Emit async-iterators with runtime-async when possible by jcouv · Pull Request #81314 · dotnet/roslyn

jcouv · 2025-11-18T16:07:26Z

Relates to test plan #75960

stephentoub · 2025-11-26T13:10:25Z

docs/compilers/CSharp/Runtime Async-Streams Design.md

+The `GetAsyncEnumerator` method either returns the current instance if it can be reused,
+or creates a new instance of the state machine class.
+
+Assuming that the unspeakble state machine class is named `Unspeakable`, `GetAsyncEnumerator` is emitted as:


Nit:

Suggested change

Assuming that the unspeakble state machine class is named `Unspeakable`, `GetAsyncEnumerator` is emitted as:

Assuming that the unspeakable state machine class is named `Unspeakable`, `GetAsyncEnumerator` is emitted as:

``` #Resolved

stephentoub · 2025-11-26T13:13:27Z

docs/compilers/CSharp/Runtime Async-Streams Design.md

+
+#### Lowering of `yield return`
+
+`yield return` is disallowed in finally, in try with catch and in catch.  


Unrelated to this PR, we were going to look at removing some of these restrictions, in particular for yields inside of a try with a catch, right? #Resolved

Yes, there's a proposal for that, tracked independently: dotnet/csharplang#8414
It's not clear whether this would make it into C# 15 yet

stephentoub · 2025-11-26T13:14:43Z

docs/compilers/CSharp/Runtime Async-Streams Design.md

+
+## Open issues
+
+Question: AsyncIteratorStateMachineAttribute, or IteratorStateMachineAttribute, or other attribute on kickoff method?


Why would this need to differ from what we do today? #Resolved

I'm not sure yet. We introduced a different attribute for each state machine design so far.
Edit&Continue looks at this attribute and also relies on Symreader which also looks at the attribute. Symreader uses it to find the MoveNext method, but here the naming convention is different (it's "MoveNextAsync").
I didn't investigate the debug symbols or EnC yet. Need to talk to Tomas ;-)

docs/compilers/CSharp/Runtime Async-Streams Design.md

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

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

src/Compilers/CSharp/Portable/Compiler/MethodCompiler.cs

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

333fred

Done review pass (commit 1). Tests are not looked at yet.

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

.../Lowering/RuntimeAsyncIteratorRewriter/RuntimeAsyncIteratorRewriter.MoveNextAsyncRewriter.cs

docs/compilers/CSharp/Runtime Async-Streams Design.md

...pilers/CSharp/Portable/Lowering/RuntimeAsyncIteratorRewriter/RuntimeAsyncIteratorRewriter.cs

src/Compilers/Core/Portable/SpecialMember.cs

...rs/CSharp/Portable/Lowering/RuntimeAsyncIteratorRewriter/RuntimeAsyncIteratorStateMachine.cs

src/Compilers/CSharp/Portable/Lowering/StateMachineRewriter/SynthesizedStateMachineMethod.cs

jjonescz · 2025-12-02T09:30:57Z

docs/compilers/CSharp/Runtime Async-Streams Design.md

+
+Async methods that return `IAsyncEnumerable<T>` or `IAsyncEnumerator<T>` are transformed by the compiler into state machines.
+States are created for each `await` and `yield`.
+Runtime-async support was added in .NET 10 as a preview feature and reduces the overhead of async methods by letting the runtime handling `await` suspensions.


Suggested change

Runtime-async support was added in .NET 10 as a preview feature and reduces the overhead of async methods by letting the runtime handling `await` suspensions.

Runtime-async support was added in .NET 10 as a preview feature and reduces the overhead of async methods by letting the runtime handle `await` suspensions.

``` #Resolved

333fred

Mostly LGTM. Just a couple of small comments.

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

...pilers/CSharp/Portable/Lowering/RuntimeAsyncIteratorRewriter/RuntimeAsyncIteratorRewriter.cs

RikkiGibson · 2025-12-09T18:57:22Z

putting on the queue.

RikkiGibson

Finished reviewing impl. Overall LGTM. Will take a break then get thru tests today.

RikkiGibson · 2026-02-06T21:19:41Z

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

+                }
+
+                var getAsyncEnumerator = (MethodSymbol?)GetWellKnownTypeMember(WellKnownMember.System_Collections_Generic_IAsyncEnumerable_T__GetAsyncEnumerator);
+                if (getAsyncEnumerator is null || !iAsyncEnumerable.GetMembers(WellKnownMemberNames.GetAsyncEnumeratorMethodName).Contains(getAsyncEnumerator))


Does the iAsyncEnumerable.GetMembers().Contains() check do something here? I thought a well-known member has a strong association with a well-known containing type, and, GetWellKnownTypeMember() won't return a symbol which is "disjunct" from that. #Pending

You're right, that was unnecessarily paranoid :-P

RikkiGibson · 2026-02-06T21:41:56Z

src/Compilers/CSharp/Portable/Lowering/StateMachineRewriter/MethodToStateMachineRewriter.cs

        /// <param name="locals">The set of locals declared in the original version of this statement</param>
        /// <param name="wrapped">A delegate to return the translation of the body of this statement</param>
-        private BoundStatement PossibleIteratorScope(ImmutableArray<LocalSymbol> locals, Func<BoundStatement> wrapped)
+        private BoundStatement PossibleIteratorScope(ImmutableArray<LocalSymbol> locals, Func<MethodToStateMachineRewriter, BoundBlock, BoundStatement> wrapped, BoundBlock node)


non-blocking: consider adjusting the doc comment to reflect the change in signature #Pending

RikkiGibson · 2026-02-06T21:54:30Z

...pilers/CSharp/Portable/Lowering/RuntimeAsyncIteratorRewriter/RuntimeAsyncIteratorRewriter.cs

+
+            ensureSpecialMember(SpecialMember.System_Runtime_CompilerServices_AsyncHelpers__Await_T, bag);
+
+            Symbol ensureSpecialMember(SpecialMember member, BindingDiagnosticBag bag)


non-blocking: consider inlining this local function. #ByDesign

RikkiGibson · 2026-02-06T21:55:48Z

...pilers/CSharp/Portable/Lowering/RuntimeAsyncIteratorRewriter/RuntimeAsyncIteratorRewriter.cs

+        {
+            return rewriter.Rewrite();
+        }
+        catch (SyntheticBoundNodeFactory.MissingPredefinedMember ex)


Does reaching this catch(), indicate a check is missing in VerifyPresenceOfRequiredAPIs above? #Resolved

Yes, we'd have to have missed some API during VerifyPresenceOfRequiredAPIs

RikkiGibson · 2026-02-06T22:02:36Z

...pilers/CSharp/Portable/Lowering/RuntimeAsyncIteratorRewriter/RuntimeAsyncIteratorRewriter.cs

+        MethodSymbol IAsyncEnumeratorOfElementType_MoveNextAsync = GetMoveNextAsyncMethod();
+        OpenMoveNextMethodImplementation(IAsyncEnumeratorOfElementType_MoveNextAsync, runtimeAsync: true);
+
+        var rewritter = new MoveNextAsyncRewriter(


nit: rewriter #Pending

RikkiGibson · 2026-02-06T22:20:46Z

.../Lowering/RuntimeAsyncIteratorRewriter/RuntimeAsyncIteratorRewriter.MoveNextAsyncRewriter.cs

+                // _current = default;
+                blockBuilder.Add(GenerateClearCurrent());
+
+                // throw null;


nit: This comment feels slightly misleading. A "throw with no expression" is being used rather than a "throw with null expression", right?

Suggested change

// throw null;

// throw;

``` #Pending

RikkiGibson · 2026-02-06T22:59:08Z

.../Lowering/RuntimeAsyncIteratorRewriter/RuntimeAsyncIteratorRewriter.MoveNextAsyncRewriter.cs

+            var resultTrue = new BoundReturnStatement(F.Syntax, RefKind.None, F.Literal(true), @checked: false) { WasCompilerGenerated = true };
+            blockBuilder.Add(resultTrue);
+
+            // <next_state_label>: ;


nit:

Suggested change

// <next_state_label>: ;

// <next_state_resume_label>: ;

``` #Pending

RikkiGibson · 2026-02-06T23:03:06Z

.../Lowering/RuntimeAsyncIteratorRewriter/RuntimeAsyncIteratorRewriter.MoveNextAsyncRewriter.cs

+        public override BoundNode? VisitReturnStatement(BoundReturnStatement node)
+        {
+            Debug.Assert(_currentDisposalLabel is not null);
+            return F.Block(SetDisposeMode(), F.Goto(_currentDisposalLabel));


I was slightly confused by this. User cannot write return; statements in an async iterator, right? So is this for handling return statements which were inserted by compiler in an earlier rewriting stage? #Resolved

I was confused by this too. Yes, it's for implicitly inserted return statement. This could probably be improved, but would affect async and (non runtime-async) async-iterator scenarios, so I kept the same approach

RikkiGibson · 2026-02-06T23:06:04Z

.../Lowering/RuntimeAsyncIteratorRewriter/RuntimeAsyncIteratorRewriter.MoveNextAsyncRewriter.cs

+            return F.Block(SetDisposeMode(), F.Goto(_currentDisposalLabel));
+        }
+
+        /// <inheritdoc cref="AsyncIteratorMethodToStateMachineRewriter.VisitTryStatement"/>


The reuse issue seems tricky. Seems we are in disjunct part of the class hierarchy here, so, simply sharing "we override certain methods" without sharing other stuff, requires a whole process of extracting a static method and calling it from two overrides. It looks like you probably made a judgment call in each case on whether to extract a static method or just copy the override to this type.

I would slightly prefer extracting this 'VisitTryStatement' to static method, but, don't feel strongly. #WontFix

Gave it a shot and I didn't find a good way to make it work. The two challenges are the base call to VisitTryStatement and the call to AppendConditionalJumpToCurrentDisposalLabel (which is used in a couple of places, so didn't want to inline)

RikkiGibson · 2026-02-06T23:45:25Z

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncIteratorTests.cs

                    "void C.<M>d__0.System.Threading.Tasks.Sources.IValueTaskSource<System.Boolean>.OnCompleted(System.Action<System.Object> continuation, System.Object state, System.Int16 token, System.Threading.Tasks.Sources.ValueTaskSourceOnCompletedFlags flags)",
+                    "void C.<M>d__0.System.Threading.Tasks.Sources.IValueTaskSource.GetResult(System.Int16 token)",


nit: In general it is good to manually reorder to preserve the diff within reason. Alternatively, we could add an ordering step prior to ToTestDisplayStrings() #Pending

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncIteratorTests.cs

jcouv self-assigned this Nov 18, 2025

jcouv added Area-Compilers Feature - Runtime Async labels Nov 18, 2025

jcouv force-pushed the async-iterator2 branch 4 times, most recently from 2765f0d to 8542bed Compare November 24, 2025 23:33

Emit async-iterators with runtime-async when possible

cfbdbfb

jcouv force-pushed the async-iterator2 branch from 8542bed to cfbdbfb Compare November 25, 2025 19:03

stephentoub reviewed Nov 26, 2025

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jcouv marked this pull request as ready for review December 1, 2025 17:08

jcouv requested a review from a team as a code owner December 1, 2025 17:08

jcouv mentioned this pull request Dec 1, 2025

Test Plan for Runtime Async #75960

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333fred reviewed Dec 1, 2025

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jjonescz reviewed Dec 2, 2025

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Julien Couvreur added 2 commits December 3, 2025 13:35

Address feedback

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Fix link

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jcouv requested a review from 333fred December 3, 2025 21:47

333fred reviewed Dec 3, 2025

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src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs Outdated Show resolved Hide resolved

...pilers/CSharp/Portable/Lowering/RuntimeAsyncIteratorRewriter/RuntimeAsyncIteratorRewriter.cs Outdated Show resolved Hide resolved

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jcouv requested a review from RikkiGibson December 4, 2025 17:19

333fred mentioned this pull request Jan 31, 2026

Fix sequence point and instrumentation issues with runtime async #82236

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jcouv enabled auto-merge (squash) February 11, 2026 18:07

jcouv merged commit 5127522 into dotnet:features/runtime-async-streams Feb 11, 2026
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jcouv deleted the async-iterator2 branch February 11, 2026 21:02

	Assuming that the unspeakble state machine class is named `Unspeakable`, `GetAsyncEnumerator` is emitted as:
	Assuming that the unspeakable state machine class is named `Unspeakable`, `GetAsyncEnumerator` is emitted as:
	``` #Resolved


		#### Lowering of `yield return`

		`yield return` is disallowed in finally, in try with catch and in catch.


		## Open issues

		Question: AsyncIteratorStateMachineAttribute, or IteratorStateMachineAttribute, or other attribute on kickoff method?

	Runtime-async support was added in .NET 10 as a preview feature and reduces the overhead of async methods by letting the runtime handling `await` suspensions.
	Runtime-async support was added in .NET 10 as a preview feature and reduces the overhead of async methods by letting the runtime handle `await` suspensions.
	``` #Resolved


		ensureSpecialMember(SpecialMember.System_Runtime_CompilerServices_AsyncHelpers__Await_T, bag);

		Symbol ensureSpecialMember(SpecialMember member, BindingDiagnosticBag bag)

	// <next_state_label>: ;
	// <next_state_resume_label>: ;
	``` #Pending

		"void C.<M>d__0.System.Threading.Tasks.Sources.IValueTaskSource<System.Boolean>.OnCompleted(System.Action<System.Object> continuation, System.Object state, System.Int16 token, System.Threading.Tasks.Sources.ValueTaskSourceOnCompletedFlags flags)",
		"void C.<M>d__0.System.Threading.Tasks.Sources.IValueTaskSource.GetResult(System.Int16 token)",

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