Optimizer: don't inline named functions in debug builds

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

@@ -22,3 +22,4 @@
 
 * Added warning FS3884 when a function or delegate value is used as an interpolated string argument. ([PR #19289](https://github.com/dotnet/fsharp/pull/19289))
 * Add `#version;;` directive to F# Interactive to display version and environment information. ([Issue #13307](https://github.com/dotnet/fsharp/issues/13307), [PR #19332](https://github.com/dotnet/fsharp/pull/19332))
+* Optimizer: don't inline named functions in debug builds ([PR #19548](https://github.com/dotnet/fsharp/pull/19548)

src/Compiler/CodeGen/IlxGen.fs

@@ -269,6 +269,8 @@ type IlxGenOptions =
 
         /// When set to true, the IlxGen will delay generation of method bodies and generated them later in parallel (parallelized across files)
         parallelIlxGenEnabled: bool
+
+        inlineNamedFunctions: bool
     }
 
 /// Compilation environment for compiling a fragment of an assembly
@@ -5586,8 +5588,13 @@ and GenTraitCall (cenv: cenv) cgbuf eenv (traitInfo: TraitConstraintInfo, argExp
 
     | None ->
 
-        // If witnesses are available, we should now always find trait witnesses in scope
-        assert not generateWitnesses
+        // When inlineNamedFunctions is true, all trait calls should be resolved via witnesses in scope.
+        // When inlineNamedFunctions is false, inline functions are kept as calls rather than inlined.
+        // Their witness arguments may contain TraitCall operations for constraints that were resolved
+        // without a witness (e.g., when the constraint is satisfied by a known concrete type).
+        // In such cases, generateWitnesses can be true (because other witnesses are in scope) but
+        // the specific trait's witness is not found. Fall through to the constraint solver to resolve it.
+        assert (not generateWitnesses || not cenv.options.inlineNamedFunctions)
 
         let exprOpt =
             CommitOperationResult(ConstraintSolver.CodegenWitnessExprForTraitConstraint cenv.tcVal g cenv.amap m traitInfo argExprs)

src/Compiler/CodeGen/IlxGen.fsi

@@ -61,6 +61,9 @@ type internal IlxGenOptions =
 
         /// When set to true, the IlxGen will delay generation of method bodies and generate them later in parallel (parallelized across files)
         parallelIlxGenEnabled: bool
+
+        /// Indicates if inline named functions are being inlined or emitted as calls
+        inlineNamedFunctions: bool
     }
 
 /// The results of the ILX compilation of one fragment of an assembly

src/Compiler/Driver/CompilerConfig.fs

@@ -600,6 +600,8 @@ type TcConfigBuilder =
 
         mutable strictIndentation: bool option
 
+        mutable inlineNamedFunctions: bool option
+
         mutable exename: string option
 
         // If true - the compiler will copy FSharp.Core.dll along the produced binaries
@@ -853,6 +855,7 @@ type TcConfigBuilder =
             dumpSignatureData = false
             realsig = false
             strictIndentation = None
+            inlineNamedFunctions = None
             compilationMode = TcGlobals.CompilationMode.Unset
         }
 
@@ -1253,6 +1256,7 @@ type TcConfig private (data: TcConfigBuilder, validate: bool) =
     member _.fsiMultiAssemblyEmit = data.fsiMultiAssemblyEmit
     member _.FxResolver = data.FxResolver
     member _.strictIndentation = data.strictIndentation
+    member _.inlineNamedFunctions = data.inlineNamedFunctions
     member _.primaryAssembly = data.primaryAssembly
     member _.noFeedback = data.noFeedback
     member _.stackReserveSize = data.stackReserveSize

src/Compiler/Driver/CompilerConfig.fsi

@@ -472,6 +472,8 @@ type TcConfigBuilder =
 
         mutable strictIndentation: bool option
 
+        mutable inlineNamedFunctions: bool option
+
         mutable exename: string option
 
         mutable copyFSharpCore: CopyFSharpCoreFlag
@@ -814,6 +816,8 @@ type TcConfig =
 
     member strictIndentation: bool option
 
+    member inlineNamedFunctions: bool option
+
     member GetTargetFrameworkDirectories: unit -> string list
 
     /// Get the loaded sources that exist and issue a warning for the ones that don't

src/Compiler/Driver/CompilerOptions.fs

@@ -1207,6 +1207,14 @@ let languageFlags tcConfigB =
             None,
             Some(FSComp.SR.optsStrictIndentation (formatOptionSwitch (Option.defaultValue false tcConfigB.strictIndentation)))
         )
+
+        CompilerOption(
+            "inline-named-functions",
+            tagNone,
+            OptionSwitch(fun switch -> tcConfigB.inlineNamedFunctions <- Some(switch = OptionSwitch.On)),
+            None,
+            Some(FSComp.SR.optsInlineNamedFunctions ())
+        )
     ]
 
 // OptionBlock: Advanced user options

src/Compiler/Driver/OptimizeInputs.fs

@@ -327,13 +327,17 @@ let ApplyAllOptimizations
             // Only do abstractBigTargets in the first phase, and only when TLR is on.
             abstractBigTargets = tcConfig.doTLR
             reportingPhase = true
+            inlineNamedFunctions =
+                tcConfig.inlineNamedFunctions
+                |> Option.defaultValue (not tcConfig.debuginfo || tcConfig.optSettings.LocalOptimizationsEnabled)
         }
 
     // Only do these two steps in the first phase.
     let extraAndFinalLoopSettings =
         { firstLoopSettings with
             abstractBigTargets = false
             reportingPhase = false
+            inlineNamedFunctions = false
         }
 
     let addPhaseDiagnostics (f: PhaseFunc) (info: Phase) =
@@ -578,6 +582,9 @@ let GenerateIlxCode
             isInteractiveItExpr = isInteractiveItExpr
             alwaysCallVirt = tcConfig.alwaysCallVirt
             parallelIlxGenEnabled = tcConfig.parallelIlxGen
+            inlineNamedFunctions =
+                tcConfig.inlineNamedFunctions
+                |> Option.defaultValue (not tcConfig.debuginfo || tcConfig.optSettings.LocalOptimizationsEnabled)
         }
 
     ilxGenerator.GenerateCode(ilxGenOpts, optimizedImpls, topAttrs.assemblyAttrs, topAttrs.netModuleAttrs)

src/Compiler/FSComp.txt

@@ -1560,6 +1560,7 @@ optsSetLangVersion,"Specify language version such as 'latest' or 'preview'."
 optsDisableLanguageFeature,"Disable a specific language feature by name."
 optsSupportedLangVersions,"Supported language versions:"
 optsStrictIndentation,"Override indentation rules implied by the language version (%s by default)"
+optsInlineNamedFunctions,"Inline named 'inline' functions"
 nativeResourceFormatError,"Stream does not begin with a null resource and is not in '.RES' format."
 nativeResourceHeaderMalformed,"Resource header beginning at offset %s is malformed."
 formatDashItem," - %s"