Async Profiler: Instrument custom awaiters in V1 dispatcher.

[lateralusX]

Summary

Extends the async profiler (V1 / StateMachine) so that continuations awaiting a custom awaiter are wrapped in an AsyncStateMachineDispatcher just like the built-in ones, making them visible to instrumentation.

Motivation

The V1 dispatcher is created on the awaiter's box-aware path (IStateMachineBoxAwareAwaiter.AwaitUnsafeOnCompleted), which only the framework awaiters (Task/ValueTask/YieldAwaitable) implement. For an arbitrary user awaiter, the async method builders fall back to the generic AwaitOnCompleted<TAwaiter, TStateMachine> / AwaitUnsafeOnCompleted paths, which registered box.MoveNextAction without creating a dispatcher. As a result, continuations that resumed through a custom awaiter never produced Create/Resume/Complete async-profiler events — leaving a gap in the captured async causality chain.

What changed

• AsyncTaskMethodBuilder and PoolingAsyncValueTaskMethodBuilder: in the generic AwaitOnCompleted and the
• AsyncStateMachineDispatcher: removed the dedicated _moveNextAction field and now backs MoveNextAction with the inherited Task.m_action slot ((Action)(m_action ??= new Action(MoveNext))), matching how the framework builder boxes expose their action.

Tests

• New [ConditionalTheory] StateMachineAsync_CustomAwaiter_EmitsCreateResumeComplete covering the full matrix of (pooling vs. non-pooling) × (ICriticalNotifyCompletion vs. INotifyCompletion) via four [InlineData] cases, each with a dedicated custom-awaiter marker method, asserting the expected Create/Resume/Complete events are emitted.

Notes / risk

• Gated behind AsyncInstrumentation.IsActive + IsEnabled.AsyncProfiler, so there is no behavioral or allocation change when the profiler is disabled (the common case).
• No change to NativeAOT behavior (V1 remains disabled there via AsyncStateMachineDispatcherInfo.IsSupported).

[Copilot]

Pull request overview

This PR extends async-profiler (V1 / state-machine) instrumentation so that custom awaiters (i.e., awaiters that don’t hit the Task/ValueTask/Yield “known awaiter” fast paths) still get their continuations wrapped in an AsyncStateMachineDispatcher, enabling Create/Resume/Complete events to be emitted for those awaits.

Changes:

• Wrap the state machine box in an AsyncStateMachineDispatcher from the generic builder fallback paths (AwaitOnCompleted and the non-known awaiter branch of AwaitUnsafeOnCompleted) when async-profiler instrumentation is active.
• Align AsyncStateMachineDispatcher.MoveNextAction with framework state-machine boxes by backing it with the inherited Task.m_action slot.
• Add new tests covering custom awaiters across the (pooling vs non-pooling) × (INotifyCompletion vs ICriticalNotifyCompletion) matrix, asserting the expected Create → Resume → Complete chain.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Runtime/tests/System.Threading.Tasks.Tests/System.Runtime.CompilerServices/AsyncProfilerV1Tests.cs	Adds coverage ensuring custom awaiters still produce V1 dispatcher events and callstack markers across pooling/non-pooling and notify/critical notify cases.
src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/PoolingAsyncValueTaskMethodBuilderT.cs	Creates a dispatcher for the pooling builder’s AwaitOnCompleted fallback path when async-profiler instrumentation is enabled.
src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncTaskMethodBuilderT.cs	Creates a dispatcher for AwaitOnCompleted and for the “unknown awaiter” branch of AwaitUnsafeOnCompleted under async-profiler instrumentation.
src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncStateMachineDispatcher.cs	Changes MoveNextAction caching to use Task.m_action, matching the existing AsyncStateMachineBox<TStateMachine> pattern.

[dotnet-policy-service]

Tagging subscribers to this area: @steveisok, @tommcdon, @dotnet/dotnet-diag
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-runtime-compilerservices
See info in area-owners.md if you want to be subscribed.

[lateralusX]

/ba-g known issue #130177