Support NotNullIfNotNullAttribute

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

@@ -103,6 +103,7 @@
 * Debug: rework for expressions stepping ([PR #19894](https://github.com/dotnet/fsharp/pull/19894))
 * Debug: rework conditional erasure, fix stepping over literals ([PR #19897](https://github.com/dotnet/fsharp/pull/19897))
 * Debug: fix if and match condition sequence points ([PR #19932](https://github.com/dotnet/fsharp/pull/19932))
+* Support common types of `NotNullIfNotNullAttribute` usage. If a method parameter is marked with `NotNullIfNotNullAttribute`, the compiler will now honor this attribute and mark the return type as non-null. ([PR #19977](https://github.com/dotnet/fsharp/pull/19977))
 * Checker: recover on checking language version ([PR ##19970](https://github.com/dotnet/fsharp/pull/19970))
 
 ### Improved

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

@@ -3,6 +3,7 @@
 * Warn (FS3884) when a function or delegate value is used as an interpolated string argument, since it will be formatted via `ToString` rather than being applied. ([PR #19289](https://github.com/dotnet/fsharp/pull/19289))
 * Added `MethodOverloadsCache` language feature (preview) that caches overload resolution results for repeated method calls, significantly improving compilation performance. ([PR #19072](https://github.com/dotnet/fsharp/pull/19072))
 * Added `ErrorOnMissingSignatureAttribute` preview language feature: makes FS3888 (compiler-semantic attribute on the `.fs` but not on the `.fsi`) an error instead of a warning. ([Issue #19560](https://github.com/dotnet/fsharp/issues/19560), [PR #19880](https://github.com/dotnet/fsharp/pull/19880))
+* Support common types of `NotNullIfNotNullAttribute` usage. If a method parameter is marked with `NotNullIfNotNullAttribute`, the compiler will now honor this attribute and mark the return type as non-null. ([PR #19977](https://github.com/dotnet/fsharp/pull/19977))
 
 ### Fixed
 

src/Compiler/AbstractIL/il.fs

@@ -1257,6 +1257,7 @@ type WellKnownILAttributes =
     | RequiredMemberAttribute = (1u <<< 22)
     | NullableContextAttribute = (1u <<< 23)
     | AttributeUsageAttribute = (1u <<< 24)
+    | NotNullIfNotNullAttribute = (1u <<< 25)
     | NotComputed = (1u <<< 31)
 
 type internal ILAttributesStoredRepr =

src/Compiler/AbstractIL/il.fsi

@@ -907,6 +907,7 @@ type WellKnownILAttributes =
     | RequiredMemberAttribute = (1u <<< 22)
     | NullableContextAttribute = (1u <<< 23)
     | AttributeUsageAttribute = (1u <<< 24)
+    | NotNullIfNotNullAttribute = (1u <<< 25)
     | NotComputed = (1u <<< 31)
 
 /// Represents the efficiency-oriented storage of ILAttributes in another item.

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -3331,6 +3331,46 @@ let GetMethodArgs arg =
 
     unnamedCallerArgs, namedCallerArgs
 
+let NotNullIfNotNullParamNames g (minfo: MethInfo) =
+    match minfo with
+    | ILMeth(ilMethInfo = ilminfo) when ilminfo.RawMetadata.Return.CustomAttrsStored.HasWellKnownAttribute (g, WellKnownILAttributes.NotNullIfNotNullAttribute) ->
+        ilminfo.RawMetadata.Return.CustomAttrs.AsArray()
+        |> Array.toList
+        |> List.choose (fun attr ->
+            if classifyILAttrib attr &&& WellKnownILAttributes.NotNullIfNotNullAttribute <> WellKnownILAttributes.None then
+                match decodeILAttribData attr with
+                | [ ILAttribElem.String (Some paramName) ], _ -> Some paramName
+                | _ -> None
+            else
+                None)
+    | FSMeth(valRef = vref) ->
+        match vref.ValReprInfo with
+        | Some (ValReprInfo(result = retInfo)) when ArgReprInfoHasWellKnownAttribute g WellKnownValAttributes.NotNullIfNotNullAttribute retInfo ->
+            retInfo.Attribs.AsList()
+            |> List.choose (fun attrib ->
+                if classifyValAttrib g attrib &&& WellKnownValAttributes.NotNullIfNotNullAttribute <> WellKnownValAttributes.None then
+                    match attrib with
+                    | Attrib(unnamedArgs = [ AttribStringArg paramName ]) -> Some paramName
+                    | _ -> None
+                else
+                    None)
+        | _ -> []
+    | _ -> []
+
+// Resolve the caller argument bound to 'paramName' and return the type of its type-checked expression.
+let TryGetCallerArgType g (minfo: MethInfo) (callerArgs: CallerArgs<_>) paramName =
+    // First try to find a named argument with the given name
+    callerArgs.Named
+    |> List.tryPick (List.tryPick (fun (CallerNamedArg(id, arg)) -> if id.idText = paramName then Some arg else None))
+    |> Option.orElseWith (fun () ->
+        // If there is no matching named argument, find the argument in the same position as the parameter with the given name
+        minfo.GetParamNames()
+        |> Seq.concat
+        |> Seq.tryFindIndex (fun nm -> match nm with Some nm -> nm = paramName | _ -> false)
+        |> Option.bind (fun idx -> Seq.concat callerArgs.Unnamed |> Seq.tryItem idx)
+    )
+    |> Option.map (fun arg -> tyOfExpr g arg.Expr)
+
 //-------------------------------------------------------------------------
 // Helpers dealing with sequence expressions
 //-------------------------------------------------------------------------
@@ -10270,12 +10310,26 @@ and TcMethodApplication_UniqueOverloadInference
 
     let arityFilteredCandidates = candidateMethsAndProps
 
-    let makeOneCalledMeth (minfo, pinfoOpt, usesParamArrayConversion) =
+    let makeOneCalledMeth (minfo: MethInfo, pinfoOpt, usesParamArrayConversion) =
         let minst = FreshenMethInfo mItem minfo
         let callerTyArgs =
             match tyArgsOpt with
             | Some tyargs -> minfo.AdjustUserTypeInstForFSharpStyleIndexedExtensionMembers tyargs
             | None -> minst
+
+        // If the return value is [<NotNullIfNotNull>], give the return a fresh nullness inference variable here so that
+        // unique-overload inference does not prematurely commit the result to the declared (nullable) nullness. The real
+        // nullness is resolved post argument type-checking (see below), once the argument types are known.
+        let minfo =
+            if not minfo.IsConstructor && g.checkNullness && g.langVersion.SupportsFeature LanguageFeature.NotNullIfNotNull then
+                match NotNullIfNotNullParamNames g minfo with
+                | [ _ ] ->
+                    let retTy = minfo.GetFSharpReturnType(cenv.amap, mMethExpr, callerTyArgs)
+                    MethInfoWithModifiedReturnType(minfo, replaceNullnessOfTy (NewNullnessVar()) retTy)
+                | _ -> minfo
+            else
+                minfo
+
         CalledMeth<SynExpr>(cenv.infoReader, Some(env.NameEnv), isCheckingAttributeCall, FreshenMethInfo, mMethExpr, ad, minfo, minst, callerTyArgs, pinfoOpt, callerObjArgTys, callerArgs, usesParamArrayConversion, true, objTyOpt, staticTyOpt)
 
     let preArgumentTypeCheckingCalledMethGroup =
@@ -10533,6 +10587,28 @@ and TcMethodApplication
                     match tyArgsOpt with
                     | Some tyargs -> minfo.AdjustUserTypeInstForFSharpStyleIndexedExtensionMembers tyargs
                     | None -> minst
+
+                let minfo =
+                    if not minfo.IsConstructor && g.checkNullness && g.langVersion.SupportsFeature LanguageFeature.NotNullIfNotNull then
+                        // 'minfo' may already carry a placeholder return nullness from unique-overload inference (phase 1);
+                        // strip it back to the base method before applying the real (argument-derived) nullness.
+                        let baseMinfo = match minfo with MethInfoWithModifiedReturnType(inner, _) -> inner | _ -> minfo
+                        match NotNullIfNotNullParamNames g baseMinfo with
+                        | [ paramName ] ->
+                            match TryGetCallerArgType g baseMinfo callerArgs paramName with
+                            | Some callerArgTy ->
+                                let retTy = baseMinfo.GetFSharpReturnType(cenv.amap, mMethExpr, callerTyArgs)
+                                let argNullness =
+                                    if TypeNullIsTrueValue g callerArgTy || TypeNullIsExtraValueNew g mMethExpr callerArgTy then
+                                        g.knownWithNull
+                                    else
+                                        nullnessOfTy g callerArgTy
+                                MethInfoWithModifiedReturnType(baseMinfo, replaceNullnessOfTy argNullness retTy)
+                            | None -> baseMinfo
+                        | _ -> baseMinfo
+                    else
+                        minfo
+
                 CalledMeth<Expr>(cenv.infoReader, Some(env.NameEnv), isCheckingAttributeCall, FreshenMethInfo, mMethExpr, ad, minfo, minst, callerTyArgs, pinfoOpt, callerObjArgTys, callerArgs, usesParamArrayConversion, true, objTyOpt, staticTyOpt))
 
         // Commit unassociated constraints prior to member overload resolution where there is ambiguity

src/Compiler/Checking/MethodCalls.fs

@@ -1250,6 +1250,14 @@ let rec BuildMethodCall tcVal g amap isMutable m isProp minfo valUseFlags minst
             let expr = mkCoerceExpr (expr, retTy, m, exprTy)
             expr, retTy
 
+        | MethInfoWithModifiedReturnType((FSMeth(_, _, vref, _) as innerMeth), retTy) ->
+            // Build the inner call directly, without re-invoking TakeObjAddrForMethodCall.
+            let vExpr, vExprTy = tcVal vref valUseFlags (innerMeth.DeclaringTypeInst @ minst) m
+            let expr, exprTy = BuildFSharpMethodApp g m vref vExpr vExprTy allArgs
+
+            let expr = mkCoerceExpr (expr, retTy, m, exprTy)
+            expr, retTy
+
         | MethInfoWithModifiedReturnType _ ->
             failwith "MethInfoWithModifiedReturnType: unexpected inner method kind"
 

src/Compiler/Checking/NicePrint.fs

@@ -1723,7 +1723,7 @@ module InfoMemberPrinting =
 
         let layout,paramLayouts =
             match denv.showCsharpCodeAnalysisAttributes, minfo with
-            | true, ILMeth(_g,mi,_e) -> 
+            | true, (ILMeth(_, mi, _) | MethInfoWithModifiedReturnType(ILMeth(_, mi, _), _)) ->
                 let methodLayout = 
                     // Render Method attributes and [return:..] attributes on separate lines above (@@) the method definition
                     PrintTypes.layoutCsharpCodeAnalysisIlAttributes denv (minfo.GetCustomAttrs()) (squareAngleL >> (@@)) layout

src/Compiler/DependencyManager/AssemblyResolveHandler.fs

@@ -54,7 +54,7 @@ type AssemblyResolveHandlerCoreclr(assemblyProbingPaths: AssemblyResolutionProbe
 
             let assemblyPathOpt =
                 assemblyPaths
-                |> Seq.tryFind (fun path -> Path.GetFileNameWithoutExtension(path) = simpleName)
+                |> Seq.tryFind (fun path -> String.Equals(Path.GetFileNameWithoutExtension(path), simpleName))
 
             match assemblyPathOpt with
             | Some path -> loadAssembly path
@@ -84,7 +84,7 @@ type AssemblyResolveHandlerDeskTop(assemblyProbingPaths: AssemblyResolutionProbe
 
             let assemblyPathOpt =
                 assemblyPaths
-                |> Seq.tryFind (fun path -> Path.GetFileNameWithoutExtension(path) = simpleName)
+                |> Seq.tryFind (fun path -> String.Equals(Path.GetFileNameWithoutExtension(path), simpleName))
 
             match assemblyPathOpt with
             | Some path -> Assembly.LoadFrom path

src/Compiler/FSComp.txt

@@ -1822,3 +1822,4 @@ featurePreprocessorElif,"#elif preprocessor directive"
 3889,tastNamespaceAndTypeWithSameNameInAssembly,"The namespace '%s' clashes with the type '%s'."
 featureExceptionFieldSerializationSupport,"emit GetObjectData and field-restoring deserialization constructor for exception types"
 featureErrorOnMissingSignatureAttribute,"error (rather than warning) when an enforced compiler-semantic attribute is present in the .fs but missing from the .fsi"
+featureNotNullIfNotNull,"honor the 'NotNullIfNotNull' attribute on a method's return value"

src/Compiler/Facilities/LanguageFeatures.fs

@@ -110,6 +110,7 @@ type LanguageFeature =
     | PreprocessorElif
     | ExceptionFieldSerializationSupport
     | ErrorOnMissingSignatureAttribute
+    | NotNullIfNotNull
 
 /// LanguageVersion management
 type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array) =
@@ -254,6 +255,7 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
                 LanguageFeature.WarnWhenFunctionValueUsedAsInterpolatedStringArg, languageVersion110
                 LanguageFeature.PreprocessorElif, languageVersion110
                 LanguageFeature.ExceptionFieldSerializationSupport, languageVersion110
+                LanguageFeature.NotNullIfNotNull, languageVersion110
 
                 // Difference between languageVersion110 and preview - 11.0 gets turned on automatically by picking a preview .NET 11 SDK
                 // previewVersion is only when "preview" is specified explicitly in project files  and users also need a preview SDK
@@ -459,6 +461,7 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
         | LanguageFeature.PreprocessorElif -> FSComp.SR.featurePreprocessorElif ()
         | LanguageFeature.ExceptionFieldSerializationSupport -> FSComp.SR.featureExceptionFieldSerializationSupport ()
         | LanguageFeature.ErrorOnMissingSignatureAttribute -> FSComp.SR.featureErrorOnMissingSignatureAttribute ()
+        | LanguageFeature.NotNullIfNotNull -> FSComp.SR.featureNotNullIfNotNull ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -101,6 +101,7 @@ type LanguageFeature =
     | PreprocessorElif
     | ExceptionFieldSerializationSupport
     | ErrorOnMissingSignatureAttribute
+    | NotNullIfNotNull
 
 /// LanguageVersion management
 type LanguageVersion =

src/Compiler/TypedTree/TypedTreeOps.Attributes.fs

@@ -183,6 +183,7 @@ module internal ILExtensions =
                     WellKnownILAttributes.SetsRequiredMembersAttribute
                 | "System.ObsoleteAttribute" -> WellKnownILAttributes.ObsoleteAttribute
                 | "System.Diagnostics.CodeAnalysis.ExperimentalAttribute" -> WellKnownILAttributes.ExperimentalAttribute
+                | "System.Diagnostics.CodeAnalysis.NotNullIfNotNullAttribute" -> WellKnownILAttributes.NotNullIfNotNullAttribute
                 | "System.AttributeUsageAttribute" -> WellKnownILAttributes.AttributeUsageAttribute
                 | _ -> WellKnownILAttributes.None
 
@@ -592,6 +593,11 @@ module internal AttributeHelpers =
                 | "ConditionalAttribute" -> WellKnownValAttributes.ConditionalAttribute
                 | _ -> WellKnownValAttributes.None
 
+            | [| "System"; "Diagnostics"; "CodeAnalysis"; name |] ->
+                match name with
+                | "NotNullIfNotNullAttribute" -> WellKnownValAttributes.NotNullIfNotNullAttribute
+                | _ -> WellKnownValAttributes.None
+
             | [| "System"; name |] ->
                 match name with
                 | "ThreadStaticAttribute" -> WellKnownValAttributes.ThreadStaticAttribute

src/Compiler/TypedTree/WellKnownAttribs.fs

@@ -116,6 +116,7 @@ type internal WellKnownValAttributes =
     | NoEagerConstraintApplicationAttribute = (1uL <<< 38)
     | ValueAsStaticPropertyAttribute = (1uL <<< 39)
     | TailCallAttribute = (1uL <<< 40)
+    | NotNullIfNotNullAttribute = (1uL <<< 41)
     | NotComputed = (1uL <<< 63)
 
 module internal Flags =

src/Compiler/TypedTree/WellKnownAttribs.fsi

@@ -114,6 +114,7 @@ type internal WellKnownValAttributes =
     | NoEagerConstraintApplicationAttribute = (1uL <<< 38)
     | ValueAsStaticPropertyAttribute = (1uL <<< 39)
     | TailCallAttribute = (1uL <<< 40)
+    | NotNullIfNotNullAttribute = (1uL <<< 41)
     | NotComputed = (1uL <<< 63)
 
 module internal Flags =

src/Compiler/Utilities/range.fs

@@ -334,7 +334,7 @@ type Range(code1: int64, code2: int64) =
     member m.FileName = fileOfFileIndex m.FileIndex
 
     member internal m.ShortFileName =
-        Path.GetFileName(fileOfFileIndex m.FileIndex) |> nonNull
+        Path.GetFileName(fileOfFileIndex m.FileIndex) |> Unchecked.nonNull
 
     member m.ApplyLineDirectives() =
         match LineDirectives.store.TryFind m.FileIndex with