Codegen for module initializers feature

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6070,4 +6070,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_GeneratorFailedDuringInitialization_Title" xml:space="preserve">
     <value>Generator failed to initialize.</value>
   </data>
+  <data name="IDS_FeatureModuleInitializers" xml:space="preserve">
+    <value>module initializers</value>
+  </data>
 </root>

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

@@ -16,6 +16,7 @@
 using System.Reflection.Metadata;
 using System.Threading;
 using System.Threading.Tasks;
+using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CodeGen;
 using Microsoft.CodeAnalysis.CSharp.Emit;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
@@ -2015,6 +2016,14 @@ private protected override bool IsSymbolAccessibleWithinCore(
             throw new NotImplementedException();
         }
 
+        private ConcurrentSet<MethodSymbol>? _moduleInitializerMethods;
+
+        internal void AddModuleInitializerMethod(MethodSymbol method)
+        {
+            Debug.Assert(!_declarationDiagnosticsFrozen);
+            LazyInitializer.EnsureInitialized(ref _moduleInitializerMethods).Add(method);
+        }
+
         #endregion
 
         #region Binding
@@ -2802,6 +2811,8 @@ internal override bool CompileMethods(
                     filterOpt: filterOpt,
                     cancellationToken: cancellationToken);
 
+                GenerateModuleInitializer(moduleBeingBuilt, methodBodyDiagnosticBag);
+
                 bool hasMethodBodyError = !FilterAndAppendAndFreeDiagnostics(diagnostics, ref methodBodyDiagnosticBag);
 
                 if (hasDeclarationErrors || hasMethodBodyError)
@@ -2813,6 +2824,31 @@ internal override bool CompileMethods(
             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);
+
+                // PROTOTYPE(module-initializers): require deterministic order
+                foreach (var method in _moduleInitializerMethods)
+                {
+                    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/Errors/MessageID.cs

@@ -192,6 +192,7 @@ internal enum MessageID
         IDS_FeatureMemberNotNull = MessageBase + 12768,
         IDS_FeatureNativeInt = MessageBase + 12769,
         IDS_FeatureTargetTypedObjectCreation = MessageBase + 12770,
+        IDS_FeatureModuleInitializers = MessageBase + 12771,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -305,6 +306,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureTargetTypedObjectCreation: // syntax check
                 case MessageID.IDS_FeatureMemberNotNull:
                 case MessageID.IDS_FeatureNativeInt:
+                case MessageID.IDS_FeatureModuleInitializers: // semantic check on method attribute
                     return LanguageVersion.Preview;
 
                 // C# 8.0 features.

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

@@ -482,6 +482,12 @@ 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);
+                // PROTOTYPE(module-initializers): diagnostics
+                DeclaringCompilation.AddModuleInitializerMethod(this);
+            }
             else
             {
                 var compilation = this.DeclaringCompilation;

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

@@ -944,6 +944,11 @@
         <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

@@ -944,6 +944,11 @@
         <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

@@ -944,6 +944,11 @@
         <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

@@ -944,6 +944,11 @@
         <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

@@ -944,6 +944,11 @@
         <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

@@ -944,6 +944,11 @@
         <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

@@ -944,6 +944,11 @@
         <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

@@ -944,6 +944,11 @@
         <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

@@ -942,6 +942,11 @@
         <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

@@ -944,6 +944,11 @@
         <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

@@ -944,6 +944,11 @@
         <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

@@ -944,6 +944,11 @@
         <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

@@ -944,6 +944,11 @@
         <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/ModuleInitializersTests.cs

@@ -0,0 +1,141 @@
+// 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.Reflection;
+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
+{
+    [CompilerTrait(CompilerFeature.ModuleInitializers)]
+    public sealed class ModuleInitializersTests : CSharpTestBase
+    {
+        private static readonly CSharpParseOptions s_parseOptions = TestOptions.RegularPreview;
+
+        [Fact]
+        public static void LastLanguageVersionNotSupportingModuleInitializersIs8()
+        {
+            var source =
+@"using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    internal static void M() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: TestOptions.Regular8);
+
+            compilation.VerifyDiagnostics(
+                // (5,6): error CS8652: The feature 'module initializers' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     [ModuleInitializer]
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "ModuleInitializer").WithArguments("module initializers").WithLocation(5, 6)
+                );
+        }
+
+        [Fact]
+        public static void FirstLanguageVersionSupportingModuleInitializersIs9()
+        {
+            var source =
+@"using System.Runtime.CompilerServices;
+
+class C
+{
+    [ModuleInitializer]
+    internal static void M() { }
+}
+
+namespace System.Runtime.CompilerServices { class ModuleInitializerAttribute : System.Attribute { } }
+";
+            var compilation = CreateCompilation(source, parseOptions: TestOptions.RegularPreview);
+
+            compilation.VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void ModuleTypeStaticConstructorIsNotEmittedWhenNoMethodIsMarkedWithModuleInitializerAttribute()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+class C
+{
+    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,
+                options: new CSharpCompilationOptions(OutputKind.ConsoleApplication, metadataImportOptions: MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    Assert.Null(rootModuleType.GetMember(".cctor"));
+                },
+                expectedOutput: @"
+Program.Main");
+        }
+
+        [Fact]
+        public void ModuleTypeStaticConstructorCallsMethodMarkedWithModuleInitializerAttribute()
+        {
+            string source = @"
+using System;
+using System.Runtime.CompilerServices;
+
+class C
+{
+    [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,
+                options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
+                symbolValidator: module =>
+                {
+                    var rootModuleType = (TypeSymbol)module.GlobalNamespace.GetMember("<Module>");
+                    var staticConstructor = (PEMethodSymbol)rootModuleType.GetMember(".cctor");
+
+                    Assert.NotNull(staticConstructor);
+                    Assert.Equal(MethodKind.StaticConstructor, staticConstructor.MethodKind);
+
+                    var expectedFlags =
+                        MethodAttributes.Private
+                        | MethodAttributes.Static
+                        | MethodAttributes.SpecialName
+                        | MethodAttributes.RTSpecialName
+                        | MethodAttributes.HideBySig;
+
+                    Assert.Equal(expectedFlags, staticConstructor.Flags);
+                },
+                expectedOutput: @"
+C.M
+Program.Main");
+        }
+    }
+}