Support NotNullIfNotNullAttribute

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

@@ -96,6 +96,7 @@
 * 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))
 
 ### Changed
 

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

@@ -3330,6 +3330,46 @@ 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 =
@@ -10505,6 +10559,27 @@ 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 =
+                                    match GetTyparTyIfSupportsNull g callerArgTy with
+                                    | ValueSome _ -> g.knownWithNull
+                                    | ValueNone -> 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

@@ -1821,3 +1821,4 @@ featurePreprocessorElif,"#elif preprocessor directive"
 3888,implAttributeMissingFromSignature,"The attribute '%s' is present on '%s' in the implementation but not in the signature, which takes precedence for tooling and consumers. Add the attribute to the signature, to ensure the attribute is not ignored by the compiler."
 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