Add support for assembly/module-level [Experimental] and contextual suppression

src/Compilers/CSharp/Portable/Symbols/AssemblySymbol.cs

@@ -276,15 +276,6 @@ public sealed override bool IsExtern
             }
         }
 
-        /// <summary>
-        /// Returns data decoded from Obsolete attribute or null if there is no Obsolete attribute.
-        /// This property returns ObsoleteAttributeData.Uninitialized if attribute arguments haven't been decoded yet.
-        /// </summary>
-        internal sealed override ObsoleteAttributeData ObsoleteAttributeData
-        {
-            get { return null; }
-        }
-
         public override ImmutableArray<SyntaxReference> DeclaringSyntaxReferences
         {
             get

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEAssemblySymbol.cs

@@ -66,6 +66,8 @@ internal sealed class PEAssemblySymbol : MetadataOrSourceAssemblySymbol
 
 #nullable enable
         private DiagnosticInfo? _lazyCachedCompilerFeatureRequiredDiagnosticInfo = CSDiagnosticInfo.EmptyErrorInfo;
+
+        private ObsoleteAttributeData? _lazyObsoleteAttributeData = ObsoleteAttributeData.Uninitialized;
 #nullable disable
 
         internal PEAssemblySymbol(PEAssembly assembly, DocumentationProvider documentationProvider, bool isLinked, MetadataImportOptions importOptions)
@@ -311,5 +313,31 @@ internal PEModuleSymbol PrimaryModule
 
         public override bool HasUnsupportedMetadata
             => GetCompilerFeatureRequiredDiagnostic()?.Code == (int)ErrorCode.ERR_UnsupportedCompilerFeature || base.HasUnsupportedMetadata;
+
+        internal sealed override ObsoleteAttributeData? ObsoleteAttributeData
+        {
+            get
+            {
+                if (_lazyObsoleteAttributeData == ObsoleteAttributeData.Uninitialized)
+                {
+                    Interlocked.CompareExchange(ref _lazyObsoleteAttributeData, computeObsoleteAttributeData(), ObsoleteAttributeData.Uninitialized);
+                }
+
+                return _lazyObsoleteAttributeData;
+
+                ObsoleteAttributeData? computeObsoleteAttributeData()
+                {
+                    foreach (var attrData in GetAttributes())
+                    {
+                        if (attrData.IsTargetAttribute(this, AttributeDescription.ExperimentalAttribute))
+                        {
+                            return attrData.DecodeExperimentalAttribute();
+                        }
+                    }
+
+                    return null;
+                }
+            }
+        }
     }
 }

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEModuleSymbol.cs

@@ -118,6 +118,8 @@ internal enum RefSafetyRulesAttributeVersion
 
 #nullable enable
         private DiagnosticInfo? _lazyCachedCompilerFeatureRequiredDiagnosticInfo = CSDiagnosticInfo.EmptyErrorInfo;
+
+        private ObsoleteAttributeData? _lazyObsoleteAttributeData = ObsoleteAttributeData.Uninitialized;
 #nullable disable
 
         internal PEModuleSymbol(PEAssemblySymbol assemblySymbol, PEModule module, MetadataImportOptions importOptions, int ordinal)
@@ -857,5 +859,31 @@ RefSafetyRulesAttributeVersion getAttributeVersion()
                 }
             }
         }
+
+        internal sealed override ObsoleteAttributeData? ObsoleteAttributeData
+        {
+            get
+            {
+                if (_lazyObsoleteAttributeData == ObsoleteAttributeData.Uninitialized)
+                {
+                    Interlocked.CompareExchange(ref _lazyObsoleteAttributeData, computeObsoleteAttributeData(), ObsoleteAttributeData.Uninitialized);
+                }
+
+                return _lazyObsoleteAttributeData;
+
+                ObsoleteAttributeData? computeObsoleteAttributeData()
+                {
+                    foreach (var attrData in GetAttributes())
+                    {
+                        if (attrData.IsTargetAttribute(this, AttributeDescription.ExperimentalAttribute))
+                        {
+                            return attrData.DecodeExperimentalAttribute();
+                        }
+                    }
+
+                    return null;
+                }
+            }
+        }
     }
 }

src/Compilers/CSharp/Portable/Symbols/MissingAssemblySymbol.cs

@@ -209,5 +209,8 @@ internal sealed override IEnumerable<NamedTypeSymbol> GetAllTopLevelForwardedTyp
         {
             return SpecializedCollections.EmptyEnumerable<NamedTypeSymbol>();
         }
+
+#nullable enable
+        internal sealed override ObsoleteAttributeData? ObsoleteAttributeData => null;
     }
 }

src/Compilers/CSharp/Portable/Symbols/MissingModuleSymbol.cs

@@ -198,6 +198,10 @@ public sealed override bool AreLocalsZeroed
         }
 
         internal sealed override bool UseUpdatedEscapeRules => false;
+
+#nullable enable
+        internal sealed override ObsoleteAttributeData? ObsoleteAttributeData => null;
+#nullable disable
     }
 
     internal sealed class MissingModuleSymbolWithName : MissingModuleSymbol

src/Compilers/CSharp/Portable/Symbols/ModuleSymbol.cs

@@ -185,15 +185,6 @@ public sealed override bool IsExtern
             }
         }
 
-        /// <summary>
-        /// Returns data decoded from Obsolete attribute or null if there is no Obsolete attribute.
-        /// This property returns ObsoleteAttributeData.Uninitialized if attribute arguments haven't been decoded yet.
-        /// </summary>
-        internal sealed override ObsoleteAttributeData ObsoleteAttributeData
-        {
-            get { return null; }
-        }
-
         public override ImmutableArray<SyntaxReference> DeclaringSyntaxReferences
         {
             get

src/Compilers/CSharp/Portable/Symbols/ObsoleteAttributeHelpers.cs

@@ -4,10 +4,12 @@
 
 #nullable disable
 
+using System;
 using System.Diagnostics;
 using System.Reflection.Metadata;
 using System.Threading;
 using Microsoft.CodeAnalysis.CSharp.Symbols.Metadata.PE;
+using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.CSharp.Symbols
 {
@@ -54,7 +56,7 @@ internal static ObsoleteAttributeData GetObsoleteDataFromMetadata(EntityHandle t
         /// symbol's Obsoleteness is Unknown. False, if we are certain that no symbol in the parent
         /// hierarchy is Obsolete.
         /// </returns>
-        private static ThreeState GetObsoleteContextState(Symbol symbol, bool forceComplete)
+        private static ThreeState GetObsoleteContextState(Symbol symbol, bool forceComplete, Func<Symbol, ThreeState> getStateFromSymbol)
         {
             while ((object)symbol != null)
             {
@@ -73,7 +75,7 @@ private static ThreeState GetObsoleteContextState(Symbol symbol, bool forceCompl
                     symbol.ForceCompleteObsoleteAttribute();
                 }
 
-                var state = symbol.ObsoleteState;
+                var state = getStateFromSymbol(symbol);
                 if (state != ThreeState.False)
                 {
                     return state;
@@ -95,13 +97,35 @@ private static ThreeState GetObsoleteContextState(Symbol symbol, bool forceCompl
 
         internal static ObsoleteDiagnosticKind GetObsoleteDiagnosticKind(Symbol symbol, Symbol containingMember, bool forceComplete = false)
         {
+            if (symbol is NamedTypeSymbol namedTypeSymbol && isExperimentalSymbol(namedTypeSymbol))
+            {
+                // Skip for System.Diagnostics.CodeAnalysis.ExperimentalAttribute to mitigate cycles
+                return ObsoleteDiagnosticKind.NotObsolete;
+            }
+
+            if (symbol is MethodSymbol { MethodKind: MethodKind.Constructor } method && isExperimentalSymbol(method.ContainingType))
+            {
+                // Skip for constructors of System.Diagnostics.CodeAnalysis.ExperimentalAttribute to mitigate cycles
+                return ObsoleteDiagnosticKind.NotObsolete;
+            }
+
+            Debug.Assert(symbol.ContainingModule.ObsoleteKind is not ObsoleteAttributeKind.Uninitialized);
+            Debug.Assert(symbol.ContainingAssembly.ObsoleteKind is not ObsoleteAttributeKind.Uninitialized);
+
+            if (symbol.ContainingModule.ObsoleteKind is ObsoleteAttributeKind.Experimental
+                || symbol.ContainingAssembly.ObsoleteKind is ObsoleteAttributeKind.Experimental)
+            {
+                return getExperimentalDiagnosticKind(containingMember, forceComplete);
+            }
+
             switch (symbol.ObsoleteKind)
             {
                 case ObsoleteAttributeKind.None:
                     return ObsoleteDiagnosticKind.NotObsolete;
                 case ObsoleteAttributeKind.WindowsExperimental:
-                case ObsoleteAttributeKind.Experimental:
                     return ObsoleteDiagnosticKind.Diagnostic;
+                case ObsoleteAttributeKind.Experimental:
+                    return getExperimentalDiagnosticKind(containingMember, forceComplete);
                 case ObsoleteAttributeKind.Uninitialized:
                     // If we haven't cracked attributes on the symbol at all or we haven't
                     // cracked attribute arguments enough to be able to report diagnostics for
@@ -110,7 +134,7 @@ internal static ObsoleteDiagnosticKind GetObsoleteDiagnosticKind(Symbol symbol,
                     return ObsoleteDiagnosticKind.Lazy;
             }
 
-            switch (GetObsoleteContextState(containingMember, forceComplete))
+            switch (GetObsoleteContextState(containingMember, forceComplete, getStateFromSymbol: static (symbol) => symbol.ObsoleteState))
             {
                 case ThreeState.False:
                     return ObsoleteDiagnosticKind.Diagnostic;
@@ -123,6 +147,30 @@ internal static ObsoleteDiagnosticKind GetObsoleteDiagnosticKind(Symbol symbol,
                     // later stage
                     return ObsoleteDiagnosticKind.LazyPotentiallySuppressed;
             }
+
+            static bool isExperimentalSymbol(NamedTypeSymbol namedTypeSymbol)
+            {
+                return namedTypeSymbol.Arity == 0
+                    && namedTypeSymbol.HasNameQualifier("System.Diagnostics.CodeAnalysis")
+                    && namedTypeSymbol.Name.Equals("ExperimentalAttribute", StringComparison.Ordinal);
+            }
+
+            static ObsoleteDiagnosticKind getExperimentalDiagnosticKind(Symbol containingMember, bool forceComplete)
+            {
+                switch (GetObsoleteContextState(containingMember, forceComplete, getStateFromSymbol: static (symbol) => symbol.ExperimentalState))
+                {
+                    case ThreeState.False:
+                        return ObsoleteDiagnosticKind.Diagnostic;
+                    case ThreeState.True:
+                        // If we are in a context that is already experimental, there is no point reporting
+                        // more obsolete diagnostics.
+                        return ObsoleteDiagnosticKind.Suppressed;
+                    default:
+                        // If the context is unknown, then store the symbol so that we can do this check at a
+                        // later stage
+                        return ObsoleteDiagnosticKind.LazyPotentiallySuppressed;
+                }
+            }
         }
 
         /// <summary>
@@ -137,7 +185,7 @@ internal static DiagnosticInfo CreateObsoleteDiagnostic(Symbol symbol, BinderFla
 
             static DiagnosticInfo createObsoleteDiagnostic(Symbol symbol, BinderFlags location)
             {
-                var data = symbol.ObsoleteAttributeData;
+                var data = symbol.ObsoleteAttributeData ?? symbol.ContainingModule.ObsoleteAttributeData ?? symbol.ContainingAssembly.ObsoleteAttributeData;
                 Debug.Assert(data != null);
 
                 if (data == null)

src/Compilers/CSharp/Portable/Symbols/Retargeting/RetargetingAssemblySymbol.cs

@@ -284,6 +284,8 @@ internal override bool GetGuidString(out string guidString)
         }
 
 #nullable enable
+        internal sealed override ObsoleteAttributeData? ObsoleteAttributeData
+            => _underlyingAssembly.ObsoleteAttributeData;
 
         internal override NamedTypeSymbol? TryLookupForwardedMetadataTypeWithCycleDetection(ref MetadataTypeName emittedName, ConsList<AssemblySymbol>? visitedAssemblies)
         {

src/Compilers/CSharp/Portable/Symbols/Retargeting/RetargetingModuleSymbol.cs

@@ -318,5 +318,9 @@ public sealed override bool AreLocalsZeroed
         }
 
         internal override bool UseUpdatedEscapeRules => _underlyingModule.UseUpdatedEscapeRules;
+
+#nullable enable
+        internal sealed override ObsoleteAttributeData? ObsoleteAttributeData
+            => _underlyingModule.ObsoleteAttributeData;
     }
 }

src/Compilers/CSharp/Portable/Symbols/Source/SourceAssemblySymbol.cs

@@ -11,10 +11,8 @@
 using System.Diagnostics;
 using System.Linq;
 using System.Reflection;
-using System.Security.Cryptography;
 using System.Threading;
 using Microsoft.CodeAnalysis.CSharp.Emit;
-using Microsoft.CodeAnalysis.CSharp.Symbols;
 using Microsoft.CodeAnalysis.CSharp.Symbols.Metadata.PE;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.PooledObjects;
@@ -1382,7 +1380,7 @@ private void LoadAndValidateNetModuleAttributes(ref CustomAttributesBag<CSharpAt
                 // This affects only diagnostics.
                 for (int i = _modules.Length - 1; i > 0; i--)
                 {
-                    var peModuleSymbol = (Metadata.PE.PEModuleSymbol)_modules[i];
+                    var peModuleSymbol = (PEModuleSymbol)_modules[i];
 
                     foreach (NamedTypeSymbol forwarded in peModuleSymbol.GetForwardedTypes())
                     {
@@ -2553,6 +2551,11 @@ private void DecodeWellKnownAttribute(ref DecodeWellKnownAttributeArguments<Attr
 
                 arguments.GetOrCreateData<CommonAssemblyWellKnownAttributeData>().AssemblyAlgorithmIdAttributeSetting = algorithmId;
             }
+            else if (attribute.IsTargetAttribute(this, AttributeDescription.ExperimentalAttribute))
+            {
+                var obsoleteData = attribute.DecodeExperimentalAttribute();
+                arguments.GetOrCreateData<CommonAssemblyWellKnownAttributeData>().ObsoleteAttributeData = obsoleteData;
+            }
         }
 
         // Checks that the integral arguments for the given well-known attribute are non-negative.
@@ -2860,6 +2863,19 @@ private static string DefaultValue(TypeSymbol type)
             return null;
         }
 
+        internal sealed override ObsoleteAttributeData? ObsoleteAttributeData
+        {
+            get
+            {
+                // [assembly: Experimental] may have been specified in the assembly or one of the modules
+                var result = GetSourceDecodedWellKnownAttributeData()?.ObsoleteAttributeData
+                    ?? GetNetModuleDecodedWellKnownAttributeData()?.ObsoleteAttributeData;
+
+                Debug.Assert(result is null || result is { Kind: ObsoleteAttributeKind.Experimental });
+                return result;
+            }
+        }
+
 #nullable disable
 
         internal override IEnumerable<NamedTypeSymbol> GetAllTopLevelForwardedTypes()

src/Compilers/CSharp/Portable/Symbols/Source/SourceModuleSymbol.cs

@@ -531,6 +531,10 @@ protected override void DecodeWellKnownAttributeImpl(ref DecodeWellKnownAttribut
             {
                 CSharpAttributeData.DecodeSkipLocalsInitAttribute<ModuleWellKnownAttributeData>(DeclaringCompilation, ref arguments);
             }
+            else if (attribute.IsTargetAttribute(this, AttributeDescription.ExperimentalAttribute))
+            {
+                arguments.GetOrCreateData<ModuleWellKnownAttributeData>().ObsoleteAttributeData = attribute.DecodeExperimentalAttribute();
+            }
         }
 
 #nullable enable
@@ -652,5 +656,8 @@ internal override bool UseUpdatedEscapeRules
                 return _lazyUseUpdatedEscapeRules == ThreeState.True;
             }
         }
+
+        internal sealed override ObsoleteAttributeData? ObsoleteAttributeData
+            => GetDecodedWellKnownAttributeData()?.ObsoleteAttributeData;
     }
 }

src/Compilers/CSharp/Portable/Symbols/Symbol.cs

@@ -1309,6 +1309,7 @@ internal static bool GetUnificationUseSiteDiagnosticRecursive(ref DiagnosticInfo
 
         #endregion
 
+#nullable enable
         /// <summary>
         /// True if this symbol has been marked with the <see cref="ObsoleteAttribute"/> attribute. 
         /// This property returns <see cref="ThreeState.Unknown"/> if the <see cref="ObsoleteAttribute"/> attribute hasn't been cracked yet.
@@ -1331,6 +1332,26 @@ internal ThreeState ObsoleteState
             }
         }
 
+        /// <summary>
+        /// True if this symbol has been marked with the System.Diagnostics.CodeAnalysis.ExperimentalAttribute attribute. 
+        /// This property returns <see cref="ThreeState.Unknown"/> if the attribute hasn't been cracked yet.
+        /// </summary>
+        internal ThreeState ExperimentalState
+        {
+            get
+            {
+                switch (ObsoleteKind)
+                {
+                    case ObsoleteAttributeKind.Experimental:
+                        return ThreeState.True;
+                    case ObsoleteAttributeKind.Uninitialized:
+                        return ThreeState.Unknown;
+                    default:
+                        return ThreeState.False;
+                }
+            }
+        }
+
         internal ObsoleteAttributeKind ObsoleteKind
         {
             get
@@ -1344,7 +1365,8 @@ internal ObsoleteAttributeKind ObsoleteKind
         /// Returns data decoded from <see cref="ObsoleteAttribute"/> attribute or null if there is no <see cref="ObsoleteAttribute"/> attribute.
         /// This property returns <see cref="Microsoft.CodeAnalysis.ObsoleteAttributeData.Uninitialized"/> if attribute arguments haven't been decoded yet.
         /// </summary>
-        internal abstract ObsoleteAttributeData ObsoleteAttributeData { get; }
+        internal abstract ObsoleteAttributeData? ObsoleteAttributeData { get; }
+#nullable disable
 
         /// <summary>
         /// Returns true and a <see cref="string"/> from the first <see cref="GuidAttribute"/> on the symbol,