Public API for runtime async

[333fred]

Background and Motivation

Runtime async introduces a new lowering for async methods, which can have one of two patterns:

1. await expr has a directly supported type, and we just emit:

System.Runtime.CompilerServices.AsyncHelpers.Await(expr);

2. await expr has a not supported type, and we emit:

var awaiter = expr.GetAwaiterMethod();
if (!awaiter.IsCompletedProperty)
{
    System.Runtime.CompilerServices.AwaitAwaiter(awaiter);
    // or
    System.Runtime.CompilerServices.UnsafeAwaitAwaiter(awaiter);
}
awaiter.GetResultMethod()

Consumers of AwaitExpressionInfo, such as the preview features analyzer, will need to be able to see this new method symbol and take action on it.

Proposed API

namespace Microsoft.CodeAnalysis.CSharp;

public readonly struct AwaitExpressionInfo
{
    public IMethodSymbol? GetAwaiterMethod { get; }

    public IPropertySymbol? IsCompletedProperty { get; }

    public IMethodSymbol? GetResultMethod { get; }

    public bool IsDynamic { get; }

+    /// <summary>
+    /// When runtime async is enabled for this await expression, this represents either:
+    /// <list type="bullet">
+    /// <item>
+    /// A call to <c>System.Runtime.CompilerServices.AsyncHelpers.Await</c>, if this is a
+    /// supported task type. In such cases, <see cref="GetAwaiterMethod" />,
+    /// <see cref="IsCompletedProperty" />, and <see cref="GetResultMethod" /> will be
+    /// <see langword="null" />.
+    /// </item>
+    /// <item>
+    /// A call to <c>System.Runtime.CompilerServices.AsyncHelpers.AwaitAwaiter|UnsafeAwaitAwaiter</c>.
+    /// In these cases, the other properties may be non-<see langword="null" /> if the
+    /// the rest of the await expression is successfully bound.
+    /// </item>
+    /// </list>
+    /// </summary>
+    public IMethodSymbol? RuntimeAwaitMethod { get; }
}

Usage Examples

var awaitInfo = semanticModel.GetAwaitExpressionInfo(expr);
if (awaitInfo.RuntimeAwaitMethod is IMethodSymbol)
{
    // Ensure that `EnablePreviewFeatures` is on.
}

Alternative Designs

Risks

[jaredpar]

Is this necessary for .NET 10 or when runtime async goes RTM?

[333fred]

This will likely be necessary for .NET 10 so the requires preview features analyzer can be updated.

[333fred]

API Review

• Approved as proposed.