Support using a simple overload resolution for finding Await helpers from the BCL

docs/compilers/CSharp/Runtime Async Design.md

@@ -135,9 +135,11 @@ For any `await expr` with where `expr` has type `E`, the compiler will attempt t
       2. There is an identity or implicit reference conversion from `E` to the type of `P`.
    4. Otherwise, if `Mi` has a generic arity of 1 with type param `Tm`, all of the following must be true, or `Mi` is removed:
       1. The return type is `Tm`
-      2. There is an identity or implicit reference conversion from `E`'s unsubstituted definition to `P`
-      3. `E`'s type argument, `Te`, is valid to substitute for `Tm`
-6. If only one `Mi` remains, that method is used for the following rewrites. Otherwise, we instead move to [await any other type].
+      2. The generic parameter of `E` is `Te`
+      3. `Ti` satisfies any constraints on `Tm`
+      4. `Mie` is `Mi` with `Te` substituted for `Tm`, and `Pe` is the resulting parameter of `Mie`
+      5. There is an identity or implicit reference conversion from `E` to the type of `Pe`
+5. If only one `Mi` remains, that method is used for the following rewrites. Otherwise, we instead move to [await any other type].
 
 We'll generally rewrite `await expr` into `System.Runtime.CompilerServices.AsyncHelpers.Await(expr)`. A number of different example scenarios for this are covered below. The
 main interesting deviations are when `struct` rvalues need to be hoisted across an `await`, and exception handling rewriting.

src/Compilers/CSharp/Portable/Binder/ForEachLoopBinder.cs

@@ -263,7 +263,7 @@ private BoundForEachStatement BindForEachPartsWorker(BindingDiagnosticBag diagno
                 var placeholder = new BoundAwaitableValuePlaceholder(expr, builder.MoveNextInfo?.Method.ReturnType ?? CreateErrorType());
                 awaitInfo = BindAwaitInfo(placeholder, expr, diagnostics, ref hasErrors);
 
-                if (!hasErrors && (awaitInfo.GetResult ?? awaitInfo.RuntimeAsyncAwaitMethod)?.ReturnType.SpecialType != SpecialType.System_Boolean)
+                if (!hasErrors && (awaitInfo.GetResult ?? awaitInfo.RuntimeAsyncAwaitCall?.Method)?.ReturnType.SpecialType != SpecialType.System_Boolean)
                 {
                     diagnostics.Add(ErrorCode.ERR_BadGetAsyncEnumerator, expr.Location, getEnumeratorMethod.ReturnTypeWithAnnotations, getEnumeratorMethod);
                     hasErrors = true;

src/Compilers/CSharp/Portable/Binder/RefSafetyAnalysis.cs

@@ -958,6 +958,11 @@ private void GetAwaitableInstancePlaceholders(ArrayBuilder<(BoundValuePlaceholde
             {
                 placeholders.Add((placeholder, valEscapeScope));
             }
+
+            if (awaitableInfo.RuntimeAsyncAwaitCallPlaceholder is { } runtimePlaceholder)
+            {
+                placeholders.Add((runtimePlaceholder, valEscapeScope));
+            }
         }
 
         public override BoundNode? VisitImplicitIndexerAccess(BoundImplicitIndexerAccess node)

src/Compilers/CSharp/Portable/Binder/UsingStatementBinder.cs

@@ -150,7 +150,7 @@ internal static BoundStatement BindUsingStatementOrDeclarationFromParts(SyntaxNo
 
                 if (awaitableTypeOpt is null)
                 {
-                    awaitOpt = new BoundAwaitableInfo(syntax, awaitableInstancePlaceholder: null, isDynamic: true, getAwaiter: null, isCompleted: null, getResult: null, runtimeAsyncAwaitMethod: null) { WasCompilerGenerated = true };
+                    awaitOpt = new BoundAwaitableInfo(syntax, awaitableInstancePlaceholder: null, isDynamic: true, getAwaiter: null, isCompleted: null, getResult: null, runtimeAsyncAwaitCall: null, runtimeAsyncAwaitCallPlaceholder: null) { WasCompilerGenerated = true };
                 }
                 else
                 {

src/Compilers/CSharp/Portable/BoundTree/BoundAwaitableInfo.cs

@@ -10,11 +10,12 @@ partial class BoundAwaitableInfo
 {
     private partial void Validate()
     {
-        if (RuntimeAsyncAwaitMethod is not null)
+        if (RuntimeAsyncAwaitCall is not null)
         {
-            Debug.Assert(RuntimeAsyncAwaitMethod.ContainingType.ExtendedSpecialType == InternalSpecialType.System_Runtime_CompilerServices_AsyncHelpers);
+            Debug.Assert(RuntimeAsyncAwaitCall.Method.ContainingType.ExtendedSpecialType == InternalSpecialType.System_Runtime_CompilerServices_AsyncHelpers);
+            Debug.Assert(RuntimeAsyncAwaitCallPlaceholder is not null);
 
-            switch (RuntimeAsyncAwaitMethod.Name)
+            switch (RuntimeAsyncAwaitCall.Method.Name)
             {
                 case "Await":
                     Debug.Assert(GetAwaiter is null);
@@ -30,11 +31,11 @@ private partial void Validate()
                     break;
 
                 default:
-                    Debug.Fail($"Unexpected RuntimeAsyncAwaitMethod: {RuntimeAsyncAwaitMethod.Name}");
+                    Debug.Fail($"Unexpected RuntimeAsyncAwaitCall: {RuntimeAsyncAwaitCall.Method.Name}");
                     break;
             }
         }
 
-        Debug.Assert(GetAwaiter is not null || RuntimeAsyncAwaitMethod is not null || IsDynamic || HasErrors);
+        Debug.Assert(GetAwaiter is not null || RuntimeAsyncAwaitCall is not null || IsDynamic || HasErrors);
     }
 }

src/Compilers/CSharp/Portable/BoundTree/BoundNodes.xml

@@ -702,11 +702,13 @@
     <Field Name="GetAwaiter" Type="BoundExpression?" Null="allow"/>
     <Field Name="IsCompleted" Type="PropertySymbol?" Null="allow"/>
     <Field Name="GetResult" Type="MethodSymbol?" Null="allow"/>
-    <!-- Refers to the runtime async helper method we use for awaiting. Either this is an instance of an AsyncHelpers.Await method symbol, and 
-         GetAwaiter, IsCompleted, and GetResult are null, or this is AsyncHelpers.AwaitAwaiter/UnsafeAwaitAwaiter, and the other
+    <!-- Refers to the runtime async helper call we use for awaiting. Either this is an instance of a BoundCall of an AsyncHelpers.Await method, and 
+         GetAwaiter, IsCompleted, and GetResult are null, or this is an instance of a AsyncHelpers.AwaitAwaiter/UnsafeAwaitAwaiter BoundCall, and the other
          fields are not null. -->
     <!-- PROTOTYPE: Look at consumers of this API and see if we can assert that this is null when it should be -->
-    <Field Name="RuntimeAsyncAwaitMethod" Type="MethodSymbol?" Null="allow"/>
+    <Field Name="RuntimeAsyncAwaitCall" Type="BoundCall?" Null="allow"/>
+    <!-- Never null when RuntimeAsyncAwaitCall is not null -->
+    <Field Name="RuntimeAsyncAwaitCallPlaceholder" Type="BoundAwaitableValuePlaceholder?" Null="allow"/>
   </Node>
 
   <Node Name="BoundAwaitExpression" Base="BoundExpression">

src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs

@@ -2249,16 +2249,16 @@ internal bool ReturnsAwaitableToVoidOrInt(MethodSymbol method, BindingDiagnostic
             var syntax = method.ExtractReturnTypeSyntax();
             var dumbInstance = new BoundLiteral(syntax, ConstantValue.Null, namedType);
             var binder = GetBinder(syntax);
-            var success = binder.GetAwaitableExpressionInfo(dumbInstance, out BoundExpression? result, out MethodSymbol? runtimeAwaitMethod, syntax, diagnostics);
+            var success = binder.GetAwaitableExpressionInfo(dumbInstance, out BoundExpression? result, out BoundCall? runtimeAwaitCall, syntax, diagnostics);
 
             RoslynDebug.Assert(!namedType.IsDynamic());
             if (!success)
             {
                 return false;
             }
 
-            Debug.Assert(result is { Type: not null } || runtimeAwaitMethod is { ReturnType: not null });
-            var returnType = result?.Type ?? runtimeAwaitMethod!.ReturnType;
+            Debug.Assert(result is { Type: not null } || runtimeAwaitCall is { Type: not null });
+            var returnType = result?.Type ?? runtimeAwaitCall!.Type;
             return returnType.IsVoidType() || returnType.SpecialType == SpecialType.System_Int32;
         }
 

src/Compilers/CSharp/Portable/Lowering/AsyncRewriter/RuntimeAsyncRewriter.cs

@@ -50,7 +50,10 @@ private RuntimeAsyncRewriter(SyntheticBoundNodeFactory factory)
         Debug.Assert(nodeType is not null);
 
         var awaitableInfo = node.AwaitableInfo;
-        var runtimeAsyncAwaitMethod = awaitableInfo.RuntimeAsyncAwaitMethod;
+        var runtimeAsyncAwaitCall = awaitableInfo.RuntimeAsyncAwaitCall;
+        Debug.Assert(runtimeAsyncAwaitCall is not null);
+        Debug.Assert(awaitableInfo.RuntimeAsyncAwaitCallPlaceholder is not null);
+        var runtimeAsyncAwaitMethod = runtimeAsyncAwaitCall.Method;
         Debug.Assert(runtimeAsyncAwaitMethod is not null);
         Debug.Assert(ReferenceEquals(
             runtimeAsyncAwaitMethod.ContainingType.OriginalDefinition,
@@ -61,7 +64,11 @@ private RuntimeAsyncRewriter(SyntheticBoundNodeFactory factory)
         {
             // This is the direct await case, with no need for the full pattern.
             // System.Runtime.CompilerServices.RuntimeHelpers.Await(awaitedExpression)
-            return _factory.Call(receiver: null, runtimeAsyncAwaitMethod, VisitExpression(node.Expression));
+            var expr = VisitExpression(node.Expression);
+            _placeholderMap.Add(awaitableInfo.RuntimeAsyncAwaitCallPlaceholder, expr);
+            var call = Visit(awaitableInfo.RuntimeAsyncAwaitCall);
+            _placeholderMap.Remove(awaitableInfo.RuntimeAsyncAwaitCallPlaceholder);
+            return call;
         }
         else
         {
@@ -108,11 +115,11 @@ private BoundExpression RewriteCustomAwaiterAwait(BoundAwaitExpression node)
         var isCompletedCall = _factory.Call(tmp, isCompletedMethod);
 
         // UnsafeAwaitAwaiter(_tmp) OR AwaitAwaiter(_tmp)
-        Debug.Assert(awaitableInfo.RuntimeAsyncAwaitMethod is not null);
-        var awaitCall = _factory.Call(
-            receiver: null,
-            awaitableInfo.RuntimeAsyncAwaitMethod,
-            tmp);
+        Debug.Assert(awaitableInfo.RuntimeAsyncAwaitCall is not null);
+        Debug.Assert(awaitableInfo.RuntimeAsyncAwaitCallPlaceholder is not null);
+        _placeholderMap.Add(awaitableInfo.RuntimeAsyncAwaitCallPlaceholder, tmp);
+        var awaitCall = (BoundCall)Visit(awaitableInfo.RuntimeAsyncAwaitCall);
+        _placeholderMap.Remove(awaitableInfo.RuntimeAsyncAwaitCallPlaceholder);
 
         // if (!_tmp.IsCompleted) awaitCall
         var ifNotCompleted = _factory.If(_factory.Not(isCompletedCall), _factory.ExpressionStatement(awaitCall));