Add --disableLanguageFeature CLI switch and MSBuild property to selectively disable language features

docs/release-notes/.FSharp.Compiler.Service/11.0.0.md

@@ -20,6 +20,7 @@
 * Add FSharpCodeCompletionOptions ([PR #19030](https://github.com/dotnet/fsharp/pull/19030))
 * Type checker: recover on checking binding parameter constraints ([#19046](https://github.com/dotnet/fsharp/pull/19046))
 * Debugger: provide breakpoint ranges for short lambdas ([#19067](https://github.com/dotnet/fsharp/pull/19067))
+* Add `--disableLanguageFeature` command line switch to selectively disable specific F# language features ([PR #NNNNN](https://github.com/dotnet/fsharp/pull/NNNNN))
 
 ### Changed
 

src/Compiler/Driver/CompilerConfig.fs

@@ -644,6 +644,8 @@ type TcConfigBuilder =
 
         mutable langVersion: LanguageVersion
 
+        mutable disabledLanguageFeatures: Set<LanguageFeature>
+
         mutable xmlDocInfoLoader: IXmlDocumentationInfoLoader option
 
         mutable exiter: Exiter
@@ -836,6 +838,7 @@ type TcConfigBuilder =
             pathMap = PathMap.empty
             applyLineDirectives = true
             langVersion = LanguageVersion.Default
+            disabledLanguageFeatures = Set.empty
             implicitIncludeDir = implicitIncludeDir
             defaultFSharpBinariesDir = defaultFSharpBinariesDir
             reduceMemoryUsage = reduceMemoryUsage
@@ -984,6 +987,9 @@ type TcConfigBuilder =
                     WarnOn = ListSet.insert (=) n tcConfigB.diagnosticsOptions.WarnOn
                 }
 
+    member tcConfigB.SupportsFeature(feature: LanguageFeature) =
+        tcConfigB.langVersion.SupportsFeature(feature)
+
     member tcConfigB.AddIncludePath(m, path, pathIncludedFrom) =
         let absolutePath = ComputeMakePathAbsolute pathIncludedFrom path
 

src/Compiler/Driver/CompilerConfig.fsi

@@ -512,6 +512,8 @@ type TcConfigBuilder =
 
         mutable langVersion: LanguageVersion
 
+        mutable disabledLanguageFeatures: Set<LanguageFeature>
+
         mutable xmlDocInfoLoader: IXmlDocumentationInfoLoader option
 
         mutable exiter: Exiter
@@ -548,6 +550,8 @@ type TcConfigBuilder =
 
     member TurnWarningOn: range * string -> unit
 
+    member SupportsFeature: LanguageFeature -> bool
+
     member AddIncludePath: range * string * string -> unit
 
     member AddCompilerToolsByPath: string -> unit

src/Compiler/Driver/CompilerOptions.fs

@@ -1177,11 +1177,27 @@ let languageFlags tcConfigB =
         CompilerOption(
             "langversion",
             tagLangVersionValues,
-            OptionString(fun switch -> tcConfigB.langVersion <- setLanguageVersion switch),
+            OptionString(fun switch ->
+                tcConfigB.langVersion <- setLanguageVersion switch
+                tcConfigB.langVersion.SetDisabledFeatures(tcConfigB.disabledLanguageFeatures)),
             None,
             Some(FSComp.SR.optsSetLangVersion ())
         )
 
+        // -disableLanguageFeature:<string>  Disable a specific language feature by name (repeatable)
+        CompilerOption(
+            "disableLanguageFeature",
+            tagString,
+            OptionStringList(fun featureName ->
+                match LanguageVersion.TryParseFeature(featureName) with
+                | Some feature ->
+                    tcConfigB.disabledLanguageFeatures <- tcConfigB.disabledLanguageFeatures.Add(feature)
+                    tcConfigB.langVersion.SetDisabledFeatures(tcConfigB.disabledLanguageFeatures)
+                | None -> error (Error(FSComp.SR.optsUnrecognizedLanguageFeature featureName, rangeCmdArgs))),
+            None,
+            Some(FSComp.SR.optsDisableLanguageFeature ())
+        )
+
         CompilerOption(
             "checked",
             tagNone,

src/Compiler/FSComp.txt

@@ -1570,6 +1570,8 @@ optsCheckNulls,"Enable nullness declarations and checks (%s by default)"
 fSharpBannerVersion,"%s for F# %s"
 optsGetLangVersions,"Display the allowed values for language version."
 optsSetLangVersion,"Specify language version such as 'latest' or 'preview'."
+optsDisableLanguageFeature,"Disable a specific language feature by name."
+3879,optsUnrecognizedLanguageFeature,"Unrecognized language feature name: '%s'. Use a valid feature name such as 'NameOf' or 'StringInterpolation'."
 optsSupportedLangVersions,"Supported language versions:"
 optsStrictIndentation,"Override indentation rules implied by the language version (%s by default)"
 nativeResourceFormatError,"Stream does not begin with a null resource and is not in '.RES' format."

src/Compiler/Facilities/LanguageFeatures.fs

@@ -279,11 +279,19 @@ type LanguageVersion(versionText) =
 
     let specifiedString = versionToString specified
 
+    let mutable disabledFeatures: Set<LanguageFeature> = Set.empty
+
     /// Check if this feature is supported by the selected langversion
     member _.SupportsFeature featureId =
-        match features.TryGetValue featureId with
-        | true, v -> v <= specified
-        | false, _ -> false
+        if disabledFeatures.Contains featureId then
+            false
+        else
+            match features.TryGetValue featureId with
+            | true, v -> v <= specified
+            | false, _ -> false
+
+    /// Set the disabled features for this language version
+    member _.SetDisabledFeatures(disabled: Set<LanguageFeature>) = disabledFeatures <- disabled
 
     /// Has preview been explicitly specified
     member _.IsExplicitlySpecifiedAs50OrBefore() =
@@ -422,6 +430,104 @@ type LanguageVersion(versionText) =
         | true, v -> versionToString v
         | _ -> invalidArg "feature" "Internal error: Unable to find feature."
 
+    /// Try to parse a feature name string to a LanguageFeature option
+    static member TryParseFeature(featureName: string) =
+        let normalized = featureName.Trim().ToLowerInvariant()
+
+        match normalized with
+        | "singleunderscorepattern" -> Some LanguageFeature.SingleUnderscorePattern
+        | "wildcardinforloop" -> Some LanguageFeature.WildCardInForLoop
+        | "relaxwhitespace" -> Some LanguageFeature.RelaxWhitespace
+        | "relaxwhitespace2" -> Some LanguageFeature.RelaxWhitespace2
+        | "strictindentation" -> Some LanguageFeature.StrictIndentation
+        | "nameof" -> Some LanguageFeature.NameOf
+        | "implicityield" -> Some LanguageFeature.ImplicitYield
+        | "opentypedeclaration" -> Some LanguageFeature.OpenTypeDeclaration
+        | "dotlessfloat32literal" -> Some LanguageFeature.DotlessFloat32Literal
+        | "packagemanagement" -> Some LanguageFeature.PackageManagement
+        | "fromendslicing" -> Some LanguageFeature.FromEndSlicing
+        | "fixedindexslice3d4d" -> Some LanguageFeature.FixedIndexSlice3d4d
+        | "andbang" -> Some LanguageFeature.AndBang
+        | "resumablestatemachines" -> Some LanguageFeature.ResumableStateMachines
+        | "nullableoptionalinterop" -> Some LanguageFeature.NullableOptionalInterop
+        | "defaultinterfacememberconsumption" -> Some LanguageFeature.DefaultInterfaceMemberConsumption
+        | "witnesspassing" -> Some LanguageFeature.WitnessPassing
+        | "additionaltypedirectedconversions" -> Some LanguageFeature.AdditionalTypeDirectedConversions
+        | "interfaceswithmultiplegenericinstantiation" -> Some LanguageFeature.InterfacesWithMultipleGenericInstantiation
+        | "stringinterpolation" -> Some LanguageFeature.StringInterpolation
+        | "overloadsforcustomoperations" -> Some LanguageFeature.OverloadsForCustomOperations
+        | "expandedmeasurables" -> Some LanguageFeature.ExpandedMeasurables
+        | "nullnesschecking" -> Some LanguageFeature.NullnessChecking
+        | "structactivepattern" -> Some LanguageFeature.StructActivePattern
+        | "printfbinaryformat" -> Some LanguageFeature.PrintfBinaryFormat
+        | "indexernotationwithoutdot" -> Some LanguageFeature.IndexerNotationWithoutDot
+        | "refcellnotationinformationals" -> Some LanguageFeature.RefCellNotationInformationals
+        | "usebindingvaluediscard" -> Some LanguageFeature.UseBindingValueDiscard
+        | "unionispropertiesvisible" -> Some LanguageFeature.UnionIsPropertiesVisible
+        | "nonvariablepatternstorightofaspatterns" -> Some LanguageFeature.NonVariablePatternsToRightOfAsPatterns
+        | "attributestorightofmodulekeyword" -> Some LanguageFeature.AttributesToRightOfModuleKeyword
+        | "mlcompatrevisions" -> Some LanguageFeature.MLCompatRevisions
+        | "betterexceptionprinting" -> Some LanguageFeature.BetterExceptionPrinting
+        | "delegatetypenameresolutionfix" -> Some LanguageFeature.DelegateTypeNameResolutionFix
+        | "reallylonglists" -> Some LanguageFeature.ReallyLongLists
+        | "errorondeprecatedrequirequalifiedaccess" -> Some LanguageFeature.ErrorOnDeprecatedRequireQualifiedAccess
+        | "requiredpropertiessupport" -> Some LanguageFeature.RequiredPropertiesSupport
+        | "initpropertiessupport" -> Some LanguageFeature.InitPropertiesSupport
+        | "lowercaseduwhenrequirequalifiedaccess" -> Some LanguageFeature.LowercaseDUWhenRequireQualifiedAccess
+        | "interfaceswithabstractstaticmembers" -> Some LanguageFeature.InterfacesWithAbstractStaticMembers
+        | "selftypeconstraints" -> Some LanguageFeature.SelfTypeConstraints
+        | "accessorfunctionshorthand" -> Some LanguageFeature.AccessorFunctionShorthand
+        | "matchnotallowedforunioncasewithnodata" -> Some LanguageFeature.MatchNotAllowedForUnionCaseWithNoData
+        | "csharpextensionattributenotrequired" -> Some LanguageFeature.CSharpExtensionAttributeNotRequired
+        | "errorfornonvirtualmembersoverrides" -> Some LanguageFeature.ErrorForNonVirtualMembersOverrides
+        | "warningwheninliningmethodimplnoinlinemarkedfunction" -> Some LanguageFeature.WarningWhenInliningMethodImplNoInlineMarkedFunction
+        | "escapedotnetformattablestrings" -> Some LanguageFeature.EscapeDotnetFormattableStrings
+        | "arithmeticinliterals" -> Some LanguageFeature.ArithmeticInLiterals
+        | "errorreportingonstaticclasses" -> Some LanguageFeature.ErrorReportingOnStaticClasses
+        | "trywithinseqexpression" -> Some LanguageFeature.TryWithInSeqExpression
+        | "warningwhencopyandupdaterecordchangesallfields" -> Some LanguageFeature.WarningWhenCopyAndUpdateRecordChangesAllFields
+        | "staticmembersininterfaces" -> Some LanguageFeature.StaticMembersInInterfaces
+        | "noninlineliteralsasprintfformat" -> Some LanguageFeature.NonInlineLiteralsAsPrintfFormat
+        | "nestedcopyandupdate" -> Some LanguageFeature.NestedCopyAndUpdate
+        | "extendedstringinterpolation" -> Some LanguageFeature.ExtendedStringInterpolation
+        | "warningwhenmultiplerecdtypechoice" -> Some LanguageFeature.WarningWhenMultipleRecdTypeChoice
+        | "improvedimpliedargumentnames" -> Some LanguageFeature.ImprovedImpliedArgumentNames
+        | "diagnosticforobjinference" -> Some LanguageFeature.DiagnosticForObjInference
+        | "constraintintersectiononflexibletypes" -> Some LanguageFeature.ConstraintIntersectionOnFlexibleTypes
+        | "staticletinrecordsdustemptypes" -> Some LanguageFeature.StaticLetInRecordsDusEmptyTypes
+        | "warningwhentailrecattributebutnontailrecusage" -> Some LanguageFeature.WarningWhenTailRecAttributeButNonTailRecUsage
+        | "unmanagedconstraintcsharpinterop" -> Some LanguageFeature.UnmanagedConstraintCsharpInterop
+        | "whilebang" -> Some LanguageFeature.WhileBang
+        | "reusesamefieldssinstructunions" -> Some LanguageFeature.ReuseSameFieldsInStructUnions
+        | "extendedfixedbindings" -> Some LanguageFeature.ExtendedFixedBindings
+        | "preferstringgetpinnablereference" -> Some LanguageFeature.PreferStringGetPinnableReference
+        | "preferextensionmethodoverplainproperty" -> Some LanguageFeature.PreferExtensionMethodOverPlainProperty
+        | "warningindexedpropertiesgetsetsametype" -> Some LanguageFeature.WarningIndexedPropertiesGetSetSameType
+        | "warningwhentailcallattrononrec" -> Some LanguageFeature.WarningWhenTailCallAttrOnNonRec
+        | "booleanreturningandreturntypedirectedpartialactivepattern" ->
+            Some LanguageFeature.BooleanReturningAndReturnTypeDirectedPartialActivePattern
+        | "enforceattributetargets" -> Some LanguageFeature.EnforceAttributeTargets
+        | "lowerinterpolatedstringtoconcat" -> Some LanguageFeature.LowerInterpolatedStringToConcat
+        | "lowerintegralrangestofastloops" -> Some LanguageFeature.LowerIntegralRangesToFastLoops
+        | "allowaccessmodifierstautopropertiesgettersandsetters" ->
+            Some LanguageFeature.AllowAccessModifiersToAutoPropertiesGettersAndSetters
+        | "lowersimplemappingsincomprehensionstofastloops" -> Some LanguageFeature.LowerSimpleMappingsInComprehensionsToFastLoops
+        | "parsedhashdirectiveargumentnonquotes" -> Some LanguageFeature.ParsedHashDirectiveArgumentNonQuotes
+        | "emptybodiedcomputationexpressions" -> Some LanguageFeature.EmptyBodiedComputationExpressions
+        | "allowobjectexpressionwithoutoverrides" -> Some LanguageFeature.AllowObjectExpressionWithoutOverrides
+        | "dontwarnunuppercaseidentifiersinbindingpatterns" -> Some LanguageFeature.DontWarnOnUppercaseIdentifiersInBindingPatterns
+        | "usetypesubsumptioncache" -> Some LanguageFeature.UseTypeSubsumptionCache
+        | "deprecateplaceswheresecanbemitted" -> Some LanguageFeature.DeprecatePlacesWhereSeqCanBeOmitted
+        | "supportvalueoptionsasoptionalparameters" -> Some LanguageFeature.SupportValueOptionsAsOptionalParameters
+        | "warnwhenunitpassedtoobjarg" -> Some LanguageFeature.WarnWhenUnitPassedToObjArg
+        | "usebangbindingvaluediscard" -> Some LanguageFeature.UseBangBindingValueDiscard
+        | "betteranonymousrecordparsing" -> Some LanguageFeature.BetterAnonymousRecordParsing
+        | "scopednowarn" -> Some LanguageFeature.ScopedNowarn
+        | "erroroninvaliddeclsintypedefinitions" -> Some LanguageFeature.ErrorOnInvalidDeclsInTypeDefinitions
+        | "allowtypedletuseandbang" -> Some LanguageFeature.AllowTypedLetUseAndBang
+        | "returnfromfinal" -> Some LanguageFeature.ReturnFromFinal
+        | _ -> None
+
     override x.Equals(yobj: obj) =
         match yobj with
         | :? LanguageVersion as y -> x.SpecifiedVersion = y.SpecifiedVersion

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -115,6 +115,9 @@ type LanguageVersion =
     /// Does the selected LanguageVersion support the specified feature
     member SupportsFeature: LanguageFeature -> bool
 
+    /// Set the disabled features for this language version
+    member SetDisabledFeatures: Set<LanguageFeature> -> unit
+
     /// Get the list of valid versions
     static member ValidVersions: string[]
 
@@ -136,4 +139,7 @@ type LanguageVersion =
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString: feature: LanguageFeature -> string
 
+    /// Try to parse a feature name string to a LanguageFeature option
+    static member TryParseFeature: featureName: string -> LanguageFeature option
+
     static member Default: LanguageVersion