Don't track current field of state machines (#2979)

There was a performance problem where we were tracking all
possible returned values throughout the method, since we were
treating them the same as hoisted locals. I'm sure it's still
possible to come up with testcases that show bad performance (for
example assigning many arrays to a hoisted local in the same
iterator), but this should at least fix the immediate issue
encountered in runtime.

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