Implement custom awaitable support (#78071)

This adds support for awaiting task-like types that are not natively supported by runtime async. Closes #77897.

Author: 333fred

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

@@ -2,7 +2,10 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Runtime.InteropServices;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
 
 namespace Microsoft.CodeAnalysis.CSharp;
@@ -21,16 +24,19 @@ public static BoundStatement Rewrite(
         }
 
         var rewriter = new RuntimeAsyncRewriter(compilationState.Compilation, new SyntheticBoundNodeFactory(method, node.Syntax, compilationState, diagnostics));
-        return (BoundStatement)rewriter.Visit(node);
+        var result = (BoundStatement)rewriter.Visit(node);
+        return SpillSequenceSpiller.Rewrite(result, method, compilationState, diagnostics);
     }
 
     private readonly CSharpCompilation _compilation;
     private readonly SyntheticBoundNodeFactory _factory;
+    private readonly Dictionary<BoundAwaitableValuePlaceholder, BoundExpression> _placeholderMap;
 
     private RuntimeAsyncRewriter(CSharpCompilation compilation, SyntheticBoundNodeFactory factory)
     {
         _compilation = compilation;
         _factory = factory;
+        _placeholderMap = [];
     }
 
     private NamedTypeSymbol Task
@@ -53,10 +59,11 @@ private NamedTypeSymbol ValueTaskT
         get => field ??= _compilation.GetWellKnownType(WellKnownType.System_Threading_Tasks_ValueTask_T);
     } = null!;
 
-    public BoundExpression VisitExpression(BoundExpression node)
+    [return: NotNullIfNotNull(nameof(node))]
+    public BoundExpression? VisitExpression(BoundExpression? node)
     {
         var result = Visit(node);
-        return (BoundExpression)result;
+        return (BoundExpression?)result;
     }
 
     public override BoundNode? VisitAwaitExpression(BoundAwaitExpression node)
@@ -88,8 +95,7 @@ public BoundExpression VisitExpression(BoundExpression node)
         }
         else
         {
-            // PROTOTYPE: when it's not a method with Task/TaskT/ValueTask/ValueTaskT returns, use the helpers
-            return base.VisitAwaitExpression(node);
+            return RewriteCustomAwaiterAwait(node);
         }
 
         // PROTOTYPE: Make sure that we report an error in initial binding if these are missing
@@ -112,4 +118,81 @@ public BoundExpression VisitExpression(BoundExpression node)
         // System.Runtime.CompilerServices.RuntimeHelpers.Await(awaitedExpression)
         return _factory.Call(receiver: null, awaitMethod, VisitExpression(node.Expression));
     }
+
+    private BoundExpression RewriteCustomAwaiterAwait(BoundAwaitExpression node)
+    {
+        // await expr
+        // becomes
+        // var _tmp = expr.GetAwaiter();
+        // if (!_tmp.IsCompleted)
+        //    UnsafeAwaitAwaiterFromRuntimeAsync(_tmp) OR AwaitAwaiterFromRuntimeAsync(_tmp);
+        // _tmp.GetResult()
+
+        // PROTOTYPE: await dynamic will need runtime checks, see AsyncMethodToStateMachine.GenerateAwaitOnCompletedDynamic
+
+        var expr = VisitExpression(node.Expression);
+
+        var awaitableInfo = node.AwaitableInfo;
+        var awaitablePlaceholder = awaitableInfo.AwaitableInstancePlaceholder;
+        if (awaitablePlaceholder is not null)
+        {
+            _placeholderMap.Add(awaitablePlaceholder, expr);
+        }
+
+        // expr.GetAwaiter()
+        var getAwaiter = VisitExpression(awaitableInfo.GetAwaiter);
+        Debug.Assert(getAwaiter is not null);
+
+        if (awaitablePlaceholder is not null)
+        {
+            _placeholderMap.Remove(awaitablePlaceholder);
+        }
+
+        // var _tmp = expr.GetAwaiter();
+        var tmp = _factory.StoreToTemp(getAwaiter, out BoundAssignmentOperator store, kind: SynthesizedLocalKind.Awaiter);
+
+        // _tmp.IsCompleted
+        Debug.Assert(awaitableInfo.IsCompleted is not null);
+        var isCompletedMethod = awaitableInfo.IsCompleted.GetMethod;
+        Debug.Assert(isCompletedMethod is not null);
+        var isCompletedCall = _factory.Call(tmp, isCompletedMethod);
+
+        // UnsafeAwaitAwaiterFromRuntimeAsync(_tmp) OR AwaitAwaiterFromRuntimeAsync(_tmp)
+        var discardedUseSiteInfo = CompoundUseSiteInfo<AssemblySymbol>.Discarded;
+        var useUnsafeAwait = _factory.Compilation.Conversions.ClassifyImplicitConversionFromType(
+            tmp.Type,
+            _factory.Compilation.GetWellKnownType(WellKnownType.System_Runtime_CompilerServices_ICriticalNotifyCompletion),
+            ref discardedUseSiteInfo).IsImplicit;
+
+        // PROTOTYPE: Make sure that we report an error in initial binding if these are missing
+        var awaitMethod = (MethodSymbol?)_compilation.GetWellKnownTypeMember(useUnsafeAwait
+            ? WellKnownMember.System_Runtime_CompilerServices_RuntimeHelpers__UnsafeAwaitAwaiterFromRuntimeAsync_TAwaiter
+            : WellKnownMember.System_Runtime_CompilerServices_RuntimeHelpers__AwaitAwaiterFromRuntimeAsync_TAwaiter);
+
+        Debug.Assert(awaitMethod is { Arity: 1 });
+
+        var awaitCall = _factory.Call(
+            receiver: null,
+            awaitMethod.Construct(tmp.Type),
+            tmp);
+
+        // if (!_tmp.IsCompleted) awaitCall
+        var ifNotCompleted = _factory.If(_factory.Not(isCompletedCall), _factory.ExpressionStatement(awaitCall));
+
+        // _tmp.GetResult()
+        var getResultMethod = awaitableInfo.GetResult;
+        Debug.Assert(getResultMethod is not null);
+        var getResultCall = _factory.Call(tmp, getResultMethod);
+
+        // final sequence
+        return _factory.SpillSequence(
+            locals: [tmp.LocalSymbol],
+            sideEffects: [_factory.ExpressionStatement(store), ifNotCompleted],
+            result: getResultCall);
+    }
+
+    public override BoundNode VisitAwaitableValuePlaceholder(BoundAwaitableValuePlaceholder node)
+    {
+        return _placeholderMap[node];
+    }
 }