Merge branch 'master' of https://github.com/dotnet/roslyn into target-typing-merge2020-07-17

eng/Version.Details.xml

@@ -3,9 +3,9 @@
   <ProductDependencies>
   </ProductDependencies>
   <ToolsetDependencies>
-    <Dependency Name="Microsoft.DotNet.Arcade.Sdk" Version="1.0.0-beta.20326.2">
+    <Dependency Name="Microsoft.DotNet.Arcade.Sdk" Version="1.0.0-beta.20365.6">
       <Uri>https://github.com/dotnet/arcade</Uri>
-      <Sha>ed69753a3ffbdaa08365252c710d57a64d17f859</Sha>
+      <Sha>7cc59275eade471e29b88a797275818b0d513d0f</Sha>
     </Dependency>
     <Dependency Name="Microsoft.Net.Compilers.Toolset" Version="3.8.0-1.20361.1">
       <Uri>https://github.com/dotnet/roslyn</Uri>

eng/common/templates/post-build/post-build.yml

@@ -355,6 +355,18 @@ stages:
     shippingFeed: 'https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json'
     symbolsFeed: 'https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools-symbols/nuget/v3/index.json'
 
+- template: \eng\common\templates\post-build\channels\generic-public-channel.yml
+  parameters:
+    artifactsPublishingAdditionalParameters: ${{ parameters.artifactsPublishingAdditionalParameters }}
+    publishInstallersAndChecksums: ${{ parameters.publishInstallersAndChecksums }}
+    symbolPublishingAdditionalParameters: ${{ parameters.symbolPublishingAdditionalParameters }}
+    stageName: 'VS16_8_Publishing'
+    channelName: 'VS 16.8'
+    channelId: 1154
+    transportFeed: 'https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools-transport/nuget/v3/index.json'
+    shippingFeed: 'https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json'
+    symbolsFeed: 'https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools-symbols/nuget/v3/index.json'
+
 - template: \eng\common\templates\post-build\channels\generic-public-channel.yml
   parameters:
     artifactsPublishingAdditionalParameters: ${{ parameters.artifactsPublishingAdditionalParameters }}

global.json

@@ -7,6 +7,6 @@
     "xcopy-msbuild": "16.4.0-alpha"
   },
   "msbuild-sdks": {
-    "Microsoft.DotNet.Arcade.Sdk": "1.0.0-beta.20326.2"
+    "Microsoft.DotNet.Arcade.Sdk": "1.0.0-beta.20365.6"
   }
 }

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6327,4 +6327,19 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_DoesNotOverrideBaseEqualityContract" xml:space="preserve">
     <value>'{0}' does not override expected property from '{1}'.</value>
   </data>
+  <data name="IDS_FeatureModuleInitializers" xml:space="preserve">
+    <value>module initializers</value>
+  </data>
+  <data name="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType" xml:space="preserve">
+    <value>Module initializer method '{0}' must be accessible at the module level</value>
+  </data>
+  <data name="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid" xml:space="preserve">
+    <value>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</value>
+  </data>
+  <data name="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric" xml:space="preserve">
+    <value>Module initializer method '{0}' must not be generic and must not be contained in a generic type</value>
+  </data>
+  <data name="ERR_ModuleInitializerMethodMustBeOrdinary" xml:space="preserve">
+    <value>A module initializer must be an ordinary member method</value>
+  </data>
 </root>

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

@@ -17,6 +17,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Cci;
+using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CodeGen;
 using Microsoft.CodeAnalysis.CSharp.Emit;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
@@ -2057,6 +2058,14 @@ internal PointerTypeSymbol CreatePointerTypeSymbol(TypeSymbol elementType, Nulla
             throw new NotImplementedException();
         }
 
+        private ConcurrentSet<MethodSymbol>? _moduleInitializerMethods;
+
+        internal void AddModuleInitializerMethod(MethodSymbol method)
+        {
+            Debug.Assert(!_declarationDiagnosticsFrozen);
+            LazyInitializer.EnsureInitialized(ref _moduleInitializerMethods).Add(method);
+        }
+
         #endregion
 
         #region Binding
@@ -2857,6 +2866,11 @@ internal override StrongNameKeys StrongNameKeys
                     filterOpt: filterOpt,
                     cancellationToken: cancellationToken);
 
+                if (!hasDeclarationErrors && !CommonCompiler.HasUnsuppressableErrors(methodBodyDiagnosticBag))
+                {
+                    GenerateModuleInitializer(moduleBeingBuilt, methodBodyDiagnosticBag);
+                }
+
                 bool hasMethodBodyError = !FilterAndAppendAndFreeDiagnostics(diagnostics, ref methodBodyDiagnosticBag);
 
                 if (hasDeclarationErrors || hasMethodBodyError)
@@ -2868,6 +2882,30 @@ internal override StrongNameKeys StrongNameKeys
             return true;
         }
 
+        private void GenerateModuleInitializer(PEModuleBuilder moduleBeingBuilt, DiagnosticBag methodBodyDiagnosticBag)
+        {
+            Debug.Assert(_declarationDiagnosticsFrozen);
+
+            if (_moduleInitializerMethods is object)
+            {
+                var ilBuilder = new ILBuilder(moduleBeingBuilt, new LocalSlotManager(slotAllocator: null), OptimizationLevel.Release, areLocalsZeroed: false);
+
+                foreach (MethodSymbol method in _moduleInitializerMethods.OrderBy<MethodSymbol>(LexicalOrderSymbolComparer.Instance))
+                {
+                    ilBuilder.EmitOpCode(ILOpCode.Call, stackAdjustment: 0);
+
+                    ilBuilder.EmitToken(
+                        moduleBeingBuilt.Translate(method, methodBodyDiagnosticBag, needDeclaration: true),
+                        CSharpSyntaxTree.Dummy.GetRoot(),
+                        methodBodyDiagnosticBag);
+                }
+
+                ilBuilder.EmitRet(isVoid: true);
+                ilBuilder.Realize();
+                moduleBeingBuilt.RootModuleType.SetStaticConstructorBody(ilBuilder.RealizedIL);
+            }
+        }
+
         internal override bool GenerateResourcesAndDocumentationComments(
             CommonPEModuleBuilder moduleBuilder,
             Stream? xmlDocStream,

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

@@ -8,10 +8,11 @@
 
 namespace Microsoft.CodeAnalysis.CSharp
 {
-    /// <summary> This is an implementation of a special symbol comparer, which is supposed to be used  for 
-    /// sorting original definition symbols (explicitly or explicitly declared in source  within the same 
-    /// container) in lexical order of their declarations. It will not work on  anything that uses non-source locations. 
-    /// </summary>        
+    /// <summary>
+    /// This is an implementation of a special symbol comparer, which is supposed to be used for sorting
+    /// original definition symbols (explicitly or implicitly declared in source within the same compilation)
+    /// in lexical order of their declarations. It will not work on anything that uses non-source locations.
+    /// </summary>
     internal class LexicalOrderSymbolComparer : IComparer<Symbol>
     {
         public static readonly LexicalOrderSymbolComparer Instance = new LexicalOrderSymbolComparer();
@@ -30,6 +31,7 @@ public int Compare(Symbol x, Symbol y)
 
             var xSortKey = x.GetLexicalSortKey();
             var ySortKey = y.GetLexicalSortKey();
+            Debug.Assert((object)x.DeclaringCompilation == y.DeclaringCompilation);
 
             comparison = LexicalSortKey.Compare(xSortKey, ySortKey);
             if (comparison != 0)

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

@@ -1820,13 +1820,10 @@ internal enum ErrorCode
         ERR_CannotConvertAddressOfToDelegate = 8811,
         ERR_AddressOfToNonFunctionPointer = 8812,
 
-        ERR_8813 = 8813, // used by features/module-initializers
-
-        ERR_8814 = 8814, // used by features/module-initializers
-
-        ERR_8815 = 8815, // used by features/module-initializers
-
-        ERR_8816 = 8816, // used by features/module-initializers
+        ERR_ModuleInitializerMethodMustBeOrdinary = 8813,
+        ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType = 8814,
+        ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid = 8815,
+        ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric = 8816,
 
         ERR_PartialMethodReturnTypeDifference = 8817,
         ERR_PartialMethodRefReturnDifference = 8818,

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

@@ -207,7 +207,8 @@ internal enum MessageID
         IDS_FeatureInitOnlySetters = MessageBase + 12781,
         IDS_FeatureRecords = MessageBase + 12782,
         IDS_FeatureNullPointerConstantPattern = MessageBase + 12783,
-        IDS_FeatureTargetTypedConditional = MessageBase + 12784,
+        IDS_FeatureModuleInitializers = MessageBase + 12784,
+        IDS_FeatureTargetTypedConditional = MessageBase + 12785,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -334,6 +335,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureRecords:
                 case MessageID.IDS_FeatureTargetTypedConditional:  // semantic check
                 case MessageID.IDS_FeatureStaticAnonymousFunction: // syntax check
+                case MessageID.IDS_FeatureModuleInitializers: // semantic check on method attribute
                     return LanguageVersion.Preview;
 
                 // C# 8.0 features.

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

@@ -9,6 +9,7 @@
 using System.Linq;
 using System.Runtime.InteropServices;
 using System.Threading;
+using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Roslyn.Utilities;
 
@@ -486,6 +487,11 @@ private void DecodeWellKnownAttributeAppliedToMethod(ref DecodeWellKnownAttribut
                 MessageID.IDS_FeatureMemberNotNull.CheckFeatureAvailability(arguments.Diagnostics, arguments.AttributeSyntaxOpt);
                 CSharpAttributeData.DecodeMemberNotNullWhenAttribute<MethodWellKnownAttributeData>(ContainingType, ref arguments);
             }
+            else if (attribute.IsTargetAttribute(this, AttributeDescription.ModuleInitializerAttribute))
+            {
+                MessageID.IDS_FeatureModuleInitializers.CheckFeatureAvailability(arguments.Diagnostics, arguments.AttributeSyntaxOpt);
+                DecodeModuleInitializerAttribute(arguments);
+            }
             else
             {
                 var compilation = this.DeclaringCompilation;
@@ -762,6 +768,46 @@ private void DecodeDllImportAttribute(ref DecodeWellKnownAttributeArguments<Attr
                     preserveSig);
             }
         }
+
+        private void DecodeModuleInitializerAttribute(DecodeWellKnownAttributeArguments<AttributeSyntax, CSharpAttributeData, AttributeLocation> arguments)
+        {
+            Debug.Assert(arguments.AttributeSyntaxOpt is object);
+
+            if (MethodKind != MethodKind.Ordinary)
+            {
+                arguments.Diagnostics.Add(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, arguments.AttributeSyntaxOpt.Location);
+                return;
+            }
+
+            Debug.Assert(ContainingType is object);
+            var hasError = false;
+
+            HashSet<DiagnosticInfo>? useSiteDiagnostics = null;
+            if (!AccessCheck.IsSymbolAccessible(this, ContainingAssembly, ref useSiteDiagnostics))
+            {
+                arguments.Diagnostics.Add(ErrorCode.ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType, arguments.AttributeSyntaxOpt.Location, Name);
+                hasError = true;
+            }
+
+            arguments.Diagnostics.Add(arguments.AttributeSyntaxOpt, useSiteDiagnostics);
+
+            if (!IsStatic || ParameterCount > 0 || !ReturnsVoid)
+            {
+                arguments.Diagnostics.Add(ErrorCode.ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid, arguments.AttributeSyntaxOpt.Location, Name);
+                hasError = true;
+            }
+
+            if (IsGenericMethod || ContainingType.IsGenericType)
+            {
+                arguments.Diagnostics.Add(ErrorCode.ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric, arguments.AttributeSyntaxOpt.Location, Name);
+                hasError = true;
+            }
+
+            if (!hasError && !CallsAreOmitted(arguments.AttributeSyntaxOpt.SyntaxTree))
+            {
+                DeclaringCompilation.AddModuleInitializerMethod(this);
+            }
+        }
 #nullable restore
 
         internal sealed override void PostDecodeWellKnownAttributes(ImmutableArray<CSharpAttributeData> boundAttributes, ImmutableArray<AttributeSyntax> allAttributeSyntaxNodes, DiagnosticBag diagnostics, AttributeLocation symbolPart, WellKnownAttributeData decodedData)

src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf

@@ -502,6 +502,26 @@
         <target state="translated">Chybějící vzor</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">Ve stejném adresáři nemůže být více konfiguračních souborů analyzátoru ({0}).</target>
@@ -1291,6 +1311,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">skrývání názvů ve vnořených funkcích</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf

@@ -502,6 +502,26 @@
         <target state="translated">Muster fehlt.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">Dasselbe Verzeichnis ({0}) darf nicht mehrere Konfigurationsdateien des Analysetools enthalten.</target>
@@ -1291,6 +1311,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">Namensshadowing in geschachtelten Funktionen</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf

@@ -502,6 +502,26 @@
         <target state="translated">Falta un patrón.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">No es posible que un mismo directorio ("{0}") contenga varios archivos de configuración del analizador.</target>
@@ -1291,6 +1311,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">sombreado de nombres en funciones anidadas</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf

@@ -502,6 +502,26 @@
         <target state="translated">Modèle manquant</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">Plusieurs fichiers config d'analyseur ne peuvent pas figurer dans le même répertoire ('{0}').</target>
@@ -1291,6 +1311,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">ombrage des noms dans les fonctions imbriquées</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf

@@ -502,6 +502,26 @@
         <target state="translated">Criterio mancante</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">La stessa directory ('{0}') non può contenere più file di configurazione dell'analizzatore.</target>
@@ -1291,6 +1311,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">shadowing dei nomi nelle funzioni annidate</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf

@@ -502,6 +502,26 @@
         <target state="translated">パターンがありません</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">複数のアナライザー構成ファイルを同じディレクトリに入れることはできません ('{0}')。</target>
@@ -1291,6 +1311,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">入れ子になった関数での名前シャドウイング</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf

@@ -502,6 +502,26 @@
         <target state="translated">패턴이 없습니다.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">분석기 구성 파일 여러 개가 동일한 디렉터리('{0}')에 있을 수 없습니다.</target>
@@ -1291,6 +1311,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">중첩된 함수의 이름 섀도잉</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf

@@ -502,6 +502,26 @@
         <target state="translated">Brak wzorca</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">Wiele plików konfiguracji analizatora nie może znajdować się w tym samym katalogu („{0}”).</target>
@@ -1291,6 +1311,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">zasłanianie nazw w funkcjach zagnieżdżonych</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf

@@ -502,6 +502,26 @@
         <target state="translated">Padrão ausente</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">Não é possível que haja vários arquivos de configuração do analisador no mesmo diretório ('{0}').</target>
@@ -1289,6 +1309,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">sombreamento de nome em funções aninhadas</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf

@@ -502,6 +502,26 @@
         <target state="translated">Отсутствует шаблон</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">В одном каталоге ("{0}") не может находиться несколько файлов конфигурации анализатора.</target>
@@ -1291,6 +1311,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">скрытие имен во вложенных функциях</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf

@@ -502,6 +502,26 @@
         <target state="translated">Desen eksik</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">Birden çok çözümleyici yapılandırma dosyası aynı dizinde ('{0}') olamaz.</target>
@@ -1291,6 +1311,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">iç içe işlevlerde ad gölgeleme</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf

@@ -502,6 +502,26 @@
         <target state="translated">模式缺失</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">多个分析器配置文件不能位于同一目录({0})中。</target>
@@ -1291,6 +1311,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">在嵌套函数中的名称映射</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf

@@ -502,6 +502,26 @@
         <target state="translated">缺少模式</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric">
+        <source>Module initializer method '{0}' must not be generic and must not be contained in a generic type</source>
+        <target state="new">Module initializer method '{0}' must not be generic and must not be contained in a generic type</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType">
+        <source>Module initializer method '{0}' must be accessible at the module level</source>
+        <target state="new">Module initializer method '{0}' must be accessible at the module level</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeOrdinary">
+        <source>A module initializer must be an ordinary member method</source>
+        <target state="new">A module initializer must be an ordinary member method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid">
+        <source>Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</source>
+        <target state="new">Module initializer method '{0}' must be static, must have no parameters, and must return 'void'</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_MultipleAnalyzerConfigsInSameDir">
         <source>Multiple analyzer config files cannot be in the same directory ('{0}').</source>
         <target state="translated">多個分析器組態檔無法處於相同目錄 ('{0}') 中。</target>
@@ -1291,6 +1311,11 @@
         <target state="new">MemberNotNull attribute</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureModuleInitializers">
+        <source>module initializers</source>
+        <target state="new">module initializers</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureNameShadowingInNestedFunctions">
         <source>name shadowing in nested functions</source>
         <target state="translated">巢狀函式中的名稱鏡像處理</target>

src/Compilers/CSharp/Test/Symbol/Symbols/ModuleInitializers/AccessibilityTests.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
+using Microsoft.CodeAnalysis.Test.Utilities;
+using Roslyn.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.CSharp.UnitTests.Symbols.ModuleInitializers
+{
+    [CompilerTrait(CompilerFeature.ModuleInitializers)]
+    public sealed class AccessibilityTests : CSharpTestBase
+    {
+        private static readonly CSharpParseOptions s_parseOptions = TestOptions.RegularPreview;
+
+        [Theory]
+        [InlineData("private")]
+        [InlineData("protected")]
+        [InlineData("private protected")]
+        public void DisallowedMethodAccessibility(string keywords)
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    " + keywords + @" static void M() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8796: Module initializer method 'M' must be accessible at the module level
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType, "ModuleInitializer").WithArguments("M").WithLocation(6, 6)
+                );
+        }
+
+        [Theory]
+        [InlineData("public")]
+        [InlineData("internal")]
+        [InlineData("protected internal")]
+        public void AllowedMethodAccessibility(string keywords)
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    " + keywords + @" static void M() => Console.WriteLine(""C.M"");
+}
+
+class Program 
+{
+    static void Main() => Console.WriteLine(""Program.Main"");
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(source, parseOptions: s_parseOptions, expectedOutput: @"
+C.M
+Program.Main");
+        }
+
+        [Theory]
+        [InlineData("public")]
+        [InlineData("internal")]
+        public void AllowedTopLevelTypeAccessibility(string keywords)
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+" + keywords + @" class C
+{
+    [ModuleInitializer]
+    public static void M() => Console.WriteLine(""C.M"");
+}
+
+class Program 
+{
+    static void Main() => Console.WriteLine(""Program.Main"");
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(source, parseOptions: s_parseOptions, expectedOutput: @"
+C.M
+Program.Main");
+        }
+
+        [Theory]
+        [InlineData("private")]
+        [InlineData("protected")]
+        [InlineData("private protected")]
+        public void DisallowedNestedTypeAccessibility(string keywords)
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+public class C
+{
+    " + keywords + @" class Nested
+    {
+        [ModuleInitializer]
+        public static void M() { }
+    }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (8,10): error CS8796: Module initializer method 'M' must be accessible at the module level
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType, "ModuleInitializer").WithArguments("M").WithLocation(8, 10)
+                );
+        }
+
+        [Fact]
+        public void ModuleInitializerOnPrivatePartialMethod()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+partial class C
+{
+    [ModuleInitializer] // 1
+    static partial void M1();
+
+    [ModuleInitializer] // 2
+    static partial void M2();
+    static partial void M2() { }
+
+    static partial void M3();
+    [ModuleInitializer] // 3
+    static partial void M3() { }
+
+    [ModuleInitializer] // 4
+    static partial void M4();
+    [ModuleInitializer] // 5
+    static partial void M4() { }
+}
+
+class Program
+{
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (7,6): error CS8814: Module initializer method 'M1' must be accessible at the module level
+                //     [ModuleInitializer] // 1
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType, "ModuleInitializer").WithArguments("M1").WithLocation(7, 6),
+                // (10,6): error CS8814: Module initializer method 'M2' must be accessible at the module level
+                //     [ModuleInitializer] // 2
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType, "ModuleInitializer").WithArguments("M2").WithLocation(10, 6),
+                // (15,6): error CS8814: Module initializer method 'M3' must be accessible at the module level
+                //     [ModuleInitializer] // 3
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType, "ModuleInitializer").WithArguments("M3").WithLocation(15, 6),
+                // (18,6): error CS8814: Module initializer method 'M4' must be accessible at the module level
+                //     [ModuleInitializer] // 4
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType, "ModuleInitializer").WithArguments("M4").WithLocation(18, 6),
+                // (20,6): error CS0579: Duplicate 'ModuleInitializer' attribute
+                //     [ModuleInitializer] // 5
+                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "ModuleInitializer").WithArguments("ModuleInitializer").WithLocation(20, 6)
+                );
+        }
+
+        [Fact]
+        public void ModuleInitializerOnPrivatePartialMethod_AllowMultiple()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+partial class C
+{
+    [ModuleInitializer] // 1
+    static partial void M1();
+
+    [ModuleInitializer] // 2
+    static partial void M2();
+    static partial void M2() { }
+
+    static partial void M3();
+    [ModuleInitializer] // 3
+    static partial void M3() { }
+
+    [ModuleInitializer] // 4
+    static partial void M4();
+    [ModuleInitializer] // 5
+    static partial void M4() { }
+}
+
+class Program
+{
+}
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.All, AllowMultiple = true)]
+    class ModuleInitializerAttribute : System.Attribute { }
+}
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (7,6): error CS8814: Module initializer method 'M1' must be accessible at the module level
+                //     [ModuleInitializer] // 1
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType, "ModuleInitializer").WithArguments("M1").WithLocation(7, 6),
+                // (10,6): error CS8814: Module initializer method 'M2' must be accessible at the module level
+                //     [ModuleInitializer] // 2
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType, "ModuleInitializer").WithArguments("M2").WithLocation(10, 6),
+                // (15,6): error CS8814: Module initializer method 'M3' must be accessible at the module level
+                //     [ModuleInitializer] // 3
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType, "ModuleInitializer").WithArguments("M3").WithLocation(15, 6),
+                // (18,6): error CS8814: Module initializer method 'M4' must be accessible at the module level
+                //     [ModuleInitializer] // 4
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType, "ModuleInitializer").WithArguments("M4").WithLocation(18, 6),
+                // (20,6): error CS8814: Module initializer method 'M4' must be accessible at the module level
+                //     [ModuleInitializer] // 5
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeAccessibleOutsideTopLevelType, "ModuleInitializer").WithArguments("M4").WithLocation(20, 6)
+                );
+        }
+
+        [Fact]
+        public void ModuleInitializerOnPublicPartialMethod()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+partial class C
+{
+    [ModuleInitializer]
+    public static partial void M1();
+    public static partial void M1() { Console.Write(1); }
+
+    public static partial void M2();
+    [ModuleInitializer]
+    public static partial void M2() { Console.Write(2); }
+}
+
+class Program
+{
+    public static void Main()
+    {
+        Console.Write(3);
+    }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(source, parseOptions: s_parseOptions, expectedOutput: @"123");
+        }
+
+        [Fact]
+        public void DuplicateModuleInitializerOnPublicPartialMethod()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+partial class C
+{
+    [ModuleInitializer]
+    public static partial void M1();
+    [ModuleInitializer] // 1
+    public static partial void M1() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                    // (9,6): error CS0579: Duplicate 'ModuleInitializer' attribute
+                    //     [ModuleInitializer] // 1
+                    Diagnostic(ErrorCode.ERR_DuplicateAttribute, "ModuleInitializer").WithArguments("ModuleInitializer").WithLocation(9, 6)
+                );
+        }
+
+        [Fact]
+        public void DuplicateModuleInitializerOnPublicPartialMethod_AllowMultiple()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+partial class C
+{
+    [ModuleInitializer]
+    public static partial void M1();
+    [ModuleInitializer]
+    public static partial void M1() { Console.Write(1); }
+}
+
+class Program
+{
+    static void Main()
+    {
+        Console.Write(2);
+    }
+}
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.All, AllowMultiple = true)]
+    class ModuleInitializerAttribute : System.Attribute { }
+}
+";
+            CompileAndVerify(source, expectedOutput: "12", parseOptions: s_parseOptions);
+        }
+
+        [Theory]
+        [InlineData("public")]
+        [InlineData("internal")]
+        [InlineData("protected internal")]
+        public void AllowedNestedTypeAccessibility(string keywords)
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+public class C
+{
+    " + keywords + @" class Nested
+    {
+        [ModuleInitializer]
+        public static void M() => Console.WriteLine(""C.M"");
+    }
+}
+
+class Program 
+{
+    static void Main() => Console.WriteLine(""Program.Main"");
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(source, parseOptions: s_parseOptions, expectedOutput: @"
+C.M
+Program.Main");
+        }
+
+        [Fact]
+        public void ImplicitPublicInterfaceMethodAccessibility()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+interface I
+{
+    [ModuleInitializer]
+    static void M() => Console.WriteLine(""I.M"");
+}
+
+class Program 
+{
+    static void Main() => Console.WriteLine(""Program.Main"");
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(
+                source,
+                parseOptions: s_parseOptions,
+                targetFramework: TargetFramework.NetStandardLatest,
+                expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? @"
+I.M
+Program.Main" : null,
+                verify: ExecutionConditionUtil.IsMonoOrCoreClr ? Verification.Passes : Verification.Skipped);
+        }
+
+        [Fact]
+        public void ImplicitPublicInterfaceNestedTypeAccessibility()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+interface I
+{
+    class Nested
+    {
+        [ModuleInitializer]
+        internal static void M() => Console.WriteLine(""C.M"");
+    }
+}
+
+class Program 
+{
+    static void Main() => Console.WriteLine(""Program.Main"");
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(source, parseOptions: s_parseOptions, expectedOutput: @"
+C.M
+Program.Main");
+        }
+    }
+}

src/Compilers/CSharp/Test/Symbol/Symbols/ModuleInitializers/GenericsTests.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
+using Microsoft.CodeAnalysis.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.CSharp.UnitTests.Symbols.ModuleInitializers
+{
+    [CompilerTrait(CompilerFeature.ModuleInitializers)]
+    public sealed class GenericsTests : CSharpTestBase
+    {
+        private static readonly CSharpParseOptions s_parseOptions = TestOptions.RegularPreview;
+
+        [Fact]
+        public void MustNotBeGenericMethod()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    internal static void M<T>() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8798: Module initializer method 'M' must not be generic and must not be contained in a generic type
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric, "ModuleInitializer").WithArguments("M").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void MustNotBeContainedInGenericType()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C<T>
+{
+    [ModuleInitializer]
+    internal static void M() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8798: Module initializer method 'M' must not be generic and must not be contained in a generic type
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric, "ModuleInitializer").WithArguments("M").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void MustNotBeGenericAndContainedInGenericType()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C<T>
+{
+    [ModuleInitializer]
+    internal static void M<U>() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8816: Module initializer method 'M' must not be generic and must not be contained in a generic type
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric, "ModuleInitializer").WithArguments("M").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void MustNotBeContainedInGenericTypeWithParametersDeclaredByContainingGenericType()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C<T>
+{
+    internal class Nested
+    {
+        [ModuleInitializer]
+        internal static void M() { }
+    }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (8,10): error CS8798: Module initializer method 'M' must not be generic and must not be contained in a generic type
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodAndContainingTypesMustNotBeGeneric, "ModuleInitializer").WithArguments("M").WithLocation(8, 10)
+                );
+        }
+    }
+}

src/Compilers/CSharp/Test/Symbol/Symbols/ModuleInitializers/IgnoredTests.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.CodeAnalysis.CSharp.Symbols;
+using Microsoft.CodeAnalysis.CSharp.Symbols.Metadata.PE;
+using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
+using Microsoft.CodeAnalysis.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.CSharp.UnitTests.Symbols.ModuleInitializers
+{
+    [CompilerTrait(CompilerFeature.ModuleInitializers)]
+    public sealed class IgnoredTests : CSharpTestBase
+    {
+        [Fact]
+        public void IgnoredOnReturnValue()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [return: ModuleInitializer]
+    internal static void M()
+    {
+    }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(
+                source,
+                options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    Assert.Equal(MetadataImportOptions.All, ((PEModuleSymbol)module).ImportOptions);
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    Assert.Null(rootModuleType.GetMember(".cctor"));
+                });
+        }
+
+        [Fact]
+        public void IgnoredOnMethodParameter()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    internal static void M([ModuleInitializer] int p)
+    {
+    }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(
+                source,
+                options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    Assert.Equal(MetadataImportOptions.All, ((PEModuleSymbol)module).ImportOptions);
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    Assert.Null(rootModuleType.GetMember(".cctor"));
+                });
+        }
+
+        [Fact]
+        public void IgnoredOnGenericParameter()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    internal static void M<[ModuleInitializer] T>()
+    {
+    }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(
+                source,
+                options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    Assert.Equal(MetadataImportOptions.All, ((PEModuleSymbol)module).ImportOptions);
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    Assert.Null(rootModuleType.GetMember(".cctor"));
+                });
+        }
+
+        [Fact]
+        public void IgnoredOnClass()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+[ModuleInitializer]
+class C
+{
+    internal static void M() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(
+                source,
+                options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    Assert.Equal(MetadataImportOptions.All, ((PEModuleSymbol)module).ImportOptions);
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    Assert.Null(rootModuleType.GetMember(".cctor"));
+                });
+        }
+
+        [Fact]
+        public void IgnoredOnEvent()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    public event System.Action E;
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(
+                source,
+                options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    Assert.Equal(MetadataImportOptions.All, ((PEModuleSymbol)module).ImportOptions);
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    Assert.Null(rootModuleType.GetMember(".cctor"));
+                });
+        }
+
+        [Fact]
+        public void IgnoredOnProperty()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    public int P { get; set; }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(
+                source,
+                options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    Assert.Equal(MetadataImportOptions.All, ((PEModuleSymbol)module).ImportOptions);
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    Assert.Null(rootModuleType.GetMember(".cctor"));
+                });
+        }
+
+        [Fact]
+        public void IgnoredOnIndexer()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    public int this[int p] => p;
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(
+                source,
+                options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    Assert.Equal(MetadataImportOptions.All, ((PEModuleSymbol)module).ImportOptions);
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    Assert.Null(rootModuleType.GetMember(".cctor"));
+                });
+        }
+
+        [Fact]
+        public void IgnoredOnField()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    public int F;
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(
+                source,
+                options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    Assert.Equal(MetadataImportOptions.All, ((PEModuleSymbol)module).ImportOptions);
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    Assert.Null(rootModuleType.GetMember(".cctor"));
+                });
+        }
+
+        [Fact]
+        public void IgnoredOnModule()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+[module: ModuleInitializer]
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(
+                source,
+                options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    Assert.Equal(MetadataImportOptions.All, ((PEModuleSymbol)module).ImportOptions);
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    Assert.Null(rootModuleType.GetMember(".cctor"));
+                });
+        }
+
+        [Fact]
+        public void IgnoredOnAssembly()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+[assembly: ModuleInitializer]
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(
+                source,
+                options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    Assert.Equal(MetadataImportOptions.All, ((PEModuleSymbol)module).ImportOptions);
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    Assert.Null(rootModuleType.GetMember(".cctor"));
+                });
+        }
+
+        [Fact]
+        public void IgnoredWhenConstructorArgumentIsSpecified()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer(42)]
+    internal static void M()
+    {
+    }
+}
+
+namespace System.Runtime.CompilerServices
+{
+    class ModuleInitializerAttribute : System.Attribute
+    {
+        public ModuleInitializerAttribute(int p) { }
+    }
+}
+";
+            CompileAndVerify(
+                source,
+                options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    Assert.Equal(MetadataImportOptions.All, ((PEModuleSymbol)module).ImportOptions);
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    Assert.Null(rootModuleType.GetMember(".cctor"));
+                });
+        }
+    }
+}

src/Compilers/CSharp/Test/Symbol/Symbols/ModuleInitializers/SignatureTests.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
+using Microsoft.CodeAnalysis.Test.Utilities;
+using Roslyn.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.CSharp.UnitTests.Symbols.ModuleInitializers
+{
+    [CompilerTrait(CompilerFeature.ModuleInitializers)]
+    public sealed class SignatureTests : CSharpTestBase
+    {
+        private static readonly CSharpParseOptions s_parseOptions = TestOptions.RegularPreview;
+
+        [Fact]
+        public void MustNotBeInstanceMethod()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    internal void M() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8815: Module initializer method 'M' must be static, must have no parameters, and must return 'void'
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid, "ModuleInitializer").WithArguments("M").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void MustNotBeInstanceMethodInInterface()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+interface i
+{
+    [ModuleInitializer]
+    internal void M1();
+
+    [ModuleInitializer]
+    internal void M2() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions, targetFramework: TargetFramework.NetStandardLatest);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8815: Module initializer method 'M1' must be static, must have no parameters, and must return 'void'
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid, "ModuleInitializer").WithArguments("M1").WithLocation(6, 6),
+                // (9,6): error CS8815: Module initializer method 'M2' must be static, must have no parameters, and must return 'void'
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid, "ModuleInitializer").WithArguments("M2").WithLocation(9, 6)
+                );
+        }
+
+        [Fact]
+        public void MustNotHaveParameters()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+static class C
+{
+    [ModuleInitializer]
+    internal static void M(object p) { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8815: Module initializer method 'M' must be static, must have no parameters, and must return 'void'
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid, "ModuleInitializer").WithArguments("M").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void MustNotHaveOptionalParameters()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+static class C
+{
+    [ModuleInitializer]
+    internal static void M(object p = null) { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8815: Module initializer method 'M' must be static, must have no parameters, and must return 'void'
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid, "ModuleInitializer").WithArguments("M").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void MustNotHaveParamsArrayParameters()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+static class C
+{
+    [ModuleInitializer]
+    internal static void M(params object[] p) { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8815: Module initializer method 'M' must be static, must have no parameters, and must return 'void'
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid, "ModuleInitializer").WithArguments("M").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void MustNotReturnAValue()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+static class C
+{
+    [ModuleInitializer]
+    internal static object M() => null;
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                    // (6,6): error CS8815: Module initializer method 'M' must be static, must have no parameters, and must return 'void'
+                    //     [ModuleInitializer]
+                    Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid, "ModuleInitializer").WithArguments("M").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void MayBeAsyncVoid()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+static class C
+{
+    [ModuleInitializer]
+    internal static async void M() => Console.WriteLine(""C.M"");
+}
+
+class Program 
+{
+    static void Main() => Console.WriteLine(""Program.Main"");
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            CompileAndVerify(source, parseOptions: s_parseOptions, expectedOutput: @"
+C.M
+Program.Main");
+        }
+
+        [Fact]
+        public void MayNotReturnAwaitableWithVoidResult()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+using System.Threading.Tasks;
+
+static class C
+{
+    [ModuleInitializer]
+    internal static async Task M() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8815: Module initializer method 'M' must be static, must have no parameters, and must return 'void'
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeStaticParameterlessVoid, "ModuleInitializer").WithArguments("M").WithLocation(7, 6),
+                // (8,32): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
+                //     internal static async Task M() { }
+                Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "M").WithLocation(8, 32));
+        }
+    }
+}

src/Compilers/CSharp/Test/Symbol/Symbols/ModuleInitializers/TargetsTests.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
+using Microsoft.CodeAnalysis.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.CSharp.UnitTests.Symbols.ModuleInitializers
+{
+    [CompilerTrait(CompilerFeature.ModuleInitializers)]
+    public sealed class TargetsTests : CSharpTestBase
+    {
+        private static readonly CSharpParseOptions s_parseOptions = TestOptions.RegularPreview;
+
+        [Fact]
+        public void TargetMustNotBeLocalFunction()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    internal static void M()
+    {
+        LocalFunction();
+        [ModuleInitializer]
+        static void LocalFunction() { }
+    }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (9,10): error CS8795: A module initializer must be an ordinary member method
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, "ModuleInitializer").WithLocation(9, 10)
+                );
+        }
+
+        [Fact]
+        public void IgnoredOnLocalFunctionWithBadAttributeTargets()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+class C
+{
+    internal static void M()
+    {
+        LocalFunction();
+        [ModuleInitializer]
+        static void LocalFunction() { }
+    }
+}
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Class)]
+    class ModuleInitializerAttribute : System.Attribute { }
+}
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (10,10): error CS0592: Attribute 'ModuleInitializer' is not valid on this declaration type. It is only valid on 'class' declarations.
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "ModuleInitializer").WithArguments("ModuleInitializer", "class").WithLocation(10, 10)
+                );
+        }
+
+        [Fact]
+        public void TargetMustNotBeDestructor()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    ~C() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8795: A module initializer must be an ordinary member method
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, "ModuleInitializer").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void IgnoredOnDestructorWithBadAttributeTargets()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    ~C() { }
+}
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Class)]
+    class ModuleInitializerAttribute : System.Attribute { }
+}
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (7,6): error CS0592: Attribute 'ModuleInitializer' is not valid on this declaration type. It is only valid on 'class' declarations.
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "ModuleInitializer").WithArguments("ModuleInitializer", "class").WithLocation(7, 6)
+                );
+        }
+
+        [Fact]
+        public void TargetMustNotBeOperator()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    public static C operator -(C p) => p;
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8795: A module initializer must be an ordinary member method
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, "ModuleInitializer").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void IgnoredOnOperatorWithBadAttributeTargets()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    public static C operator -(C p) => p;
+}
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Class)]
+    class ModuleInitializerAttribute : System.Attribute { }
+}
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (7,6): error CS0592: Attribute 'ModuleInitializer' is not valid on this declaration type. It is only valid on 'class' declarations.
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "ModuleInitializer").WithArguments("ModuleInitializer", "class").WithLocation(7, 6)
+                );
+        }
+
+        [Fact]
+        public void TargetMustNotBeConversionOperator()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    public static explicit operator int(C p) => default;
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8795: A module initializer must be an ordinary member method
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, "ModuleInitializer").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void IgnoredOnConversionOperatorWithBadAttributeTargets()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    public static explicit operator int(C p) => default;
+}
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Class)]
+    class ModuleInitializerAttribute : System.Attribute { }
+}
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (7,6): error CS0592: Attribute 'ModuleInitializer' is not valid on this declaration type. It is only valid on 'class' declarations.
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "ModuleInitializer").WithArguments("ModuleInitializer", "class").WithLocation(7, 6)
+                );
+        }
+
+        [Fact]
+        public void TargetMustNotBeEventAccessor()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    public event System.Action E
+    {
+        [ModuleInitializer]
+        add { }
+        [ModuleInitializer]
+        remove { }
+    }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (8,10): error CS8795: A module initializer must be an ordinary member method
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, "ModuleInitializer").WithLocation(8, 10),
+                // (10,10): error CS8795: A module initializer must be an ordinary member method
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, "ModuleInitializer").WithLocation(10, 10)
+                );
+        }
+
+        [Fact]
+        public void IgnoredOnEventAccessorsWithBadAttributeTargets()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+class C
+{
+    public event System.Action E
+    {
+        [ModuleInitializer]
+        add { }
+        [ModuleInitializer]
+        remove { }
+    }
+}
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Class)]
+    class ModuleInitializerAttribute : System.Attribute { }
+}
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (9,10): error CS0592: Attribute 'ModuleInitializer' is not valid on this declaration type. It is only valid on 'class' declarations.
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "ModuleInitializer").WithArguments("ModuleInitializer", "class").WithLocation(9, 10),
+                // (11,10): error CS0592: Attribute 'ModuleInitializer' is not valid on this declaration type. It is only valid on 'class' declarations.
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "ModuleInitializer").WithArguments("ModuleInitializer", "class").WithLocation(11, 10)
+                );
+        }
+
+        [Fact]
+        public void TargetMustNotBePropertyAccessor()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    public int P 
+    {
+        [ModuleInitializer]
+        get;
+        [ModuleInitializer]
+        set;
+    }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (8,10): error CS8795: A module initializer must be an ordinary member method
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, "ModuleInitializer").WithLocation(8, 10),
+                // (10,10): error CS8795: A module initializer must be an ordinary member method
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, "ModuleInitializer").WithLocation(10, 10)
+                );
+        }
+
+        [Fact]
+        public void IgnoredOnPropertyAccessorsWithBadAttributeTargets()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+class C
+{
+    public int P 
+    {
+        [ModuleInitializer]
+        get;
+        [ModuleInitializer]
+        set;
+    }
+}
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Class)]
+    class ModuleInitializerAttribute : System.Attribute { }
+}
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (9,10): error CS0592: Attribute 'ModuleInitializer' is not valid on this declaration type. It is only valid on 'class' declarations.
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "ModuleInitializer").WithArguments("ModuleInitializer", "class").WithLocation(9, 10),
+                // (11,10): error CS0592: Attribute 'ModuleInitializer' is not valid on this declaration type. It is only valid on 'class' declarations.
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "ModuleInitializer").WithArguments("ModuleInitializer", "class").WithLocation(11, 10)
+                );
+        }
+
+        [Fact]
+        public void TargetMustNotBeIndexerAccessor()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    public int this[int p]
+    {
+        [ModuleInitializer]
+        get => p;
+        [ModuleInitializer]
+        set { }
+    }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (8,10): error CS8795: A module initializer must be an ordinary member method
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, "ModuleInitializer").WithLocation(8, 10),
+                // (10,10): error CS8795: A module initializer must be an ordinary member method
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, "ModuleInitializer").WithLocation(10, 10)
+                );
+        }
+
+        [Fact]
+        public void IgnoredOnIndexerAccessorsWithBadAttributeTargets()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+class C
+{
+    public int this[int p]
+    {
+        [ModuleInitializer]
+        get => p;
+        [ModuleInitializer]
+        set { }
+    }
+}
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Class)]
+    class ModuleInitializerAttribute : System.Attribute { }
+}
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (9,10): error CS0592: Attribute 'ModuleInitializer' is not valid on this declaration type. It is only valid on 'class' declarations.
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "ModuleInitializer").WithArguments("ModuleInitializer", "class").WithLocation(9, 10),
+                // (11,10): error CS0592: Attribute 'ModuleInitializer' is not valid on this declaration type. It is only valid on 'class' declarations.
+                //         [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "ModuleInitializer").WithArguments("ModuleInitializer", "class").WithLocation(11, 10)
+                );
+        }
+
+        [Fact]
+        public void TargetMustNotBeStaticConstructor()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    static C() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8795: A module initializer must be an ordinary member method
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, "ModuleInitializer").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void IgnoredOnStaticConstructorWithBadAttributeTargets()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    static C() { }
+}
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Method)]
+    class ModuleInitializerAttribute : System.Attribute { }
+}
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (7,6): error CS0592: Attribute 'ModuleInitializer' is not valid on this declaration type. It is only valid on 'method' declarations.
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "ModuleInitializer").WithArguments("ModuleInitializer", "method").WithLocation(7, 6)
+                );
+        }
+
+        [Fact]
+        public void TargetMustNotBeInstanceConstructor()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    public C() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (6,6): error CS8795: A module initializer must be an ordinary member method
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_ModuleInitializerMethodMustBeOrdinary, "ModuleInitializer").WithLocation(6, 6)
+                );
+        }
+
+        [Fact]
+        public void IgnoredOnInstanceConstructorWithBadAttributeTargets()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    public C() { }
+}
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Method)]
+    class ModuleInitializerAttribute : System.Attribute { }
+}
+";
+            var compilation = CreateCompilation(source, parseOptions: s_parseOptions);
+            compilation.VerifyEmitDiagnostics(
+                // (7,6): error CS0592: Attribute 'ModuleInitializer' is not valid on this declaration type. It is only valid on 'method' declarations.
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "ModuleInitializer").WithArguments("ModuleInitializer", "method").WithLocation(7, 6)
+                );
+        }
+    }
+}

src/Compilers/CSharp/Test/Syntax/Diagnostics/DiagnosticTest.cs

@@ -31,12 +31,6 @@ public void Resources()
                 ErrorCode.Void,
                 ErrorCode.Unknown,
                 ErrorCode.WRN_ALinkWarn, // Not reported, but retained to allow configuring class of related warnings. See CSharpDiagnosticFilter.Filter.
-
-                // The following error codes are reserved by feature branches
-                ErrorCode.ERR_8813,
-                ErrorCode.ERR_8814,
-                ErrorCode.ERR_8815,
-                ErrorCode.ERR_8816,
             };
             foreach (ErrorCode code in Enum.GetValues(typeof(ErrorCode)))
             {

src/Compilers/Core/Portable/Emit/CommonPEModuleBuilder.cs

@@ -469,8 +469,6 @@ internal abstract class PEModuleBuilder<TCompilation, TSourceModuleSymbol, TAsse
         where TEmbeddedTypesManager : CommonEmbeddedTypesManager
         where TModuleCompilationState : ModuleCompilationState<TNamedTypeSymbol, TMethodSymbol>
     {
-        private readonly Cci.RootModuleType _rootModuleType = new Cci.RootModuleType();
-
         internal readonly TSourceModuleSymbol SourceModule;
         internal readonly TCompilation Compilation;
 
@@ -479,6 +477,7 @@ internal abstract class PEModuleBuilder<TCompilation, TSourceModuleSymbol, TAsse
         private HashSet<string> _namesOfTopLevelTypes;
 
         internal readonly TModuleCompilationState CompilationState;
+        public Cci.RootModuleType RootModuleType { get; } = new Cci.RootModuleType();
 
         public abstract TEmbeddedTypesManager EmbeddedTypesManagerOpt { get; }
 
@@ -562,9 +561,9 @@ public override IEnumerable<Cci.INamespaceTypeDefinition> GetTopLevelTypeDefinit
                 Dispatch(typeReferenceIndexer);
             }
 
-            AddTopLevelType(names, _rootModuleType);
-            VisitTopLevelType(typeReferenceIndexer, _rootModuleType);
-            yield return _rootModuleType;
+            AddTopLevelType(names, RootModuleType);
+            VisitTopLevelType(typeReferenceIndexer, RootModuleType);
+            yield return RootModuleType;
 
             foreach (var typeDef in GetAnonymousTypeDefinitions(context))
             {

src/Compilers/Core/Portable/InternalUtilities/ConcurrentSet.cs

@@ -4,7 +4,6 @@
 
 #nullable enable
 
-using System;
 using System.Collections;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
@@ -178,7 +177,13 @@ void ICollection<T>.Add(T item)
 
         public void CopyTo(T[] array, int arrayIndex)
         {
-            throw new NotImplementedException();
+            // PERF: Do not use dictionary.Keys here because that creates a snapshot
+            // of the collection resulting in a List<T> allocation.
+            // Instead, enumerate the set and copy over the elements.
+            foreach (var element in this)
+            {
+                array[arrayIndex++] = element;
+            }
         }
     }
 }

src/Compilers/Core/Portable/PEWriter/RootModuleStaticConstructor.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// 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.Collections.Generic;
+using System.Collections.Immutable;
+using System.Reflection;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CodeGen;
+using Microsoft.CodeAnalysis.Debugging;
+using Microsoft.CodeAnalysis.Emit;
+using Roslyn.Utilities;
+
+namespace Microsoft.Cci
+{
+    internal sealed partial class RootModuleStaticConstructor : IMethodDefinition, IMethodBody
+    {
+        public RootModuleStaticConstructor(ITypeDefinition containingTypeDefinition, ImmutableArray<byte> il)
+        {
+            ContainingTypeDefinition = containingTypeDefinition;
+            IL = il;
+        }
+
+        // IMethodDefinition implementation
+
+        public ITypeDefinition ContainingTypeDefinition { get; }
+
+        public string Name => WellKnownMemberNames.StaticConstructorName;
+
+        public IMethodBody GetBody(EmitContext context) => this;
+
+        public IEnumerable<IGenericMethodParameter> GenericParameters => SpecializedCollections.EmptyEnumerable<IGenericMethodParameter>();
+
+        public bool IsImplicitlyDeclared => true;
+
+        public bool HasDeclarativeSecurity => false;
+
+        public bool IsAbstract => false;
+
+        public bool IsAccessCheckedOnOverride => false;
+
+        public bool IsConstructor => false;
+
+        public bool IsExternal => false;
+
+        public bool IsHiddenBySignature => true;
+
+        public bool IsNewSlot => false;
+
+        public bool IsPlatformInvoke => false;
+
+        public bool IsRuntimeSpecial => true;
+
+        public bool IsSealed => false;
+
+        public bool IsSpecialName => true;
+
+        public bool IsStatic => true;
+
+        public bool IsVirtual => false;
+
+        public ImmutableArray<IParameterDefinition> Parameters => ImmutableArray<IParameterDefinition>.Empty;
+
+        public IPlatformInvokeInformation PlatformInvokeData => null;
+
+        public bool RequiresSecurityObject => false;
+
+        public bool ReturnValueIsMarshalledExplicitly => false;
+
+        public IMarshallingInformation ReturnValueMarshallingInformation => null;
+
+        public ImmutableArray<byte> ReturnValueMarshallingDescriptor => default;
+
+        public IEnumerable<SecurityAttribute> SecurityAttributes => null;
+
+        public INamespace ContainingNamespace => null;
+
+        public TypeMemberVisibility Visibility => TypeMemberVisibility.Private;
+
+        public bool AcceptsExtraArguments => false;
+
+        public ushort GenericParameterCount => 0;
+
+        public bool IsGeneric => false;
+
+        public ImmutableArray<IParameterTypeInformation> ExtraParameters => ImmutableArray<IParameterTypeInformation>.Empty;
+
+        public IGenericMethodInstanceReference AsGenericMethodInstanceReference => null;
+
+        public ISpecializedMethodReference AsSpecializedMethodReference => null;
+
+        public CallingConvention CallingConvention => CallingConvention.Default;
+
+        public ushort ParameterCount => 0;
+
+        public ImmutableArray<ICustomModifier> ReturnValueCustomModifiers => ImmutableArray<ICustomModifier>.Empty;
+
+        public ImmutableArray<ICustomModifier> RefCustomModifiers => ImmutableArray<ICustomModifier>.Empty;
+
+        public bool ReturnValueIsByRef => false;
+
+        public IDefinition AsDefinition(EmitContext context) => this;
+
+        public void Dispatch(MetadataVisitor visitor) => visitor.Visit((IMethodDefinition)this);
+
+        public IEnumerable<ICustomAttribute> GetAttributes(EmitContext context) => SpecializedCollections.EmptyEnumerable<ICustomAttribute>();
+
+        public ITypeReference GetContainingType(EmitContext context) => ContainingTypeDefinition;
+
+        public MethodImplAttributes GetImplementationAttributes(EmitContext context) => default;
+
+        public ImmutableArray<IParameterTypeInformation> GetParameters(EmitContext context) => ImmutableArray<IParameterTypeInformation>.Empty;
+
+        public IMethodDefinition GetResolvedMethod(EmitContext context) => this;
+
+        public IEnumerable<ICustomAttribute> GetReturnValueAttributes(EmitContext context) => SpecializedCollections.EmptyEnumerable<ICustomAttribute>();
+
+        public ITypeReference GetType(EmitContext context) => context.Module.GetPlatformType(PlatformType.SystemVoid, context);
+
+        // IMethodBody implementation
+
+        public ushort MaxStack => 0;
+
+        public ImmutableArray<byte> IL { get; }
+
+        public IMethodDefinition MethodDefinition => this;
+
+        public ImmutableArray<ExceptionHandlerRegion> ExceptionRegions => ImmutableArray<ExceptionHandlerRegion>.Empty;
+
+        public bool AreLocalsZeroed => false;
+
+        public bool HasStackalloc => false;
+
+        public ImmutableArray<ILocalDefinition> LocalVariables => ImmutableArray<ILocalDefinition>.Empty;
+
+        public StateMachineMoveNextBodyDebugInfo MoveNextBodyInfo => null;
+
+        public ImmutableArray<SequencePoint> SequencePoints => ImmutableArray<SequencePoint>.Empty;
+
+        public bool HasDynamicLocalVariables => false;
+
+        public ImmutableArray<LocalScope> LocalScopes => ImmutableArray<LocalScope>.Empty;
+
+        public IImportScope ImportScope => null;
+
+        public DebugId MethodId => default;
+
+        public ImmutableArray<StateMachineHoistedLocalScope> StateMachineHoistedLocalScopes => ImmutableArray<StateMachineHoistedLocalScope>.Empty;
+
+        public string StateMachineTypeName => null;
+
+        public ImmutableArray<EncHoistedLocalInfo> StateMachineHoistedLocalSlots => ImmutableArray<EncHoistedLocalInfo>.Empty;
+
+        public ImmutableArray<ITypeReference> StateMachineAwaiterSlots => ImmutableArray<ITypeReference>.Empty;
+
+        public ImmutableArray<ClosureDebugInfo> ClosureDebugInfo => ImmutableArray<ClosureDebugInfo>.Empty;
+
+        public ImmutableArray<LambdaDebugInfo> LambdaDebugInfo => ImmutableArray<LambdaDebugInfo>.Empty;
+
+        public DynamicAnalysisMethodBodyData DynamicAnalysisData => null;
+    }
+}

src/Compilers/Core/Portable/PEWriter/RootModuleType.cs

@@ -4,10 +4,11 @@
 
 #nullable enable
 
-using System;
 using System.Collections.Generic;
-using System.Runtime.InteropServices;
+using System.Collections.Immutable;
+using System.Diagnostics;
 using System.Reflection.Metadata;
+using System.Runtime.InteropServices;
 using Roslyn.Utilities;
 using EmitContext = Microsoft.CodeAnalysis.Emit.EmitContext;
 
@@ -18,6 +19,21 @@ namespace Microsoft.Cci
     /// </summary>
     internal class RootModuleType : INamespaceTypeDefinition
     {
+        private IReadOnlyList<IMethodDefinition>? _methods;
+
+        public void SetStaticConstructorBody(ImmutableArray<byte> il)
+        {
+            Debug.Assert(_methods is null);
+
+            _methods = SpecializedCollections.SingletonReadOnlyList(
+                new RootModuleStaticConstructor(containingTypeDefinition: this, il));
+        }
+
+        public IEnumerable<IMethodDefinition> GetMethods(EmitContext context)
+        {
+            return _methods ??= SpecializedCollections.EmptyReadOnlyList<IMethodDefinition>();
+        }
+
         public TypeDefinitionHandle TypeDef
         {
             get { return default(TypeDefinitionHandle); }
@@ -138,11 +154,6 @@ public LayoutKind Layout
             get { return LayoutKind.Auto; }
         }
 
-        public IEnumerable<IMethodDefinition> GetMethods(EmitContext context)
-        {
-            return SpecializedCollections.EmptyEnumerable<IMethodDefinition>();
-        }
-
         public IEnumerable<INestedTypeDefinition> GetNestedTypes(EmitContext context)
         {
             return SpecializedCollections.EmptyEnumerable<INestedTypeDefinition>();

src/Compilers/Core/Portable/Symbols/Attributes/AttributeDescription.cs

@@ -428,6 +428,8 @@ static AttributeDescription()
         private static readonly byte[][] s_signaturesOfEnumeratorCancellationAttribute = { s_signature_HasThis_Void };
         private static readonly byte[][] s_signaturesOfNativeIntegerAttribute = { s_signature_HasThis_Void, s_signature_HasThis_Void_SzArray_Boolean };
 
+        private static readonly byte[][] s_signaturesOfModuleInitializerAttribute = { s_signature_HasThis_Void };
+
         // early decoded attributes:
         internal static readonly AttributeDescription OptionalAttribute = new AttributeDescription("System.Runtime.InteropServices", "OptionalAttribute", s_signaturesOfOptionalAttribute);
         internal static readonly AttributeDescription ComImportAttribute = new AttributeDescription("System.Runtime.InteropServices", "ComImportAttribute", s_signaturesOfComImportAttribute);
@@ -559,5 +561,6 @@ static AttributeDescription()
         internal static readonly AttributeDescription EnumeratorCancellationAttribute = new AttributeDescription("System.Runtime.CompilerServices", "EnumeratorCancellationAttribute", s_signaturesOfEnumeratorCancellationAttribute);
         internal static readonly AttributeDescription SkipLocalsInitAttribute = new AttributeDescription("System.Runtime.CompilerServices", "SkipLocalsInitAttribute", s_signaturesOfSkipLocalsInitAttribute);
         internal static readonly AttributeDescription NativeIntegerAttribute = new AttributeDescription("System.Runtime.CompilerServices", "NativeIntegerAttribute", s_signaturesOfNativeIntegerAttribute);
+        internal static readonly AttributeDescription ModuleInitializerAttribute = new AttributeDescription("System.Runtime.CompilerServices", "ModuleInitializerAttribute", s_signaturesOfModuleInitializerAttribute);
     }
 }

src/Compilers/Server/VBCSCompiler/MetadataCache.cs

@@ -15,8 +15,8 @@ namespace Microsoft.CodeAnalysis.CompilerServer
 {
     internal class MetadataAndSymbolCache
     {
-        // Store 100 entries -- arbitrary number
-        private const int CacheSize = 100;
+        // Store 500 entries -- Out of ~8.7M projects, only about 4,000 had more than 500 references
+        private const int CacheSize = 500;
         private readonly ConcurrentLruCache<FileKey, Metadata> _metadataCache =
             new ConcurrentLruCache<FileKey, Metadata>(CacheSize);
 

src/Compilers/VisualBasic/Portable/Symbols/Source/SourceMethodSymbol.vb

@@ -1721,6 +1721,8 @@ lReportErrorOnTwoTokens:
                 End If
             ElseIf VerifyObsoleteAttributeAppliedToMethod(arguments, AttributeDescription.ObsoleteAttribute) Then
             ElseIf VerifyObsoleteAttributeAppliedToMethod(arguments, AttributeDescription.DeprecatedAttribute) Then
+            ElseIf arguments.Attribute.IsTargetAttribute(Me, AttributeDescription.ModuleInitializerAttribute) Then
+                arguments.Diagnostics.Add(ERRID.WRN_AttributeNotSupportedInVB, arguments.AttributeSyntaxOpt.Location, AttributeDescription.ModuleInitializerAttribute.FullName)
             Else
                 Dim methodImpl As MethodSymbol = If(Me.IsPartial, PartialImplementationPart, Me)
 

src/Compilers/VisualBasic/Test/Emit/Attributes/AttributeTests_WellKnownAttributes.vb

@@ -5813,6 +5813,413 @@ BC30657: 'sc1_method' has a return type that is not supported or parameter types
 ]]></expected>)
         End Sub
 
+#End Region
+
+#Region "ModuleInitializerAttribute"
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnMethod()
+            Dim source =
+            <compilation>
+                <file name="attr.vb"><![CDATA[
+Namespace System.Runtime.CompilerServices
+    Public Class ModuleInitializerAttribute
+        Inherits Attribute
+    End Class
+End Namespace
+
+Class Program
+    <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+    Sub S()
+    End Sub
+
+    <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+    Function F() As Integer
+        Return 1
+    End Function
+End Class
+]]>
+                </file>
+            </compilation>
+
+            Dim compilation = CreateCompilationWithMscorlib40(source)
+            CompilationUtils.AssertTheseDiagnostics(compilation,
+<expected><![CDATA[
+BC42381: 'System.Runtime.CompilerServices.ModuleInitializerAttribute' is not supported in VB.
+    <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC42381: 'System.Runtime.CompilerServices.ModuleInitializerAttribute' is not supported in VB.
+    <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+]]></expected>)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnClass()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+
+<System.Runtime.CompilerServices.ModuleInitializerAttribute>
+Class C
+End Class
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnProperty()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+
+Class C
+    <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+    Property P As Integer
+        Get
+            Return 1
+        End Get
+
+        Set
+        End Set
+    End Property
+End Class
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnAccessors()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+
+Class C
+    Property P As Integer
+        <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+        Get
+            Return 1
+        End Get
+
+        <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+        Set
+        End Set
+    End Property
+End Class
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertTheseDiagnostics(comp,
+<expected><![CDATA[
+BC42381: 'System.Runtime.CompilerServices.ModuleInitializerAttribute' is not supported in VB.
+        <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC42381: 'System.Runtime.CompilerServices.ModuleInitializerAttribute' is not supported in VB.
+        <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+]]></expected>)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnModule()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+<Module: System.Runtime.CompilerServices.ModuleInitializerAttribute>
+
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnAssembly()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+<Assembly: System.Runtime.CompilerServices.ModuleInitializerAttribute>
+
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnEnum()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+
+<System.Runtime.CompilerServices.ModuleInitializerAttribute>
+Enum E
+    Member
+End Enum
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnEnumMember()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+
+Enum E
+    <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+    Member1
+    <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+    Member2
+End Enum
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnEvent()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+
+Class C
+    <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+    Event E(ByVal i As Integer)
+End Class
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnDelegate()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+
+Class C
+    <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+    Delegate Sub D()
+End Class
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnInterface()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+
+<System.Runtime.CompilerServices.ModuleInitializerAttribute>
+Interface I
+End Interface
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnStructure()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+
+<System.Runtime.CompilerServices.ModuleInitializerAttribute>
+Structure S
+End Structure
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnReturnValue()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+
+Class C
+    Function F() As <System.Runtime.CompilerServices.ModuleInitializerAttribute> Integer
+        Return 1
+    End Function
+End Class
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnParameter()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+
+Class C
+    Sub M(<System.Runtime.CompilerServices.ModuleInitializerAttribute> ByVal i As Integer)
+    End Sub
+End Class
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
+        <Fact>
+        Public Sub ModuleInitializerAttributeOnField()
+            Dim source = <compilation>
+                             <file name="a.vb">
+                                 <![CDATA[
+Namespace System.Runtime.CompilerServices
+    Class ModuleInitializerAttribute
+        Inherits System.Attribute
+    End Class
+End Namespace
+
+Class C
+    <System.Runtime.CompilerServices.ModuleInitializerAttribute>
+    Dim i As Integer
+End Class
+]]>
+
+                             </file>
+                         </compilation>
+
+            Dim comp = CreateCompilationWithMscorlib40(source)
+
+            CompilationUtils.AssertNoDiagnostics(comp)
+        End Sub
+
 #End Region
 
         <Fact, WorkItem(807, "https://github.com/dotnet/roslyn/issues/807")>

src/Test/Utilities/Portable/Traits/CompilerFeature.cs

@@ -38,5 +38,6 @@ public enum CompilerFeature
         TopLevelStatements,
         InitOnlySetters,
         AnonymousFunctions,
+        ModuleInitializers,
     }
 }