Add awaiter implementation

Il2CppInterop.Generator/Passes/Pass61ImplementAwaiters.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using System.Text;
+using Il2CppInterop.Generator.Contexts;
+using Il2CppInterop.Generator.Utils;
+using Mono.Cecil;
+using Mono.Cecil.Cil;
+
+namespace Il2CppInterop.Generator.Passes;
+
+internal class Pass61ImplementAwaiters
+{
+    public static void DoPass(RewriteGlobalContext context)
+    {
+        var corlib = context.GetAssemblyByName("mscorlib");
+        var actionUntyped = corlib.GetTypeByName("System.Action");
+
+        var actionConversionUntyped = actionUntyped.NewType.Methods.FirstOrDefault(m => m.Name == "op_Implicit") ?? throw new MissingMethodException("Untyped action conversion");
+
+        foreach (var assemblyContext in context.Assemblies)
+        {
+            // dont actually import the references until they're needed
+            Lazy<TypeReference> actionUntypedRef = new(() => assemblyContext.NewAssembly.MainModule.ImportReference(actionUntyped.OriginalType));
+            Lazy<MethodReference> actionConversionUntypedRef = new(() => assemblyContext.NewAssembly.MainModule.ImportReference(actionConversionUntyped));
+            Lazy<TypeReference> notifyCompletionRef = new(() => assemblyContext.NewAssembly.MainModule.ImportReference(typeof(INotifyCompletion)));
+            var voidRef = assemblyContext.Imports.Module.Void();
+            foreach (var typeContext in assemblyContext.Types)
+            {
+                var interfaceImplementation = typeContext.OriginalType.Interfaces.FirstOrDefault(InterfaceImplementation => InterfaceImplementation.InterfaceType.Name == nameof(INotifyCompletion));
+                if (interfaceImplementation is null)
+                    continue;
+
+                var isGeneric = typeContext.OriginalType.ContainsGenericParameter;
+
+                var awaiterType = typeContext.OriginalType;
+
+                var originalOnComplete = typeContext.TryGetMethodByName(nameof(INotifyCompletion.OnCompleted)) ?? throw new MissingMethodException("Original OnComplete");
+
+                var onCompletedAttr = MethodAttributes.Public | MethodAttributes.Final | MethodAttributes.Virtual | MethodAttributes.HideBySig | MethodAttributes.NewSlot;
+                var onComplete = new MethodDefinition(nameof(INotifyCompletion.OnCompleted), onCompletedAttr, voidRef);
+                typeContext.NewType.Interfaces.Add(new(notifyCompletionRef.Value));
+                typeContext.NewType.Methods.Add(onComplete);
+
+                onComplete.Parameters.Add(new ParameterDefinition("continuation", ParameterAttributes.None, actionUntypedRef.Value));
+
+                var onCompleteIl = onComplete.Body.GetILProcessor();
+
+                onCompleteIl.Emit(OpCodes.Nop);
+                onCompleteIl.Emit(OpCodes.Ldarg_0);
+                onCompleteIl.Emit(OpCodes.Ldarg_1); // ldarg1 bc not static, so ldarg0 is this & ldarg1 is the parameter
+                onCompleteIl.Emit(OpCodes.Call, actionConversionUntypedRef.Value);
+                onCompleteIl.Emit(OpCodes.Call, originalOnComplete.NewMethod);
+                onCompleteIl.Emit(OpCodes.Nop);
+                onCompleteIl.Emit(OpCodes.Ret);
+            }
+        }
+    }
+}

Il2CppInterop.Generator/Runners/InteropAssemblyGenerator.cs

@@ -151,6 +151,11 @@ public void Run(GeneratorOptions options)
             Pass60AddImplicitConversions.DoPass(rewriteContext);
         }
 
+        using (new TimingCookie("Implementing awaiters"))
+        {
+            Pass61ImplementAwaiters.DoPass(rewriteContext);
+        }
+
         using (new TimingCookie("Creating properties"))
         {
             Pass70GenerateProperties.DoPass(rewriteContext);