Emit DebuggerStepThrough attribute on async Main stub (#34531)

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

@@ -5,7 +5,9 @@
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
+using Microsoft.CodeAnalysis.CSharp.Emit;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.PooledObjects;
 using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.CSharp.Symbols
@@ -345,6 +347,13 @@ internal sealed class AsyncForwardEntryPoint : SynthesizedEntryPointSymbol
                     ReturnType.SpecialType == SpecialType.System_Int32);
             }
 
+            internal override void AddSynthesizedAttributes(PEModuleBuilder moduleBuilder, ref ArrayBuilder<SynthesizedAttributeData> attributes)
+            {
+                base.AddSynthesizedAttributes(moduleBuilder, ref attributes);
+
+                AddSynthesizedAttribute(ref attributes, this.DeclaringCompilation.SynthesizeDebuggerStepThroughAttribute());
+            }
+
             public override string Name => MainName;
 
             public override ImmutableArray<ParameterSymbol> Parameters => _parameters;

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncMainTests.cs

@@ -4,6 +4,11 @@
 using Xunit;
 using System.Threading;
 using Microsoft.CodeAnalysis.Test.Utilities;
+using System.Reflection.PortableExecutable;
+using System.Reflection.Metadata;
+using System.Linq;
+using Roslyn.Test.Utilities;
+using System.Collections.Generic;
 
 namespace Microsoft.CodeAnalysis.CSharp.UnitTests.CodeGen
 {
@@ -707,7 +712,21 @@ async static Task Main()
             var entry = compilation.GetEntryPoint(CancellationToken.None);
             Assert.NotNull(entry);
             Assert.Equal("System.Threading.Tasks.Task A.Main()", entry.ToTestDisplayString());
+
+            var image = compilation.EmitToArray();
+            using var reader = new PEReader(image);
+            var metadataReader = reader.GetMetadataReader();
+            var main = metadataReader.MethodDefinitions.Where(mh => getMethodName(mh) == "<Main>").Single();
+            Assert.Equal(new[] { "MemberReference:Void System.Diagnostics.DebuggerStepThroughAttribute..ctor()" }, getMethodAttributes(main));
+
+            string getMethodName(MethodDefinitionHandle handle)
+                => metadataReader.GetString(metadataReader.GetMethodDefinition(handle).Name);
+
+            IEnumerable<string> getMethodAttributes(MethodDefinitionHandle handle)
+                => metadataReader.GetMethodDefinition(handle).GetCustomAttributes()
+                    .Select(a => metadataReader.Dump(metadataReader.GetCustomAttribute(a).Constructor));
         }
+
         [Fact]
         public void MainCanReturnTask_NoAsync()
         {