Don't track current field of state machines

src/linker/Linker.Dataflow/CompilerGeneratedNames.cs

@@ -34,6 +34,19 @@ internal static bool IsStateMachineType (string typeName)
 			return typeName.Length > i + 1 && typeName[i + 1] == 'd';
 		}
 
+		internal static bool IsStateMachineCurrentField (string fieldName)
+		{
+			if (!IsGeneratedMemberName (fieldName))
+				return false;
+
+			int i = fieldName.LastIndexOf ('>');
+			if (i == -1)
+				return false;
+
+			// Current field is <>2__current
+			return fieldName.Length > i + 1 && fieldName[i + 1] == '2';
+		}
+
 		internal static bool IsGeneratedType (string name) => IsStateMachineType (name) || IsLambdaDisplayClass (name);
 
 		internal static bool IsLambdaOrLocalFunction (string methodName) => IsLambdaMethod (methodName) || IsLocalFunction (methodName);

src/linker/Linker.Dataflow/CompilerGeneratedState.cs

@@ -55,10 +55,16 @@ static IEnumerable<TypeDefinition> GetCompilerGeneratedNestedTypes (TypeDefiniti
 
 		public static bool IsHoistedLocal (FieldDefinition field)
 		{
-			// Treat all fields on compiler-generated types as hoisted locals.
-			// This avoids depending on the name mangling scheme for hoisted locals.
-			var declaringTypeName = field.DeclaringType.Name;
-			return CompilerGeneratedNames.IsLambdaDisplayClass (declaringTypeName) || CompilerGeneratedNames.IsStateMachineType (declaringTypeName);
+			if (CompilerGeneratedNames.IsLambdaDisplayClass (field.DeclaringType.Name))
+				return true;
+
+			if (CompilerGeneratedNames.IsStateMachineType (field.DeclaringType.Name)) {
+				// Don't track the "current" field which is used for state machine return values,
+				// because this can be expensive to track.
+				return !CompilerGeneratedNames.IsStateMachineCurrentField (field.Name);
+			}
+
+			return false;
 		}
 
 		// "Nested function" refers to lambdas and local functions.

test/Mono.Linker.Tests.Cases/DataFlow/CompilerGeneratedCodeDataflow.cs

@@ -210,8 +210,8 @@ public static void Test ()
 				FlowParameterAcrossYieldReturn ();
 				FlowUnannotatedParameterAcrossYieldReturn ();
 				FlowAcrossYieldReturnWithBackwardsBranch ();
-				
-				foreach (var o in ReturnManyObjects ());
+
+				foreach (var o in ReturnManyObjects ()) ;
 			}
 		}
 

test/Mono.Linker.Tests/TestCasesRunner/ILVerifier.cs

@@ -14,13 +14,13 @@
 #nullable enable
 namespace Mono.Linker.Tests.TestCasesRunner
 {
-	class ILVerifier : ILVerify.IResolver
+	sealed class ILVerifier : ILVerify.IResolver
 	{
-		Verifier _verifier;
-		NPath _assemblyFolder;
-		NPath _frameworkFolder;
-		Dictionary<string, PEReader> _assemblyCache;
-		AssemblyLoadContext _alc;
+		readonly Verifier _verifier;
+		readonly NPath _assemblyFolder;
+		readonly NPath _frameworkFolder;
+		readonly Dictionary<string, PEReader> _assemblyCache;
+		readonly AssemblyLoadContext _alc;
 
 		public IEnumerable<VerificationResult> Results { get; private set; }
 
@@ -123,7 +123,7 @@ bool TryLoadAssemblyFromFolder (string assemblyName, NPath folder, [NotNullWhen
 		PEReader? ILVerify.IResolver.ResolveModule (AssemblyName referencingModule, string fileName)
 			=> Resolve (Path.GetFileNameWithoutExtension (fileName));
 
-		public string GetErrorMessage (VerificationResult result)
+		public static string GetErrorMessage (VerificationResult result)
 		{
 			return $"IL Verification error:\n{result.Message}";
 		}

test/Mono.Linker.Tests/TestCasesRunner/ResultChecker.cs

@@ -55,7 +55,7 @@ static void VerifyIL (NPath pathToAssembly)
 			foreach (var result in verifier.Results) {
 				if (result.Code == ILVerify.VerifierError.None)
 					continue;
-				Assert.Fail (verifier.GetErrorMessage (result));
+				Assert.Fail (ILVerifier.GetErrorMessage (result));
 			}
 		}
 
@@ -1122,7 +1122,7 @@ static bool TryGetCustomAttribute (ICustomAttributeProvider caProvider, string a
 			return false;
 		}
 
-		static IEnumerable<CustomAttribute> GetCustomAttributes (ICustomAttributeProvider caProvider, string attributeName )
+		static IEnumerable<CustomAttribute> GetCustomAttributes (ICustomAttributeProvider caProvider, string attributeName)
 		{
 			if (caProvider is AssemblyDefinition assembly && assembly.EntryPoint != null)
 				return assembly.EntryPoint.DeclaringType.CustomAttributes