Improve static compilation of state machines by majocha · Pull Request #19297 · dotnet/fsharp

majocha · 2026-02-16T12:03:36Z

Fixes #19296, #12839 also includes test cases from #14930.

Also fixes FS3511 The resumable code value(s) 'code' does not have a definition. we have no issue open for it, but it was suppressed in this repo.

Brazenly vibecoded, however I think the risk of changes here is under control, given that we have quite a few relevant projects and their tests in the regression matrix.

OK here goes AI summary:

This pull request improves the F# compiler's handling of state machine lowering, especially for cases involving resumable code and control flow constructs like if __useResumableCode. It fixes issues where inlined helpers or nested resumable code constructs could incorrectly fall back to dynamic branches at runtime, and adds comprehensive test coverage for these scenarios. The main focus is on ensuring that statically-compilable state machines are correctly recognized and optimized, and that code generation remains robust for complex patterns.

Key changes include:

Compiler logic improvements:

Enhanced the lowering pass in LowerStateMachines.fs to correctly handle resumable code bindings found inside debug points, ensuring that nested resumable code constructs are properly expanded.
Updated the reduction logic to track locally-bound resumable code continuations in the environment, allowing correct resolution and inlining of these continuations during optimization. [1] [2]
Modified the rewrite environment to avoid recursing into nested state machine expressions, preventing unintended modification of their internal logic and ensuring that only the appropriate static branches are taken for if __useResumableCode.

Test suite enhancements:

Added a new test module (FailingInlinedHelper) and test case to verify that inlined helpers containing if __useResumableCode are expanded correctly, addressing a real-world bug. [1] [2]
Expanded tests to cover additional scenarios involving for-loops over tuples and various statically-compilable task patterns, ensuring robust handling of these constructs.

Documentation/test comments:

Updated comments in the NestedTaskFailures.fs test to reflect that certain state machine failures are now fixed, clarifying the status of known issues.Fixes State machines: low-level resumable code not always expanded correctly, without warning #19296, also includes test cases from Try fix static compilation of state machines #14930

github-actions · 2026-02-16T12:04:16Z

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Change path	Release notes path	Description
`src/FSharp.Core`	docs/release-notes/.FSharp.Core/10.0.300.md	No release notes found or release notes format is not correct

✅ Found changes and release notes in following paths:

Change path Release notes path Description

src/Compiler docs/release-notes/.FSharp.Compiler.Service/10.0.300.md

vsintegration/src docs/release-notes/.VisualStudio/18.vNext.md

Improve reduction of resumable code in state machines Enhance application reduction in state machine lowering for F# computation expressions by tracking let-bound resumable code in the environment and resolving references during reduction. This enables correct handling of optimizer-generated continuations and deeper reduction of nested applications. Also, update test comments to reflect resolved state machine compilation issues.

majocha · 2026-02-16T22:11:40Z

Looks like this now to some point reinvents #14930. I'll try to include some of the relevant repros in tests. Too bad they are scattered across issues and comments.

majocha · 2026-02-16T22:16:51Z

Another observation: Lack of FS3511 warning does not mean the state machine actually compiled statically. Sometimes the fallback to dynamic implementation gives no warning.

majocha · 2026-02-17T10:02:14Z

tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/NestedTaskFailures.fs

+// The tasks below used to fail state machine compilation.  This failure was causing subsequent problems in code generation.
 // See https://github.com/dotnet/fsharp/issues/13404

-#nowarn "3511"  // state machine not statically compilable - this is a separate issue, see  https://github.com/dotnet/fsharp/issues/13404


This issue is fixed now.

majocha · 2026-02-17T10:37:11Z

tests/FSharp.Compiler.ComponentTests/Language/StateMachineTests.fs

+}
+    """
+        |> compile
+        |> verifyIL [ ".override [runtime]System.Runtime.CompilerServices.IAsyncStateMachine::MoveNext" ]


State machines sometimes fallback to dynamic implmentation without FS3511 warning. Checking for generated MoveNext is more reliable.

majocha · 2026-02-18T11:57:52Z

I tried to include in tests as many old repros as I could find across the issues.
My methodology was: paste the repro in sharplab.io, if it does not statically compile, include it in StateMachineTests.fs

Turns out a lot of those repros already compile statically. If not in sharplab (no idea what version it uses) than with current compiler from main.

3 cases remained that are fixed specifically by this PR:

State machines: low-level resumable code not always expanded correctly, without warning #19296,
large record from Unexpected warning '[FS3511] This state machine is not statically compilable' #12839,
and this:

fsharp/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/NestedTaskFailures.fs

Line 6 in 6c7aa5b

#nowarn "3511" // state machine not statically compilable - this is a separate issue, see https://github.com/dotnet/fsharp/issues/13404

T-Gro · 2026-02-18T15:25:30Z

Is it right to close #12839 with this ? It looks like an umbrella issue for all sorts of problems.
I am definitely happy if all of them are resolved 👍 , well done.

I am also thinking if there is a way we could add an end2end test here, e.g. using the library regression testing pipeline?
We do have iced tasks in already, is there something we could run as verification or a special flag/prop/check/IL detection perhaps, to spot regressions ?
(regression = code still compiles, but will fallback to dynamic implementation)

majocha · 2026-02-18T15:57:41Z

Is it right to close #12839 with this ? It looks like an umbrella issue for all sorts of problems. I am definitely happy if all of them are resolved 👍 , well done.

As far as I can tell all of them are resolved now, few have been resolved for some time.

I am also thinking if there is a way we could add an end2end test here, e.g. using the library regression testing pipeline? We do have iced tasks in already, is there something we could run as verification or a special flag/prop/check/IL detection perhaps, to spot regressions ? (regression = code still compiles, but will fallback to dynamic implementation)

I have nothing apart from the few tests I added :)

One thing that comes to mind is to disable some of the #nowarn 3511 we have in this repo, or at least make them scoped to the exact place, like using let rec in tasks. But that depends on the warning being reliable.

majocha · 2026-02-18T17:07:13Z

OK, it seems removing nowarn 3511 in this repo uncovers some more cases that need fixing.

…es not have a definition.") consider a Run(code: ResumableCode<...>) method. In some cases code can get optimized away resulting in warning 3511 "The resumable code value(s) 'code' does not have a definition."

… into resumable-fix-19296

majocha · 2026-02-19T09:08:56Z

I removed the few catch all nowarns 3511. This revealed a nice small repro of

The resumable code value(s) 'code' does not have a definition.

Let's see if it's fixed.

majocha · 2026-02-19T10:42:38Z

vsintegration/src/FSharp.Editor/CodeFixes/CodeFixHelpers.fs

    type IFSharpCodeFixProvider with

        member private provider.FixAllAsync (fixAllCtx: FixAllContext) (doc: Document) (allDiagnostics: ImmutableArray<Diagnostic>) =
            cancellableTask {


This was previously giving FS3511 with The resumable code value(s) 'code' does not have a definition.

majocha · 2026-02-19T11:16:09Z

src/FSharp.Core/FSharp.Core.fsproj

-    <!-- Turn off 3511: state machine not compilable - expected for inlined functions defining state machine generators -->
-    <OtherFlags>$(OtherFlags) --nowarn:3511</OtherFlags>


this is the only change to FSharp.Core. I guess no release notes needed.

T-Gro · 2026-02-19T16:56:55Z

Removing the nowarns here is good dogfooding 👍 .

IcedTasks have 3 of them in tests https://github.com/search?q=repo%3ATheAngryByrd%2FIcedTasks%203511&type=code that appear to be intentionally dynamic (for testing), otherwise I do not see it - all good ( I will just check the regression pipeline outputs if there are any warnings from it)

majocha added 2 commits February 16, 2026 13:00

add test

2c0cbae

fix expansion of nested if __useResumableCode

2f16b59

github-project-automation bot added this to F# Compiler and Tooling Feb 16, 2026

github-project-automation bot moved this to New in F# Compiler and Tooling Feb 16, 2026

Merge branch 'main' into resumable-fix-19296

766680e

majocha added 5 commits February 16, 2026 16:02

wip

904da82

Merge branch 'main' into resumable-fix-19296

1819aca

directly compile the test

d63dc85

fix comments

574b9d8

majocha added 2 commits February 16, 2026 23:21

add repro cases - for loop over tuples

250c218

include test and debugpoint handling from dotnet#14930

78d272e

majocha changed the title ~~Resumable state machines: fix #19296~~ Improve static compilation of state machines Feb 17, 2026

majocha added 3 commits February 17, 2026 10:53

add release notes

3491f3a

Merge branch 'main' into resumable-fix-19296

b13be95

Merge branch 'main' into resumable-fix-19296

004e577

majocha marked this pull request as ready for review February 17, 2026 09:56

majocha requested a review from a team as a code owner February 17, 2026 09:56

majocha commented Feb 17, 2026

View reviewed changes

majocha added 3 commits February 17, 2026 11:37

Merge branch 'main' into resumable-fix-19296

e6aa861

Merge branch 'main' into resumable-fix-19296

dfc53f7

add more repros to tests

273e3cf

Merge branch 'main' into resumable-fix-19296

80ad10e

majocha added 5 commits February 19, 2026 09:41

remove catch all 3511 nowarns

98e4727

fix handling outer expandVars ("The resumable code value(s) 'code' do…

7a0418a

…es not have a definition.") consider a Run(code: ResumableCode<...>) method. In some cases code can get optimized away resulting in warning 3511 "The resumable code value(s) 'code' does not have a definition."

fix double wrapped Delay in CancellableTask

d477745

Merge branch 'resumable-fix-19296' of https://github.com/majocha/fsharp…

ae1c1de

… into resumable-fix-19296

Merge branch 'main' into resumable-fix-19296

e51d979

majocha added 2 commits February 19, 2026 10:20

fix fantomas

ba1d9a5

release notes

8c45c13

majocha commented Feb 19, 2026

View reviewed changes

majocha added 2 commits February 19, 2026 14:50

Merge branch 'main' into resumable-fix-19296

1c2f077

Merge branch 'main' into resumable-fix-19296

6a6cd5b

		<!-- Turn off 3511: state machine not compilable - expected for inlined functions defining state machine generators -->
		<OtherFlags>$(OtherFlags) --nowarn:3511</OtherFlags>

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