[KeptDelegateCacheField ("0", nameof (Tmp_Bar))]
 	class UsedEventIsKept
 	{
 		public static void Main ()
diff --git a/test/Mono.Linker.Tests.Cases/Basic/UsedEventOnInterfaceIsKept.cs b/test/Mono.Linker.Tests.Cases/Basic/UsedEventOnInterfaceIsKept.cs
index 099b92ab2..47e45e75e 100644
--- a/test/Mono.Linker.Tests.Cases/Basic/UsedEventOnInterfaceIsKept.cs
+++ b/test/Mono.Linker.Tests.Cases/Basic/UsedEventOnInterfaceIsKept.cs
@@ -3,7 +3,7 @@ using Mono.Linker.Tests.Cases.Expectations.Assertions;
 
 namespace Mono.Linker.Tests.Cases.Basic
 {
-	[KeptDelegateCacheField ("0")]
+	[KeptDelegateCacheField ("0", nameof (Bar_Ping))]
 	class UsedEventOnInterfaceIsKept
 	{
 		static void Main ()
diff --git a/test/Mono.Linker.Tests.Cases/Basic/UsedEventOnInterfaceIsRemovedWhenUsedFromClass.cs b/test/Mono.Linker.Tests.Cases/Basic/UsedEventOnInterfaceIsRemovedWhenUsedFromClass.cs
index 464db7008..10555f041 100644
--- a/test/Mono.Linker.Tests.Cases/Basic/UsedEventOnInterfaceIsRemovedWhenUsedFromClass.cs
+++ b/test/Mono.Linker.Tests.Cases/Basic/UsedEventOnInterfaceIsRemovedWhenUsedFromClass.cs
@@ -3,7 +3,7 @@ using Mono.Linker.Tests.Cases.Expectations.Assertions;
 
 namespace Mono.Linker.Tests.Cases.Basic
 {
-	[KeptDelegateCacheField ("0")]
+	[KeptDelegateCacheField ("0", nameof (Bar_Ping))]
 	class UsedEventOnInterfaceIsRemovedWhenUsedFromClass
 	{
 		static void Main ()
diff --git a/test/Mono.Linker.Tests.Cases/DataFlow/NullableAnnotations.cs b/test/Mono.Linker.Tests.Cases/DataFlow/NullableAnnotations.cs
index d19a1fb82..90c24fd9f 100644
--- a/test/Mono.Linker.Tests.Cases/DataFlow/NullableAnnotations.cs
+++ b/test/Mono.Linker.Tests.Cases/DataFlow/NullableAnnotations.cs
@@ -11,6 +11,7 @@ using DAMT = System.Diagnostics.CodeAnalysis.DynamicallyAccessedMemberTypes;
 namespace Mono.Linker.Tests.Cases.DataFlow
 {
 	[ExpectedNoWarnings]
+	[KeptPrivateImplementationDetails ("ThrowSwitchExpressionException")]
 	class NullableAnnotations
 	{
 		[Kept]
diff --git a/test/Mono.Linker.Tests.Cases/DataFlow/PropertyDataFlow.cs b/test/Mono.Linker.Tests.Cases/DataFlow/PropertyDataFlow.cs
index 2128b8e41..15508baca 100644
--- a/test/Mono.Linker.Tests.Cases/DataFlow/PropertyDataFlow.cs
+++ b/test/Mono.Linker.Tests.Cases/DataFlow/PropertyDataFlow.cs
@@ -45,6 +45,9 @@ namespace Mono.Linker.Tests.Cases.DataFlow
 
 			BasePropertyAccess.Test ();
 			AccessReturnedInstanceProperty.Test ();
+
+			ExplicitIndexerAccess.Test ();
+			ImplicitIndexerAccess.Test ();
 		}
 
 		[DynamicallyAccessedMembers (DynamicallyAccessedMemberTypes.PublicConstructors)]
@@ -653,6 +656,85 @@ namespace Mono.Linker.Tests.Cases.DataFlow
 			}
 		}
 
+		class ExplicitIndexerAccess
+		{
+			[DynamicallyAccessedMembers (DynamicallyAccessedMemberTypes.PublicMethods)]
+			Type this[Index idx] {
+				get => throw new NotImplementedException ();
+				set => throw new NotImplementedException ();
+			}
+
+			[ExpectedWarning ("IL2072", "this[Index].get", nameof (DataFlowTypeExtensions.RequiresAll), ProducedBy = ProducedBy.Analyzer)]
+			[ExpectedWarning ("IL2072", "Item.get", nameof (DataFlowTypeExtensions.RequiresAll), ProducedBy = ProducedBy.Trimmer)]
+			static void TestRead (ExplicitIndexerAccess instance = null)
+			{
+				instance[new Index (1)].RequiresAll ();
+			}
+
+			[ExpectedWarning ("IL2072", nameof (GetTypeWithPublicConstructors), "this[Index].set", ProducedBy = ProducedBy.Analyzer)]
+			[ExpectedWarning ("IL2072", nameof (GetTypeWithPublicConstructors), "Item.set", ProducedBy = ProducedBy.Trimmer)]
+			static void TestWrite (ExplicitIndexerAccess instance = null)
+			{
+				instance[^1] = GetTypeWithPublicConstructors ();
+			}
+
+			public static void Test ()
+			{
+				TestRead ();
+				TestWrite ();
+			}
+		}
+
+		class ImplicitIndexerAccess
+		{
+			[DynamicallyAccessedMembers (DynamicallyAccessedMemberTypes.PublicMethods)]
+			Type this[int idx] {
+				get => throw new NotImplementedException ();
+				set => throw new NotImplementedException ();
+			}
+
+			int Length => throw new NotImplementedException ();
+
+			[ExpectedWarning ("IL2072", "this[Int32].get", nameof (DataFlowTypeExtensions.RequiresAll), ProducedBy = ProducedBy.Analyzer)]
+			[ExpectedWarning ("IL2072", "Item.get", nameof (DataFlowTypeExtensions.RequiresAll), ProducedBy = ProducedBy.Trimmer)]
+			static void TestRead (ImplicitIndexerAccess instance = null)
+			{
+				instance[new Index (1)].RequiresAll ();
+			}
+
+			[ExpectedWarning ("IL2072", nameof (GetTypeWithPublicConstructors), "this[Int32].set", ProducedBy = ProducedBy.Analyzer)]
+			[ExpectedWarning ("IL2072", nameof (GetTypeWithPublicConstructors), "Item.set", ProducedBy = ProducedBy.Trimmer)]
+			static void TestWrite (ImplicitIndexerAccess instance = null)
+			{
+				instance[^1] = GetTypeWithPublicConstructors ();
+			}
+
+			[ExpectedWarning ("IL2072", nameof (GetUnknownType), "this[Int32].set", ProducedBy = ProducedBy.Analyzer)]
+			[ExpectedWarning ("IL2072", nameof (GetUnknownType), "Item.set", ProducedBy = ProducedBy.Trimmer)]
+			static void TestNullCoalescingAssignment (ImplicitIndexerAccess instance = null)
+			{
+				instance[new Index (1)] ??= GetUnknownType ();
+			}
+
+			[ExpectedWarning ("IL2062", nameof (DataFlowTypeExtensions.RequiresAll))]
+			static void TestSpanIndexerAccess (int start = 0, int end = 3)
+			{
+				Span<byte> bytes = stackalloc byte[4] { 1, 2, 3, 4 };
+				bytes[^4] = 0; // This calls the get indexer which has a ref return.
+				int index = bytes[0];
+				Type[] types = new Type[] { GetUnknownType () };
+				types[index].RequiresAll ();
+			}
+
+			public static void Test ()
+			{
+				TestRead ();
+				TestWrite ();
+				TestNullCoalescingAssignment ();
+				TestSpanIndexerAccess ();
+			}
+		}
+
 		[return: DynamicallyAccessedMembers (DynamicallyAccessedMemberTypes.PublicParameterlessConstructor)]
 		private static Type GetTypeWithPublicParameterlessConstructor ()
 		{
diff --git a/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/OnReferenceType/BaseProvidesInterfaceMember/GenericInterfaceWithEvent.cs b/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/OnReferenceType/BaseProvidesInterfaceMember/GenericInterfaceWithEvent.cs
index e8856be3b..ec4a75670 100644
--- a/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/OnReferenceType/BaseProvidesInterfaceMember/GenericInterfaceWithEvent.cs
+++ b/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/OnReferenceType/BaseProvidesInterfaceMember/GenericInterfaceWithEvent.cs
@@ -2,7 +2,7 @@ using Mono.Linker.Tests.Cases.Expectations.Assertions;
 
 namespace Mono.Linker.Tests.Cases.Inheritance.Interfaces.OnReferenceType.BaseProvidesInterfaceMember
 {
-	[KeptDelegateCacheField ("0")]
+	[KeptDelegateCacheField ("0", nameof (EventMethod))]
 	public class GenericInterfaceWithEvent
 	{
 		public static void Main ()
diff --git a/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/OnReferenceType/BaseProvidesInterfaceMember/SimpleEvent.cs b/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/OnReferenceType/BaseProvidesInterfaceMember/SimpleEvent.cs
index 11e5860a6..98597a4b2 100644
--- a/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/OnReferenceType/BaseProvidesInterfaceMember/SimpleEvent.cs
+++ b/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/OnReferenceType/BaseProvidesInterfaceMember/SimpleEvent.cs
@@ -2,7 +2,7 @@ using Mono.Linker.Tests.Cases.Expectations.Assertions;
 
 namespace Mono.Linker.Tests.Cases.Inheritance.Interfaces.OnReferenceType.BaseProvidesInterfaceMember
 {
-	[KeptDelegateCacheField ("0")]
+	[KeptDelegateCacheField ("0", nameof (EventMethod))]
 	public class SimpleEvent
 	{
 		public static void Main ()
diff --git a/test/Mono.Linker.Tests.Cases/Reflection/ExpressionCallString.cs b/test/Mono.Linker.Tests.Cases/Reflection/ExpressionCallString.cs
index 10ac90435..5489497a8 100644
--- a/test/Mono.Linker.Tests.Cases/Reflection/ExpressionCallString.cs
+++ b/test/Mono.Linker.Tests.Cases/Reflection/ExpressionCallString.cs
@@ -10,6 +10,7 @@ namespace Mono.Linker.Tests.Cases.Reflection
 	[SetupCSharpCompilerToUse ("csc")]
 	[Reference ("System.Core.dll")]
 	[ExpectedNoWarnings]
+	[KeptPrivateImplementationDetails ("ThrowSwitchExpressionException")]
 	public class ExpressionCallString
 	{
 		public static void Main ()
diff --git a/test/Mono.Linker.Tests.Cases/Reflection/TypeUsedViaReflection.cs b/test/Mono.Linker.Tests.Cases/Reflection/TypeUsedViaReflection.cs
index 906f56ab4..5bec60d50 100644
--- a/test/Mono.Linker.Tests.Cases/Reflection/TypeUsedViaReflection.cs
+++ b/test/Mono.Linker.Tests.Cases/Reflection/TypeUsedViaReflection.cs
@@ -9,6 +9,8 @@ namespace Mono.Linker.Tests.Cases.Reflection
 {
 	[KeptMember (".cctor()")]
 	[ExpectedNoWarnings ()]
+	[KeptDelegateCacheField ("0", nameof (AssemblyResolver))]
+	[KeptDelegateCacheField ("1", nameof (GetTypeFromAssembly))]
 	public class TypeUsedViaReflection
 	{
 		public static void Main ()