Enable runtime async via compiler intrinsics by majocha · Pull Request #20235 · dotnet/fsharp

majocha · 2026-08-08T07:15:38Z

Add preview F# compiler support for .NET runtime-async methods. Compiler-recognized __runtimeAsyncReturn intrinsics mark Task/ValueTask methods and lambdas with MethodImplOptions.Async, while AsyncHelpers.Await* calls become runtime suspension points.

The optimizer preserves and specializes inline suspension fragments, rewrites suspending exception handlers and finally compensations, and reports unsupported byref or suspension patterns. The feature is gated by langversion:preview and target-runtime metadata support; FSharp.Core exposes the intrinsics only for net10.0, with builders remaining application-defined.

Consider an inline "sync" CE builder. applying __runtimeAsyncReturn to the inlined code in its Run method compiles the computation expression into a single runtime async method:

member inline _.Run([<InlineIfLambda>] code) = __runtimeAsyncReturn(code())

Resumption is handled by the runtime, calling AsyncHelpers.Await in Bind is all that is needed to make the resulting CE async:

member inline _.Bind(task, [<InlineIfLambda>] continuation) = AsyncHelpers.Await task |> continuation

runtime spec :
Runtime-async specification

interesting docs and links:

github-actions · 2026-08-08T07:15:49Z

❗ Release notes required

You can open this PR in browser to add release notes: open in github.dev

✅ Found changes and release notes in following paths:

Change path Release notes path Description

`src/FSharp.Core` docs/release-notes/.FSharp.Core/11.0.100.md

`src/Compiler` docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

`src/Compiler/Facilities/LanguageFeatures.fsi` docs/release-notes/.Language/preview.md

…c; add Language preview release notes The features dictionary lost ImplicitDIMCoverage, MethodOverloadsCache, ErrorOnMissingSignatureAttribute, DirectDelegateConstruction, AccessProtectedBaseFieldFromClosure and RecordSpreads entries, causing 54 CI test failures ('Unable to find feature' internal errors and preview features not enabled). Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

majocha · 2026-08-13T17:22:20Z

Another thing to think through is inlining. Currently there are no checks at all for use of suspending AsyncHelpers members outside of an async method. According to spec, this is illegal but the idea is that any AsyncHelpers.Await calls should be contained by or inlined into the resulting async method (see the sample runtimeTask builder in the tests here). It seems to get an efficient single method from a CE the builder needs to declare every method inline and make use of InlineIfLambda.

Currently it is up to the "expert" user to not misuse AsyncHelpers. Ideally the compiler should check for any such illegal calls only after inlining.

This is still a sketch, but it successfully compiles runtimeTask builder. The builder passes ported Tasks.fs tests, which is promising.

T-Gro · 2026-08-14T10:48:53Z

Ideally the compiler should check for any such illegal calls only after inlining.

We could have a notion of PostIlxGen checks.
Agree it must run after all optimizations.

Roslyn-async2-inspired edge cases for the runtime-async intrinsic, driven through the test-only runtimeTask CE (treated as a hypothetical library): * execution fixture (RuntimeAsync/RuntimeAsyncEdgeCases.fs): locals/loops across suspension, non-ref struct across suspension, ValueTask operand, exception propagation, IAsyncDisposable with genuinely-async DisposeAsync. * facts (RuntimeAsyncEdgeCaseTests.fs): Await overload selection per operand type, no compiler state machine (direct + CE), the C1 forbidden `tail.` prefix, and a parametrized set of currently-undiagnosed contract-forbidden patterns (await-in-finally/catch, ref-struct- and byref-across-suspension). Every asserted IL substring and runtime symptom was captured empirically on the pinned net11 preview; the forbidden patterns match docs/runtime-async.md. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 55dd72c8-46d3-4959-9677-c52d41779596

The sequence-points baseline (and ildasm) cannot render MethodImplOptions.Async (0x2000), so the lifted __runtimeAsync body shows up as a plain outer@<line> closure. Factor the metadata flag check into assertAsyncFlagOnLiftedClosureOnly and chain it onto the sequence-points fact so the exact program that emits the .bsl also proves the async marker lands only on the lifted closure, never on the user's outer/helper methods. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 55dd72c8-46d3-4959-9677-c52d41779596

majocha · 2026-08-20T07:43:30Z

Looks like the runtime feature is quickly evolving, see #19056 (comment)

runtime-async-tiering-and-tail-await-optimizations

The previous run was SIGKILL'd by the OOM-killer mid-suite (0 real test failures; 229 tests never ran). Empty commit to re-run the pipeline. Same exit-137 flake also hit unrelated PRs #20274 and #20235 at the same time. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

majocha · 2026-08-20T21:36:58Z

I wonder how to support the other allowed return types.

__runtimeAsyncReturn<'T>       : 'T -> Task<'T>
__runtimeAsyncReturnValueTask<'T> : 'T -> ValueTask<'T>
__runtimeAsyncReturnUnit       : unit -> Task
__runtimeAsyncReturnValueTaskUnit : unit -> ValueTask

and it quickly becomes a whole zoo. Do we need the non-generic versions at all? Only for potential C# interop, I guess. The upside is that the current type check is all we need to keep it correct, without any extra handling.

The other alternative it to have a unconstrained __runtimeAsyncReturn: 'T -> 'Carrier and do extra checks that the 'Carrier type is supported.

…sharp into runtime-async-intrinsic

majocha · 2026-09-07T14:12:25Z

I ordered the AI to take the sample runtime async asyncSeq and make a full FSharp.Control.TaskSeq reimplementation:

https://github.com/majocha/FSharp.Control.TaskSeq/tree/runtime-async

Remarkably it passes the whole test suite. (in Release).

It also reveals some more debug configuration bugs in this PR.

The coreclr_release job was canceled after the 120-minute limit due to a flaky infrastructure timeout (memory pressure hanging an unrelated test assembly). The same job timed out on unrelated PRs #20235 and #20393 in the last 10 days. ComponentTests (all DIM/interface tests affected by this PR) passed fully, so the merge resolution is correct. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

T-Gro

🤖🕵️ AI-assisted review. Every finding reproduced on a local build of dac78834: captured output, attribution controls, --optimize+ and --optimize-.

T-Gro · 2026-09-09T13:21:21Z

            // application of local type functions with type parameters = measure types and body = local value - inline the body
            GenExpr cenv cgbuf eenv v sequel

+        | Expr.App _ when TryGetRuntimeAsyncReturn g expr |> Option.isSome -> GenRuntimeAsyncReturnAsStartedTask cenv cgbuf eenv expr sequel


🤖🕵️ byref captured by the closure synthesized after PostInferenceChecks, so FS0406 has already run. The flag lands on f@8::Invoke, not f.

[<NoCompilerInlining>] let f (x: byref<int>) : Task<int> = let y = x __runtimeAsyncReturn (x + y)

now: compiles clean; ILVerify StackByRef @0x06; run → NRE; both opt modes

expected: rejected, or lowered without a capturing closure

no leading let, or y unused → flag on M::f, verifies clean, returns 2

fix: reject byref/byref-like/pinning free values before the lambda is built

T-Gro · 2026-09-09T13:21:21Z

+    let (|RuntimeAsyncApplication|_|) =
+        function
+        | ApplicableExpr(expr = (RuntimeAsyncReturnFunction g (vref, flags, m))) ->
+            checkLanguageFeatureAndRecover g.langVersion LanguageFeature.RuntimeAsync m


🤖🕵️ Pipe bypasses the preview gate — enforced only in syntactic application position, while downstream recognition is structural.

// --langversion:9 let f x = x |> __runtimeAsyncReturn // compiles, implAttrs=0x2008 let g x = __runtimeAsyncReturn x // error FS3350

expected: FS3350 for both

with a suspension in the body: opt+ emits the flag, opt- gives FS3916 — a different symptom, not absence

fix: gate where recognition happens, plus a re-check at the IlxGen placements — LowerStateMachineExpr already re-gates there (IlxGen.fs:3248)

imported cross-assembly inline also emits the flag under F# 9 — deliberate?

github-actions · 2026-09-09T23:33:22Z

🔍 Tooling Safety Check — Affects-Bootstrap, Affects-Build-Infra, Affects-Compiler-Output, Affects-Test-Tooling
Affects-Bootstrap: Compiler sources alter the self-hosting compiler chain.
Affects-Build-Infra: Compiler project changes participate in builds.
Affects-Compiler-Output: Code generation changes alter emitted binaries.
Affects-Test-Tooling: Component-test project infrastructure is modified.

Generated by PR Tooling Safety Check · gpt56 2.6M · ◷

majocha · 2026-09-10T07:44:24Z

Unfortunatelly to make async iterators like TaskSeq implementable efficiently, we still need resumable state machine. In fact we need to extend it to generate a runtime-async MoveNext. It would allow to do runtime-async AsyncHelpers.Await directly in a state machine step.

See:

https://github.com/dotnet/roslyn/blob/features/runtime-async-streams/docs/compilers/CSharp/Runtime%20Async-Streams%20Design.md

majocha · 2026-09-10T10:14:54Z

AI thoughts on runtime async state machines implementation:
https://gist.github.com/majocha/785b8cf30de031eec340cc4227e93061

majocha added 5 commits August 8, 2026 08:34

docs

15fff46

feature

0cc2108

tests

719d00f

surface area

f9bb483

rns

dfe4109

github-project-automation Bot added this to F# Compiler and Tooling Aug 8, 2026

github-project-automation Bot moved this to New in F# Compiler and Tooling Aug 8, 2026

majocha and others added 5 commits August 8, 2026 09:20

merge main

b06dd3d

translations

e4eb267

add pr numbers

38578d4

surface area

48d1691

T-Gro reviewed Aug 12, 2026

View reviewed changes

Comment thread tests/FSharp.Core.UnitTests/FSharp.Core.SurfaceArea.netstandard20.release.bsl Outdated

T-Gro reviewed Aug 12, 2026

View reviewed changes

Comment thread src/FSharp.Core/resumable.fs Outdated

This comment was marked as outdated.

Sign in to view

Merge branch 'main' into runtime-async-intrinsic

eb32d05

T-Gro and others added 2 commits August 18, 2026 14:59

Merge branch 'main' into runtime-async-intrinsic

d29dc42

Merge branch 'main' into runtime-async-intrinsic

ca0ca40

majocha added 4 commits August 21, 2026 00:24

restrict to net10, rename to __runtimeAsyncReturn

6f8cadc

ns surfacearea

d5bd674

Merge branch 'runtime-async-edge-tests' of https://github.com/T-Gro/f…

19117fe

…sharp into runtime-async-intrinsic

adjust edge case tests to name change and net10 FSharp.Core

5154fa9

github-actions Bot added ⚠️ Affects-Test-Tooling Tooling check: PR touches test framework infrastructure ⚠️ Affects-Compiler-Output Tooling check: PR touches IL emission or codegen ⚠️ Affects-Bootstrap Tooling check: PR touches compiler bootstrap chain labels Sep 2, 2026

This comment has been minimized.

Sign in to view

majocha added 3 commits September 3, 2026 07:15

cleanup

6f58b8f

improve debug stepping

52deaff

merge main

e33dabe

This comment has been minimized.

Sign in to view

Merge branch 'main' into runtime-async-intrinsic

d2d7555

This comment has been minimized.

Sign in to view

majocha added 7 commits September 8, 2026 07:26

icrease pool size brcause for debug runs

97bfa0e

add failing code to test fixture

24e3f3f

disalow runtime async fragments in ComputeSplitToMethodCondition

c0d0299

Merge branch 'main' into runtime-async-intrinsic

9d0fae8

Merge branch 'main' into runtime-async-intrinsic

b2df986

Merge branch 'main' into runtime-async-intrinsic

66723c2

update surface

dac7883

T-Gro reviewed Sep 9, 2026

View reviewed changes

Merge branch 'main' into runtime-async-intrinsic

866a169

github-actions Bot added the ⚠️ Affects-Build-Infra Tooling check: PR touches build infrastructure label Sep 9, 2026

majocha added 5 commits September 10, 2026 12:15

Merge branch 'main' into runtime-async-intrinsic

382ae42

Merge branch 'main' into runtime-async-intrinsic

8c9cc48

review: add failing test cases

6fa01aa

update debug surface area

004f393

address review

0f4e602

Uh oh!

Conversation

majocha commented Aug 8, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

github-actions Bot commented Aug 8, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

❗ Release notes required

Uh oh!

Uh oh!

Uh oh!

This comment was marked as outdated.

Uh oh!

majocha commented Aug 13, 2026

Uh oh!

T-Gro commented Aug 14, 2026

Uh oh!

majocha commented Aug 20, 2026

Uh oh!

majocha commented Aug 20, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

majocha commented Sep 7, 2026

Uh oh!

This comment has been minimized.

T-Gro left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

T-Gro Sep 9, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

T-Gro Sep 9, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

github-actions Bot commented Sep 9, 2026

Uh oh!

majocha commented Sep 10, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

majocha commented Sep 10, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

majocha commented Aug 8, 2026 •

edited

Loading

github-actions Bot commented Aug 8, 2026 •

edited

Loading

majocha commented Sep 10, 2026 •

edited

Loading