File types emit

src/Compilers/CSharp/Portable/Binder/Binder_Symbols.cs

@@ -2493,7 +2493,8 @@ protected AssemblySymbol GetForwardedToAssembly(string name, int arity, ref Name
 
             // NOTE: This won't work if the type isn't using CLS-style generic naming (i.e. `arity), but this code is
             // only intended to improve diagnostic messages, so false negatives in corner cases aren't a big deal.
-            var metadataName = MetadataHelpers.ComposeAritySuffixedMetadataName(name, arity);
+            // File types can't be forwarded, so we won't attempt to determine a file identifier to attach to the metadata name.
+            var metadataName = MetadataHelpers.ComposeAritySuffixedMetadataName(name, arity, associatedFileIdentifier: null);
             var fullMetadataName = MetadataHelpers.BuildQualifiedName(qualifierOpt?.ToDisplayString(SymbolDisplayFormat.QualifiedNameOnlyFormat), metadataName);
             var result = GetForwardedToAssembly(fullMetadataName, diagnostics, location);
             if ((object)result != null)

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7112,6 +7112,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_FileTypeBase" xml:space="preserve">
     <value>File type '{0}' cannot be used as a base type of non-file type '{1}'.</value>
   </data>
+  <data name="ERR_FileTypeNested" xml:space="preserve">
+    <value>File type '{0}' must be defined in a top level type; '{0}' is a nested type.</value>
+  </data>
   <data name="IDS_FeatureUnsignedRightShift" xml:space="preserve">
     <value>unsigned right shift</value>
   </data>

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

@@ -40,6 +40,16 @@ bool Cci.INamedTypeReference.MangleName
             }
         }
 
+#nullable enable
+        string? Cci.INamedTypeReference.AssociatedFileIdentifier
+        {
+            get
+            {
+                return UnderlyingNamedType.AssociatedFileIdentifier();
+            }
+        }
+#nullable disable
+
         string Cci.INamedEntity.Name
         {
             get

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

@@ -756,6 +756,16 @@ bool Cci.INamedTypeReference.MangleName
             }
         }
 
+#nullable enable
+        string? Cci.INamedTypeReference.AssociatedFileIdentifier
+        {
+            get
+            {
+                return AdaptedNamedTypeSymbol.AssociatedFileIdentifier();
+            }
+        }
+#nullable disable
+
         string Cci.INamedEntity.Name
         {
             get

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

@@ -660,7 +660,7 @@ private void ReportExportedTypeNameCollisions(ImmutableArray<Cci.ExportedType> e
                 // exported types are not emitted in EnC deltas (hence generation 0):
                 string fullEmittedName = MetadataHelpers.BuildQualifiedName(
                     ((Cci.INamespaceTypeReference)type.GetCciAdapter()).NamespaceName,
-                    Cci.MetadataWriter.GetMangledName(type.GetCciAdapter(), generation: 0));
+                    Cci.MetadataWriter.GetMetadataName(type.GetCciAdapter(), generation: 0));
 
                 // First check against types declared in the primary module
                 if (ContainsTopLevelType(fullEmittedName))

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2095,6 +2095,7 @@ internal enum ErrorCode
         ERR_FileTypeDisallowedInSignature = 9300,
         ERR_FileTypeNoExplicitAccessibility = 9301,
         ERR_FileTypeBase = 9302,
+        ERR_FileTypeNested = 9303,
 
         #endregion
 

src/Compilers/CSharp/Portable/SymbolDisplay/SymbolDisplayVisitor.Types.cs

@@ -10,6 +10,7 @@
 using System.Linq;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
+using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.SymbolDisplay;
 using Roslyn.Utilities;
 
@@ -182,17 +183,28 @@ public override void VisitNamedType(INamedTypeSymbol symbol)
             AddNullableAnnotations(symbol);
 
             if ((format.CompilerInternalOptions & SymbolDisplayCompilerInternalOptions.IncludeContainingFileForFileTypes) != 0
-                // PROTOTYPE(ft): public API?
-                && symbol.GetSymbol() is SourceMemberContainerTypeSymbol { IsFile: true } fileType)
+                && symbol is Symbols.PublicModel.Symbol { UnderlyingSymbol: NamedTypeSymbol { AssociatedSyntaxTree: SyntaxTree tree } internalSymbol })
             {
-                var tree = symbol.DeclaringSyntaxReferences[0].SyntaxTree;
-                var fileDescription = tree.FilePath is { Length: not 0 } path
-                    ? Path.GetFileNameWithoutExtension(path)
-                    : $"<tree {fileType.DeclaringCompilation.SyntaxTrees.IndexOf(tree)}>";
+                var fileDescription = getDisplayFileName(tree) is { Length: not 0 } path
+                    ? path
+                    : $"<tree {internalSymbol.DeclaringCompilation.SyntaxTrees.IndexOf(tree)}>";
 
                 builder.Add(CreatePart(SymbolDisplayPartKind.Punctuation, symbol, "@"));
                 builder.Add(CreatePart(SymbolDisplayPartKind.ModuleName, symbol, fileDescription));
             }
+
+            static string getDisplayFileName(SyntaxTree tree)
+            {
+                if (tree.FilePath.Length == 0)
+                {
+                    return "";
+                }
+
+                var pooledBuilder = PooledStringBuilder.GetInstance();
+                var sb = pooledBuilder.Builder;
+                GeneratedNames.AppendFileName(tree.FilePath, sb);
+                return pooledBuilder.ToStringAndFree();
+            }
         }
 
         private void VisitNamedTypeWithoutNullability(INamedTypeSymbol symbol)

src/Compilers/CSharp/Portable/Symbols/AnonymousTypes/PublicSymbols/AnonymousManager.TypeOrDelegatePublicSymbol.cs

@@ -73,6 +73,8 @@ internal sealed override bool MangleName
                 get { return false; }
             }
 
+            internal sealed override SyntaxTree? AssociatedSyntaxTree => null;
+
             public sealed override int Arity
             {
                 get { return 0; }

src/Compilers/CSharp/Portable/Symbols/AnonymousTypes/SynthesizedSymbols/AnonymousType.TypeOrDelegateTemplateSymbol.cs

@@ -237,6 +237,8 @@ internal sealed override bool MangleName
                 get { return this.Arity > 0; }
             }
 
+            internal sealed override SyntaxTree? AssociatedSyntaxTree => null;
+
             internal sealed override ImmutableArray<TypeWithAnnotations> TypeArgumentsWithAnnotationsNoUseSiteDiagnostics
             {
                 get { return GetTypeParametersAsTypeArguments(); }

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

@@ -578,6 +578,8 @@ internal override bool MangleName
             get { return _originalDefinition.MangleName; }
         }
 
+        internal override SyntaxTree? AssociatedSyntaxTree => _originalDefinition.AssociatedSyntaxTree;
+
         internal override DiagnosticInfo? ErrorInfo
         {
             get { return _originalDefinition.ErrorInfo; }

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

@@ -145,6 +145,8 @@ internal override bool MangleName
             }
         }
 
+        internal override SyntaxTree? AssociatedSyntaxTree => null;
+
         public override Symbol? ContainingSymbol
         {
             get

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

@@ -376,6 +376,8 @@ internal abstract override bool MangleName
             get;
         }
 
+        internal override SyntaxTree AssociatedSyntaxTree => null;
+
         internal abstract int MetadataArity
         {
             get;

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

@@ -47,6 +47,9 @@ internal override bool MangleName
                 return mangleName;
             }
         }
+
+        internal override SyntaxTree? AssociatedSyntaxTree => null;
+
         /// <summary>
         /// Get the arity of the missing type.
         /// </summary>

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

@@ -479,17 +479,24 @@ public virtual bool IsImplicitClass
         /// </summary>
         public abstract override string Name { get; }
 
+#nullable enable
         /// <summary>
         /// Return the name including the metadata arity suffix.
         /// </summary>
         public override string MetadataName
         {
             get
             {
-                return MangleName ? MetadataHelpers.ComposeAritySuffixedMetadataName(Name, Arity) : Name;
+                return MangleName ? MetadataHelpers.ComposeAritySuffixedMetadataName(Name, Arity, this.AssociatedFileIdentifier()) : Name;
             }
         }
 
+        /// <summary>
+        /// If this type is a file type, returns the syntax tree where this type is visible. Otherwise, returns null.
+        /// </summary>
+        internal abstract SyntaxTree? AssociatedSyntaxTree { get; }
+#nullable disable
+
         /// <summary>
         /// Should the name returned by Name property be mangled with [`arity] suffix in order to get metadata name.
         /// Must return False for a type with Arity == 0.

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

@@ -2,11 +2,14 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
+using Microsoft.CodeAnalysis.CSharp.Symbols.Metadata.PE;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.Symbols;
 using Roslyn.Utilities;
 
@@ -243,6 +246,7 @@ internal virtual NamedTypeSymbol LookupMetadataType(ref MetadataTypeName emitted
             Debug.Assert(!emittedTypeName.IsNull);
 
             NamespaceOrTypeSymbol scope = this;
+            Debug.Assert(scope is not MergedNamespaceSymbol);
 
             if (scope.Kind == SymbolKind.ErrorType)
             {
@@ -326,6 +330,28 @@ internal virtual NamedTypeSymbol LookupMetadataType(ref MetadataTypeName emitted
             }
 
 Done:
+            if (isTopLevel && scope is not PENamespaceSymbol && GeneratedNameParser.TryParseFileTypeName(emittedTypeName.UnmangledTypeName, out string? displayFileName, out int ordinal, out string? sourceName))
+            {
+                // also do a lookup for file types from source.
+                namespaceOrTypeMembers = scope.GetTypeMembers(sourceName);
+                foreach (var named in namespaceOrTypeMembers)
+                {
+                    if (named.AssociatedSyntaxTree is SyntaxTree tree
+                        && getDisplayName(tree) == displayFileName
+                        && named.DeclaringCompilation.GetSyntaxTreeOrdinal(tree) == ordinal
+                        && named.Arity == emittedTypeName.InferredArity)
+                    {
+                        if ((object?)namedType != null)
+                        {
+                            namedType = null;
+                            break;
+                        }
+
+                        namedType = named;
+                    }
+                }
+            }
+
             if ((object?)namedType == null)
             {
                 if (isTopLevel)
@@ -339,6 +365,13 @@ internal virtual NamedTypeSymbol LookupMetadataType(ref MetadataTypeName emitted
             }
 
             return namedType;
+
+            static string getDisplayName(SyntaxTree tree)
+            {
+                var sb = PooledStringBuilder.GetInstance();
+                GeneratedNames.AppendFileName(tree.FilePath, sb);
+                return sb.ToStringAndFree();
+            }
         }
 
         /// <summary>

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

@@ -49,6 +49,8 @@ internal override bool MangleName
             }
         }
 
+        internal override SyntaxTree? AssociatedSyntaxTree => null;
+
         public AssemblySymbol EmbeddingAssembly
         {
             get

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

@@ -41,6 +41,8 @@ internal override bool MangleName
             }
         }
 
+        internal override SyntaxTree? AssociatedSyntaxTree => null;
+
         public NamedTypeSymbol UnderlyingSymbol
         {
             get

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

@@ -73,6 +73,8 @@ internal override bool MangleName
             }
         }
 
+        internal override SyntaxTree? AssociatedSyntaxTree => null;
+
         public string Guid
         {
             get

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

@@ -48,6 +48,8 @@ internal override bool MangleName
             }
         }
 
+        internal override SyntaxTree? AssociatedSyntaxTree => null;
+
         internal override DiagnosticInfo? ErrorInfo
         {
             get

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

@@ -826,8 +826,7 @@ internal override ManagedKind GetManagedKind(ref CompoundUseSiteInfo<AssemblySym
 
         internal bool IsFile => HasFlag(DeclarationModifiers.File);
 
-        /// <summary>If this symbol is only available within a single syntax tree, returns that syntax tree. Otherwise, returns null.</summary>
-        internal SyntaxTree? AssociatedSyntaxTree => IsFile ? declaration.Declarations[0].Location.SourceTree : null;
+        internal sealed override SyntaxTree? AssociatedSyntaxTree => IsFile ? declaration.Declarations[0].Location.SourceTree : null;
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private bool HasFlag(DeclarationModifiers flag) => (_declModifiers & flag) != 0;
@@ -927,7 +926,7 @@ internal override bool MangleName
         {
             get
             {
-                return Arity > 0;
+                return Arity > 0 || IsFile;
             }
         }
 
@@ -1743,6 +1742,11 @@ protected void AfterMembersChecks(BindingDiagnosticBag diagnostics)
                 }
             }
 
+            if (IsFile && (object?)ContainingType != null)
+            {
+                diagnostics.Add(ErrorCode.ERR_FileTypeNested, location, this);
+            }
+
             return;
 
             bool hasBaseTypeOrInterface(Func<NamedTypeSymbol, bool> predicate)

src/Compilers/CSharp/Portable/Symbols/Synthesized/GeneratedNameKind.cs

@@ -30,6 +30,7 @@ internal enum GeneratedNameKind
         ReusableHoistedLocalField = '7',
         LambdaCacheField = '9',
         FixedBufferField = 'e',
+        FileType = 'F',
         AnonymousType = 'f',
         TransparentIdentifier = 'h',
         AnonymousTypeField = 'i',

src/Compilers/CSharp/Portable/Symbols/Synthesized/GeneratedNameParser.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
+using System.Text.RegularExpressions;
 
 namespace Microsoft.CodeAnalysis.CSharp.Symbols
 {
@@ -170,5 +171,35 @@ internal static bool TryParseAnonymousTypeParameterName(string typeParameterName
             propertyName = null;
             return false;
         }
+
+        // A full metadata name for a generic file type looks like:
+        // <ContainingFile>FN__ClassName`A
+        // where 'N' is the syntax tree ordinal, 'A' is the arity,
+        // and 'ClassName' is the source name of the type.
+        //
+        // The "unmangled" name of a generic file type looks like:
+        // <ContainingFile>FN__ClassName
+        private static readonly Regex s_fileTypeOrdinalPattern = new Regex(@"<([a-zA-Z_0-9]*)>F(\d)+__", RegexOptions.Compiled);
+
+        /// <remarks>
+        /// This method will work with either unmangled or mangled type names as input, but it does not remove any arity suffix if present.
+        /// </remarks>
+        internal static bool TryParseFileTypeName(string generatedName, [NotNullWhen(true)] out string? displayFileName, out int ordinal, [NotNullWhen(true)] out string? originalTypeName)
+        {
+            if (s_fileTypeOrdinalPattern.Match(generatedName) is Match { Success: true, Groups: var groups, Index: var index, Length: var length }
+                && int.TryParse(groups[2].Value, out ordinal))
+            {
+                displayFileName = groups[1].Value;
+
+                var prefixEndsAt = index + length;
+                originalTypeName = generatedName.Substring(prefixEndsAt);
+                return true;
+            }
+
+            ordinal = -1;
+            displayFileName = null;
+            originalTypeName = null;
+            return false;
+        }
     }
 }