Implement direct delegates by kerams · Pull Request #19993 · dotnet/fsharp

kerams · 2026-06-24T18:26:47Z

Description

Implements fsharp/fslang-suggestions#1083, fixes #11898. RFC with unresolved questions.

The IlxGen.fs diff is slightly weird, but basically lines 7465-7623 are new, and the remaining function body below is just existing code indented further to the right and otherwise unchanged.

I left a lot of the comments in place to make reviewing easier, but they can be certainly removed later. Test cases should cover everything mentioned in the language suggestion.

Considering this can be seen as a breaking change, we could also expose it behind an opt-in compiler flag, rather than language feature.

Checklist

Test cases added
Performance benchmarks added in case of performance changes
Release notes entry updated:

github-actions · 2026-06-24T18:27:28Z

❗ 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/Compiler docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

LanguageFeatures.fsi docs/release-notes/.Language/preview.md

kerams · 2026-06-24T18:29:13Z

There are a couple of inner functions here. Not sure where exactly to place them.

Does delegate codegen have a lot of dependencies on other parts of IlxGen? Could it with all its inner functions perhaps live in a separate file? IlxGen.Delegates.fs or so?

github-actions · 2026-06-24T19:06:11Z

🔍 Tooling Safety Check — Affects-Compiler-Output
Affects-Compiler-Output: modifies IlxGen.fs IL emission for delegate expressions

Generated by PR Tooling Safety Check · opus46 4.6M · ◷

T-Gro · 2026-06-25T11:27:20Z

+
+module DirectDelegates =
+
+    let private coreOptions compilation =


Could you pls also add a call to the runtime's IL verifier, to make sure we produce valid IL ?

Example here, there are more helpers for it.

fsharp/tests/FSharp.Compiler.ComponentTests/EmittedIL/RealInternalSignature/ClassTypeInitialization.fs

Lines 46 to 49 in f8da10d

|> verifyPEFileWithSystemDlls

|> withOutputContainsAllInOrderWithWildcards [

"All Classes and Methods in*SimpleTypesInNamespace.exe Verified."

]

T-Gro · 2026-06-25T11:33:28Z

@@ -0,0 +1,417 @@
+


Unless there are valid , by-design (or by test-case) reasons for deviations across optimize/realsig switches, it is better to have a single .bsl file covering more runs.

That way the tests hold equal codegen for both versions together, and guarantee it cannot deviate.

(the test framework allows for that - if e.g. .RealInternalSignature{b}. part is missing, it takes the common name.

There are deviations depending on language version and optimizations. Therefore we have 4 snapshots of everything and can prove the feature is on under specific circumstances only. I suppose I can remove the signature switching though.

But honestly, the language feature clearly gates this, so if you think there is no point asserting the non-preview behavior, I'll happily remove half of the baselines.

T-Gro · 2026-06-25T11:42:21Z

+            not cenv.options.localOptimizationsEnabled
+            && tmvs |> List.exists (fun (v: Val) -> not v.IsCompilerGenerated)
+        then
+            // Eta-expanded delegate in an unoptimized build: the Invoke parameters are the user's own lambda
+            // variables, so keep the closure to preserve their names for debugging. Non-eta delegates (whose
+            // parameters are synthesized by BuildNewDelegateExpr) are always made direct; eta-expanded ones
+            // are made direct only in optimized builds, where debuggability is not a concern and the
+            // forwarding call survives to here.
+            None
+        else


If we are sure we are not losing any sequence points for any forms of lambdas, maybe the new stepping test helpers from @auduchinok could be used to guard that behavior.

Yes, None here causes a fallback to existing behavior, completely unchanged.

T-Gro · 2026-06-25T11:44:44Z

+                            hasWitnesses
+                            || newobj
+                            || isSuperInit
+                            || isSelfInit
+                            || isBaseCall
+                            || not (receiverShapeOk takesInstanceArg)
+                            || receiverNotBindable ()


I would like if this block of code has better explanation and maybe even encapsulation if possible (similar one is also for ILMethod ). To make sure future extensions of runtime/language know when to extend this block.

T-Gro · 2026-06-25T11:50:56Z

+
+open System
+
+// Section D, bullet 1: the optimizer inlines small function bodies before IlxGen runs. If a delegate


Another dimension: Inlining across F# assembly boundaries

T-Gro · 2026-06-25T11:51:23Z

+// 3. eta-expanded known target, tupled application, same compiled representation
+let case3_etaTupled () = Action<int, int>(fun a b -> handlerTupled (a, b))
+
+// 15. non-eta-expanded known target via partial application


I'll make sure the numbering is unified with the RFC cases

T-Gro · 2026-06-25T11:51:31Z

+
+open System
+
+// Section D, bullet 1: the optimizer inlines small function bodies before IlxGen runs. If a delegate


T-Gro · 2026-06-25T11:52:04Z

+
+open System
+
+// Section D, bullet 2: cases 15/16 (in DelegateKnownFunction.fs / DelegateStaticMethod.fs) capture a


Clean the development-time pointers/indices please.

T-Gro · 2026-06-25T11:55:01Z

+        |> coreOptions
+        |> withLangVersionPreview
+        |> withPreviewBaseline
+        |> compile


At least a representative for each positive case should be also running, not just compile.

T-Gro · 2026-06-25T11:56:33Z

+        |> coreOptions
+        |> compile
+        |> shouldSucceed
+        |> verifyILBaseline


One more thought - for the positive scenarios, can we be more specific and instead of .bsl do a pair of "contains" (function pointer load) and "not contains" (no closure allocation).

ldftn/ldvirtftn are present in both cases, along with putting arguments on the stack. The latter also requires a dup instruction. If your goal is to minimize potential future baseline churn, I'm not sure this would help.

Yep, the goal would to minimize the chance of an unrelated change affecting the .bsl.
But the code samples are well isolated, so its not a big deal.

…est case numbering

kerams · 2026-06-26T16:49:30Z

Ready for another pass, think I addressed everything. (But alternatives and unresolved questions in the RFC need answers still:))

smoothdeveloper · 2026-06-26T18:25:08Z

Add an opt-out compiler flag (default on) alongside the language feature.

I think this is not needed anymore, Tomas has added a msbuild way as well as compiler flag to disable specific language features:

--disableLanguageFeature:DirectDelegateConstruction

or

 <ItemGroup>
  <DisabledLanguageFeatures Include="DirectDelegateConstruction" />
</ItemGroup>

You may want to check it picks up as is to allow selectively disabling the feature.

kerams added 3 commits June 24, 2026 00:39

Add language feature and baselines tests

baf99e8

Implementation

87cff9a

Merge branch 'main' of https://github.com/dotnet/fsharp into del

6682524

kerams requested a review from a team as a code owner June 24, 2026 18:26

github-project-automation Bot added this to F# Compiler and Tooling Jun 24, 2026

github-project-automation Bot moved this to New in F# Compiler and Tooling Jun 24, 2026

kerams commented Jun 24, 2026

View reviewed changes

github-actions Bot added the ⚠️ Affects-Compiler-Output Tooling check: PR touches IL emission or codegen label Jun 24, 2026

Release notes, custom delegate tests

894f6f0

T-Gro reviewed Jun 25, 2026

View reviewed changes

kerams added 4 commits June 25, 2026 18:58

Merge branch 'main' of https://github.com/dotnet/fsharp into del

67b1ca7

verifyPEFileWithSystemDlls, remove realsig keying, RFC <-> baseline t…

4270507

…est case numbering

Remove duplicate test cases

2d60fe4

Refactor

dc32fcd

Build fix

8017ff0

Build fix

cd6e3c6


		module DirectDelegates =

		let private coreOptions compilation =

	\|> verifyPEFileWithSystemDlls
	\|> withOutputContainsAllInOrderWithWildcards [
	"All Classes and Methods in*SimpleTypesInNamespace.exe Verified."
	]


		open System

		// Section D, bullet 1: the optimizer inlines small function bodies before IlxGen runs. If a delegate


		open System

		// Section D, bullet 2: cases 15/16 (in DelegateKnownFunction.fs / DelegateStaticMethod.fs) capture a

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kerams commented Jun 26, 2026

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smoothdeveloper commented Jun 26, 2026

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kerams commented Jun 24, 2026 •

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