Add NullableContextAttribute

docs/features/nullable-metadata.md

@@ -148,25 +148,36 @@ for members that are inaccessible outside the assembly (`private` members, and a
 if the assembly does not contain `InternalsVisibleToAttribute` attributes).
 
 The compiler behavior is configured from a command-line flag.
-For now a feature flag is used: `-feature:nullablePublicOnly`.
+For now a feature flag is used: `-features:nullablePublicOnly`.
 
 If private member attributes are dropped, the compiler will emit a `[module: NullablePublicOnly]` attribute.
 The presence or absence of the `NullablePublicOnlyAttribute` can be used by tools to interpret
 the nullability of private members that do not have an associated `NullableAttribute` attribute.
 
+For members that do not have explicit accessibility in metadata
+(specifically for parameters, type parameters, events, and properties),
+the compiler uses the accessibility of the container to determine whether to emit nullable attributes. 
+
 ```C#
 namespace System.Runtime.CompilerServices
 {
     [System.AttributeUsage(AttributeTargets.Module, AllowMultiple = false)]
     public sealed class NullablePublicOnlyAttribute : Attribute
     {
+        public readonly bool IncludesInternals;
+        public NullablePublicOnlyAttribute(bool includesInternals)
+        {
+            IncludesInternals = includesInternals;
+        }
     }
 }
 ```
 
 The `NullablePublicOnlyAttribute` type is for compiler use only - it is not permitted in source.
 The type declaration is synthesized by the compiler if not already included in the compilation.
 
+`IncludesInternal` is true if `internal` members are annotated in addition to `public` and `protected` members.
+
 ## Compatibility
 
 The nullable metadata does not include an explicit version number.

src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs

@@ -3305,12 +3305,34 @@ internal bool ShouldEmitNullableAttributes(Symbol symbol)
                 return true;
             }
 
+            // For symbols that do not have explicit accessibility in metadata,
+            // use the accessibility of the container.
+            symbol = getExplicitAccessibilitySymbol(symbol);
+
             if (!AccessCheck.IsEffectivelyPublicOrInternal(symbol, out bool isInternal))
             {
                 return false;
             }
 
             return !isInternal || SourceAssembly.InternalsAreVisible;
+
+            static Symbol getExplicitAccessibilitySymbol(Symbol symbol)
+            {
+                while (true)
+                {
+                    switch (symbol.Kind)
+                    {
+                        case SymbolKind.Parameter:
+                        case SymbolKind.TypeParameter:
+                        case SymbolKind.Property:
+                        case SymbolKind.Event:
+                            symbol = symbol.ContainingSymbol;
+                            break;
+                        default:
+                            return symbol;
+                    }
+                }
+            }
         }
 
         internal override AnalyzerDriver AnalyzerForLanguage(ImmutableArray<DiagnosticAnalyzer> analyzers, AnalyzerManager analyzerManager)

src/Compilers/CSharp/Portable/Emitter/Model/MostCommonNullableValueBuilder.cs

@@ -0,0 +1,95 @@
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using Microsoft.CodeAnalysis.CSharp.Symbols;
+using Microsoft.CodeAnalysis.PooledObjects;
+using Roslyn.Utilities;
+
+namespace Microsoft.CodeAnalysis.CSharp
+{
+    internal struct MostCommonNullableValueBuilder
+    {
+        private int _value0;
+        private int _value1;
+        private int _value2;
+
+        internal byte? MostCommonValue
+        {
+            get
+            {
+                int max;
+                byte b;
+                if (_value1 > _value0)
+                {
+                    max = _value1;
+                    b = 1;
+                }
+                else
+                {
+                    max = _value0;
+                    b = 0;
+                }
+                if (_value2 > max)
+                {
+                    return 2;
+                }
+                return max == 0 ? (byte?)null : b;
+            }
+        }
+
+        internal void AddValue(byte value)
+        {
+            switch (value)
+            {
+                case 0:
+                    _value0++;
+                    break;
+                case 1:
+                    _value1++;
+                    break;
+                case 2:
+                    _value2++;
+                    break;
+                default:
+                    throw ExceptionUtilities.UnexpectedValue(value);
+            }
+        }
+
+        internal void AddValue(byte? value)
+        {
+            if (value != null)
+            {
+                AddValue(value.GetValueOrDefault());
+            }
+        }
+
+        internal void AddValue(TypeWithAnnotations type)
+        {
+            var builder = ArrayBuilder<byte>.GetInstance();
+            type.AddNullableTransforms(builder);
+            AddValue(GetCommonValue(builder));
+            builder.Free();
+        }
+
+        /// <summary>
+        /// Returns the common value if all bytes are the same value.
+        /// Otherwise returns null.
+        /// </summary>
+        internal static byte? GetCommonValue(ArrayBuilder<byte> builder)
+        {
+            int n = builder.Count;
+            if (n == 0)
+            {
+                return null;
+            }
+            byte b = builder[0];
+            for (int i = 1; i < n; i++)
+            {
+                if (builder[i] != b)
+                {
+                    return null;
+                }
+            }
+            return b;
+        }
+    }
+}

src/Compilers/CSharp/Portable/Emitter/Model/PEAssemblyBuilder.cs

@@ -26,7 +26,8 @@ internal abstract class PEAssemblyBuilderBase : PEModuleBuilder, Cci.IAssemblyRe
         private SynthesizedEmbeddedAttributeSymbol _lazyIsByRefLikeAttribute;
         private SynthesizedEmbeddedAttributeSymbol _lazyIsUnmanagedAttribute;
         private SynthesizedEmbeddedNullableAttributeSymbol _lazyNullableAttribute;
-        private SynthesizedEmbeddedAttributeSymbol _lazyNullablePublicOnlyAttribute;
+        private SynthesizedEmbeddedNullableContextAttributeSymbol _lazyNullableContextAttribute;
+        private SynthesizedEmbeddedNullablePublicOnlyAttributeSymbol _lazyNullablePublicOnlyAttribute;
 
         /// <summary>
         /// The behavior of the C# command-line compiler is as follows:
@@ -83,6 +84,7 @@ internal override ImmutableArray<NamedTypeSymbol> GetEmbeddedTypes(DiagnosticBag
             builder.AddIfNotNull(_lazyIsUnmanagedAttribute);
             builder.AddIfNotNull(_lazyIsByRefLikeAttribute);
             builder.AddIfNotNull(_lazyNullableAttribute);
+            builder.AddIfNotNull(_lazyNullableContextAttribute);
             builder.AddIfNotNull(_lazyNullablePublicOnlyAttribute);
 
             return builder.ToImmutableAndFree();
@@ -195,17 +197,30 @@ internal override SynthesizedAttributeData SynthesizeNullableAttribute(WellKnown
             return base.SynthesizeNullableAttribute(member, arguments);
         }
 
-        internal override SynthesizedAttributeData SynthesizeNullablePublicOnlyAttribute()
+        internal override SynthesizedAttributeData SynthesizeNullableContextAttribute(ImmutableArray<TypedConstant> arguments)
+        {
+            if ((object)_lazyNullableContextAttribute != null)
+            {
+                return new SynthesizedAttributeData(
+                    _lazyNullableContextAttribute.Constructors[0],
+                    arguments,
+                    ImmutableArray<KeyValuePair<string, TypedConstant>>.Empty);
+            }
+
+            return base.SynthesizeNullableContextAttribute(arguments);
+        }
+
+        internal override SynthesizedAttributeData SynthesizeNullablePublicOnlyAttribute(ImmutableArray<TypedConstant> arguments)
         {
             if ((object)_lazyNullablePublicOnlyAttribute != null)
             {
                 return new SynthesizedAttributeData(
                     _lazyNullablePublicOnlyAttribute.Constructors[0],
-                    ImmutableArray<TypedConstant>.Empty,
+                    arguments,
                     ImmutableArray<KeyValuePair<string, TypedConstant>>.Empty);
             }
 
-            return base.SynthesizeNullablePublicOnlyAttribute();
+            return base.SynthesizeNullablePublicOnlyAttribute(arguments);
         }
 
         protected override SynthesizedAttributeData TrySynthesizeIsReadOnlyAttribute()
@@ -292,12 +307,18 @@ private void CreateEmbeddedAttributesIfNeeded(DiagnosticBag diagnostics)
                     diagnostics);
             }
 
+            if ((needsAttributes & EmbeddableAttributes.NullableContextAttribute) != 0)
+            {
+                CreateEmbeddedNullableContextAttributeIfNeeded(
+                    ref _lazyNullableContextAttribute,
+                    diagnostics);
+            }
+
             if ((needsAttributes & EmbeddableAttributes.NullablePublicOnlyAttribute) != 0)
             {
-                CreateEmbeddedAttributeIfNeeded(
+                CreateEmbeddedNullablePublicOnlyAttributeIfNeeded(
                     ref _lazyNullablePublicOnlyAttribute,
-                    diagnostics,
-                    AttributeDescription.NullablePublicOnlyAttribute);
+                    diagnostics);
             }
         }
 
@@ -324,6 +345,28 @@ private void CreateEmbeddedNullableAttributeIfNeeded(
             }
         }
 
+        private void CreateEmbeddedNullableContextAttributeIfNeeded(
+            ref SynthesizedEmbeddedNullableContextAttributeSymbol symbol,
+            DiagnosticBag diagnostics)
+        {
+            if (symbol is null)
+            {
+                AddDiagnosticsForExistingAttribute(AttributeDescription.NullableContextAttribute, diagnostics);
+                symbol = new SynthesizedEmbeddedNullableContextAttributeSymbol(_sourceAssembly.DeclaringCompilation, diagnostics);
+            }
+        }
+
+        private void CreateEmbeddedNullablePublicOnlyAttributeIfNeeded(
+            ref SynthesizedEmbeddedNullablePublicOnlyAttributeSymbol symbol,
+            DiagnosticBag diagnostics)
+        {
+            if (symbol is null)
+            {
+                AddDiagnosticsForExistingAttribute(AttributeDescription.NullablePublicOnlyAttribute, diagnostics);
+                symbol = new SynthesizedEmbeddedNullablePublicOnlyAttributeSymbol(_sourceAssembly.DeclaringCompilation, diagnostics);
+            }
+        }
+
         private void AddDiagnosticsForExistingAttribute(AttributeDescription description, DiagnosticBag diagnostics)
         {
             var attributeMetadataName = MetadataTypeName.FromFullName(description.FullName);

src/Compilers/CSharp/Portable/Emitter/Model/PEModuleBuilder.cs

@@ -1462,7 +1462,7 @@ internal SynthesizedAttributeData SynthesizeIsByRefLikeAttribute(Symbol symbol)
         /// is a constructed type with a nullable reference type present in its type argument tree,
         /// returns a synthesized NullableAttribute with encoded nullable transforms array.
         /// </summary>
-        internal SynthesizedAttributeData SynthesizeNullableAttribute(Symbol symbol, TypeWithAnnotations type)
+        internal SynthesizedAttributeData SynthesizeNullableAttributeIfNecessary(Symbol symbol, byte? nullableContextValue, TypeWithAnnotations type)
         {
             if ((object)Compilation.SourceModule != symbol.ContainingModule)
             {
@@ -1473,36 +1473,46 @@ internal SynthesizedAttributeData SynthesizeNullableAttribute(Symbol symbol, Typ
             var flagsBuilder = ArrayBuilder<byte>.GetInstance();
             type.AddNullableTransforms(flagsBuilder);
 
-            Debug.Assert(flagsBuilder.Any());
-            Debug.Assert(flagsBuilder.Contains(NullableAnnotationExtensions.NotAnnotatedAttributeValue) || flagsBuilder.Contains(NullableAnnotationExtensions.AnnotatedAttributeValue));
-
-            WellKnownMember constructor;
-            ImmutableArray<TypedConstant> arguments;
-            NamedTypeSymbol byteType = Compilation.GetSpecialType(SpecialType.System_Byte);
-            Debug.Assert((object)byteType != null);
-
-            if (flagsBuilder.All(flag => flag == NullableAnnotationExtensions.NotAnnotatedAttributeValue) || flagsBuilder.All(flag => flag == NullableAnnotationExtensions.AnnotatedAttributeValue))
+            SynthesizedAttributeData attribute;
+            if (!flagsBuilder.Any())
             {
-                constructor = WellKnownMember.System_Runtime_CompilerServices_NullableAttribute__ctorByte;
-                arguments = ImmutableArray.Create(new TypedConstant(byteType, TypedConstantKind.Primitive, flagsBuilder[0]));
+                attribute = null;
             }
             else
             {
-                var constantsBuilder = ArrayBuilder<TypedConstant>.GetInstance(flagsBuilder.Count);
-
-                foreach (byte flag in flagsBuilder)
+                Debug.Assert(flagsBuilder.All(f => f <= 2));
+                byte? commonValue = MostCommonNullableValueBuilder.GetCommonValue(flagsBuilder);
+                if (commonValue != null)
                 {
-                    Debug.Assert(flag == NullableAnnotationExtensions.ObliviousAttributeValue || flag == NullableAnnotationExtensions.NotAnnotatedAttributeValue || flag == NullableAnnotationExtensions.AnnotatedAttributeValue);
-                    constantsBuilder.Add(new TypedConstant(byteType, TypedConstantKind.Primitive, flag));
+                    attribute = SynthesizeNullableAttributeIfNecessary(nullableContextValue, commonValue.GetValueOrDefault());
+                }
+                else
+                {
+                    NamedTypeSymbol byteType = Compilation.GetSpecialType(SpecialType.System_Byte);
+                    var byteArrayType = ArrayTypeSymbol.CreateSZArray(byteType.ContainingAssembly, TypeWithAnnotations.Create(byteType));
+                    var value = flagsBuilder.SelectAsArray((flag, byteType) => new TypedConstant(byteType, TypedConstantKind.Primitive, flag), byteType);
+                    attribute = SynthesizeNullableAttribute(
+                        WellKnownMember.System_Runtime_CompilerServices_NullableAttribute__ctorTransformFlags,
+                        ImmutableArray.Create(new TypedConstant(byteArrayType, value)));
                 }
-
-                var byteArray = ArrayTypeSymbol.CreateSZArray(byteType.ContainingAssembly, TypeWithAnnotations.Create(byteType));
-                constructor = WellKnownMember.System_Runtime_CompilerServices_NullableAttribute__ctorTransformFlags;
-                arguments = ImmutableArray.Create(new TypedConstant(byteArray, constantsBuilder.ToImmutableAndFree()));
             }
 
             flagsBuilder.Free();
-            return SynthesizeNullableAttribute(constructor, arguments);
+            return attribute;
+        }
+
+        internal SynthesizedAttributeData SynthesizeNullableAttributeIfNecessary(byte? nullableContextValue, byte nullableValue)
+        {
+            if (nullableValue == nullableContextValue ||
+                (nullableContextValue == null && nullableValue == 0))
+            {
+                return null;
+            }
+
+            NamedTypeSymbol byteType = Compilation.GetSpecialType(SpecialType.System_Byte);
+            return SynthesizeNullableAttribute(
+                WellKnownMember.System_Runtime_CompilerServices_NullableAttribute__ctorByte,
+                ImmutableArray.Create(new TypedConstant(byteType, TypedConstantKind.Primitive, nullableValue)));
         }
 
         internal virtual SynthesizedAttributeData SynthesizeNullableAttribute(WellKnownMember member, ImmutableArray<TypedConstant> arguments)
@@ -1512,10 +1522,30 @@ internal virtual SynthesizedAttributeData SynthesizeNullableAttribute(WellKnownM
             return Compilation.TrySynthesizeAttribute(member, arguments, isOptionalUse: true);
         }
 
-        internal virtual SynthesizedAttributeData SynthesizeNullablePublicOnlyAttribute()
+        internal SynthesizedAttributeData SynthesizeNullableContextAttribute(Symbol symbol, byte value)
+        {
+            var module = Compilation.SourceModule;
+            if ((object)module != symbol && (object)module != symbol.ContainingModule)
+            {
+                // For symbols that are not defined in the same compilation (like NoPia), don't synthesize this attribute.
+                return null;
+            }
+
+            return SynthesizeNullableContextAttribute(
+                ImmutableArray.Create(new TypedConstant(Compilation.GetSpecialType(SpecialType.System_Byte), TypedConstantKind.Primitive, value)));
+        }
+
+        internal virtual SynthesizedAttributeData SynthesizeNullableContextAttribute(ImmutableArray<TypedConstant> arguments)
         {
             // For modules, this attribute should be present. Only assemblies generate and embed this type.
-            return Compilation.TrySynthesizeAttribute(WellKnownMember.System_Runtime_CompilerServices_NullablePublicOnlyAttribute__ctor);
+            // https://github.com/dotnet/roslyn/issues/30062 Should not be optional.
+            return Compilation.TrySynthesizeAttribute(WellKnownMember.System_Runtime_CompilerServices_NullableContextAttribute__ctor, arguments, isOptionalUse: true);
+        }
+
+        internal virtual SynthesizedAttributeData SynthesizeNullablePublicOnlyAttribute(ImmutableArray<TypedConstant> arguments)
+        {
+            // For modules, this attribute should be present. Only assemblies generate and embed this type.
+            return Compilation.TrySynthesizeAttribute(WellKnownMember.System_Runtime_CompilerServices_NullablePublicOnlyAttribute__ctor, arguments);
         }
 
         protected virtual SynthesizedAttributeData TrySynthesizeIsReadOnlyAttribute()
@@ -1566,10 +1596,5 @@ internal void EnsureNullableAttributeExists()
         {
             EnsureEmbeddableAttributeExists(EmbeddableAttributes.NullableAttribute);
         }
-
-        internal void EnsureNullablePublicOnlyAttributeExists()
-        {
-            EnsureEmbeddableAttributeExists(EmbeddableAttributes.NullablePublicOnlyAttribute);
-        }
     }
 }