Resumable state machines: fix "builder@" variable leak (#19630)

Author: majocha

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

@@ -36,6 +36,7 @@
 * Fix signature generation: `private` keyword placement for prefix-style type abbreviations. ([Issue #15560](https://github.com/dotnet/fsharp/issues/15560), [PR #19586](https://github.com/dotnet/fsharp/pull/19586))
 * Fix signature generation: missing `[<Class>]` attribute for types without visible constructors. ([Issue #16531](https://github.com/dotnet/fsharp/issues/16531), [PR #19586](https://github.com/dotnet/fsharp/pull/19586))
 * Fix methods being tagged as `Member` instead of `Method` in tooltips. ([Issue #10540](https://github.com/dotnet/fsharp/issues/10540), [PR #19507](https://github.com/dotnet/fsharp/pull/19507))
+* Fix Debug-mode compilation when mixing resumable and standard computation expressions. ([Issue #19625](https://github.com/dotnet/fsharp/issues/19625), [PR #19630](https://github.com/dotnet/fsharp/pull/19630))
 * IlxGen: fix missing CompilationMapping attribute for generic values ([PR #19643](https://github.com/dotnet/fsharp/pull/19643))
 
 ### Added

src/Compiler/Optimize/LowerStateMachines.fs

@@ -338,6 +338,13 @@ type LowerStateMachine(g: TcGlobals, outerResumableCodeDefns: ValMap<Expr>) =
         if sm_verbose then printfn "expanding defns and reducing %A..." expr
         //if sm_verbose then printfn "checking %A for possible resumable code application..." expr
         match expr with
+        // Reduce helper-local 'if __useResumableCode then ... else ...' after inlining,
+        // but preserve real nested state machines so their own lowering can still choose
+        // the dynamic fallback if static compilation fails.
+        | IfUseResumableStateMachinesExpr g (thenExpr, _) when Option.isNone (IsStateMachineExpr g thenExpr) ->
+            if sm_verbose then printfn "reducing helper-local 'if __useResumableCode ...' to static branch"
+            Some (remake thenExpr)
+
         // defn --> [expand_code]
         | Expr.Val (defnRef, _, _) when env.ResumableCodeDefns.ContainsVal defnRef.Deref ->
             let defn = env.ResumableCodeDefns[defnRef.Deref]
@@ -372,22 +379,13 @@ type LowerStateMachine(g: TcGlobals, outerResumableCodeDefns: ValMap<Expr>) =
     // Repeated top-down rewrite
     let makeRewriteEnv (env: env) = 
         { PreIntercept = Some (fun cont e ->
-            match e with
-            // Don't recurse into nested state machine expressions - they will be
-            // processed by their own LowerStateMachineExpr during codegen.
-            // This prevents modification of the nested machine's internal
-            // 'if __useResumableCode' patterns which select its dynamic fallback.
-            | _ when Option.isSome (IsStateMachineExpr g e) -> Some e
-            // Eliminate 'if __useResumableCode' - nested state machines are already
-            // guarded above, so any remaining occurrences at this level are from
-            // beta-reduced inline helpers and should take the static branch.
-            | IfUseResumableStateMachinesExpr g (thenExpr, _) -> Some (cont thenExpr)
-            | _ ->
-            match TryReduceExpr env e [] id with Some e2 -> Some (cont e2) | None -> None)
+            match TryReduceExpr env e [] id with
+            | Some e2 -> Some (cont e2)
+            | None -> None)
           PostTransform = (fun _ -> None)
           PreInterceptBinding = None
           RewriteQuotations=true 
-          StackGuard = StackGuard("LowerStateMachineStackGuardDepth") }
+          StackGuard = StackGuard("LowerStateMachineStackGuard") }
 
     let ConvertStateMachineLeafExpression (env: env) expr = 
         if sm_verbose then printfn "ConvertStateMachineLeafExpression for %A..." expr

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

@@ -6,15 +6,32 @@ open Xunit
 open FSharp.Test.Assert
 open FSharp.Test.Compiler
 
+module StateMachineTests =
 
-// Inlined helper containing a "if __useResumableCode ..." construct failed to expand correctly,
-// executing the dynmamic branch at runtime even when the state machine was compiled statically.
-// see https://github.com/dotnet/fsharp/issues/19296
-module FailingInlinedHelper =
-    open FSharp.Core.CompilerServices
-    open FSharp.Core.CompilerServices.StateMachineHelpers
-    open System.Runtime.CompilerServices
+    let verify3511AndRun code = 
+        Fsx code
+        |> withNoOptimize
+        |> compile
+        |> shouldFail
+        |> withWarningCode 3511
+        |> ignore
+
+        Fsx code
+        |> withNoOptimize
+        |> withOptions ["--nowarn:3511"]
+        |> compileExeAndRun
 
+    // Inlined helper containing a "if __useResumableCode ..." construct failed to expand correctly,
+    // executing the dynmamic branch at runtime even when the state machine was compiled statically.
+    // see https://github.com/dotnet/fsharp/issues/19296
+    [<Fact>]
+    let ``Nested __useResumableCode is expanded correctly`` () =
+        Fsx """
+open FSharp.Core.CompilerServices
+open FSharp.Core.CompilerServices.StateMachineHelpers
+open System.Runtime.CompilerServices
+
+module FailingInlinedHelper =
     let inline MoveOnce(x: byref<'T> when 'T :> IAsyncStateMachine and 'T :> IResumableStateMachine<'Data>) =
         x.MoveNext()
         x.Data
@@ -27,7 +44,7 @@ module FailingInlinedHelper =
             else
                 failwith "unexpected dynamic branch at runtime")
 
-    #nowarn 3513 // Resumable code invocation.
+    #nowarn 3513
     let inline repro x =
         if __useResumableCode then
             __stateMachine<int, int>
@@ -38,25 +55,10 @@ module FailingInlinedHelper =
             failwith "dynamic state machine"
     #warnon 3513
 
-module StateMachineTests =
-
-    let verify3511AndRun code = 
-        Fsx code
-        |> withNoOptimize
-        |> compile
-        |> shouldFail
-        |> withWarningCode 3511
-        |> ignore
-
-        Fsx code
-        |> withNoOptimize
-        |> withOptions ["--nowarn:3511"]
+if FailingInlinedHelper.repro 42 <> 42 then failwith "unexpected result"
+"""
         |> compileExeAndRun
-
-    [<Fact>]
-    let ``Nested __useResumableCode is expanded correctly`` () =
-        FailingInlinedHelper.repro 42
-        |> shouldEqual 42
+        |> shouldSucceed
 
     [<Fact>] // https://github.com/dotnet/fsharp/issues/13067
     let ``Local function with a flexible type``() = 
@@ -467,3 +469,34 @@ if result[0].x <> 1 then failwith $"unexpected result {result[0]}"
         |> asExe
         |> compileExeAndRun
         |> shouldSucceed
+
+    [<Fact>]
+    let ``Debug-mode: mixing resumable and standard computation expressions compiles``() =
+        FSharp """
+module ReproMixedBuilders
+open System.Threading.Tasks
+
+type TaskMaybeBuilder() =
+
+    member inline _.Zero() = Task.FromResult None
+
+    member inline _.Delay([<InlineIfLambda>] f) = task { return! f () }
+
+    member inline _.Bind(value, [<InlineIfLambda>] f) =
+        task {
+            match value with
+            | None -> return None
+            | Some result -> return! f result
+        }
+
+let taskMaybe = TaskMaybeBuilder()
+
+let trigger() =
+    taskMaybe {
+        do! None
+    }
+"""
+        |> withDebug
+        |> withNoOptimize
+        |> compile
+        |> shouldSucceed