Mark Go override as SkipLocalsInit if possible (#63277)

It can have so many locals that zero-initing is measurable.

Author: stephentoub

src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs

@@ -41,7 +41,7 @@ public partial class RegexGenerator
         };
 
         /// <summary>Generates the code for one regular expression class.</summary>
-        private static (string, ImmutableArray<Diagnostic>) EmitRegexType(RegexType regexClass)
+        private static (string, ImmutableArray<Diagnostic>) EmitRegexType(RegexType regexClass, Compilation compilation)
         {
             var sb = new StringBuilder(1024);
             var writer = new IndentedTextWriter(new StringWriter(sb));
@@ -82,7 +82,7 @@ private static (string, ImmutableArray<Diagnostic>) EmitRegexType(RegexType rege
             generatedName += ComputeStringHash(generatedName).ToString("X");
 
             // Generate the regex type
-            ImmutableArray<Diagnostic> diagnostics = EmitRegexMethod(writer, regexClass.Method, generatedName);
+            ImmutableArray<Diagnostic> diagnostics = EmitRegexMethod(writer, regexClass.Method, generatedName, compilation);
 
             while (writer.Indent != 0)
             {
@@ -149,7 +149,7 @@ static bool ExceedsMaxDepthForSimpleCodeGeneration(RegexNode node, int allowedDe
         }
 
         /// <summary>Generates the code for a regular expression method.</summary>
-        private static ImmutableArray<Diagnostic> EmitRegexMethod(IndentedTextWriter writer, RegexMethod rm, string id)
+        private static ImmutableArray<Diagnostic> EmitRegexMethod(IndentedTextWriter writer, RegexMethod rm, string id, Compilation compilation)
         {
             string patternExpression = Literal(rm.Pattern);
             string optionsExpression = Literal(rm.Options);
@@ -174,6 +174,8 @@ private static ImmutableArray<Diagnostic> EmitRegexMethod(IndentedTextWriter wri
                 return ImmutableArray.Create(Diagnostic.Create(DiagnosticDescriptors.LimitedSourceGeneration, rm.MethodSyntax.GetLocation()));
             }
 
+            bool allowUnsafe = compilation.Options is CSharpCompilationOptions { AllowUnsafe: true };
+
             writer.WriteLine($"new {id}();");
             writer.WriteLine();
             writer.WriteLine($"    private {id}()");
@@ -231,6 +233,10 @@ private static ImmutableArray<Diagnostic> EmitRegexMethod(IndentedTextWriter wri
             writer.Indent -= 4;
             writer.WriteLine($"            }}");
             writer.WriteLine();
+            if (allowUnsafe)
+            {
+                writer.WriteLine($"            [global::System.Runtime.CompilerServices.SkipLocalsInit]");
+            }
             writer.WriteLine($"            protected override void Go()");
             writer.WriteLine($"            {{");
             writer.Indent += 4;

src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.cs

@@ -49,7 +49,7 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                 {
                     Debug.Assert(state.Item1 is not null);
                     object? result = GetRegexTypeToEmit(state.Item2, state.Item1, cancellationToken);
-                    return result is RegexType regexType ? EmitRegexType(regexType) : result;
+                    return result is RegexType regexType ? EmitRegexType(regexType, state.Item2) : result;
                 })
                 .Collect();
 

src/libraries/System.Text.RegularExpressions/tests/System.Text.RegularExpressions.Generators.Tests/RegexGeneratorParserTests.cs

@@ -297,8 +297,10 @@ partial class C
             ", compile: true));
         }
 
-        [Fact]
-        public async Task Valid_ClassWithNamespace()
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public async Task Valid_ClassWithNamespace(bool allowUnsafe)
         {
             Assert.Empty(await RunGenerator(@"
                 using System.Text.RegularExpressions;
@@ -310,7 +312,7 @@ partial class C
                         private static partial Regex Valid();
                     }
                 }
-            ", compile: true));
+            ", compile: true, allowUnsafe: allowUnsafe));
         }
 
         [Fact]
@@ -557,13 +559,13 @@ partial class C
         }
 
         private async Task<IReadOnlyList<Diagnostic>> RunGenerator(
-            string code, bool compile = false, LanguageVersion langVersion = LanguageVersion.Preview, MetadataReference[]? additionalRefs = null, CancellationToken cancellationToken = default)
+            string code, bool compile = false, LanguageVersion langVersion = LanguageVersion.Preview, MetadataReference[]? additionalRefs = null, bool allowUnsafe = false, CancellationToken cancellationToken = default)
         {
             var proj = new AdhocWorkspace()
                 .AddSolution(SolutionInfo.Create(SolutionId.CreateNewId(), VersionStamp.Create()))
                 .AddProject("RegexGeneratorTest", "RegexGeneratorTest.dll", "C#")
                 .WithMetadataReferences(additionalRefs is not null ? s_refs.Concat(additionalRefs) : s_refs)
-                .WithCompilationOptions(new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary)
+                .WithCompilationOptions(new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary, allowUnsafe: allowUnsafe)
                 .WithNullableContextOptions(NullableContextOptions.Enable))
                 .WithParseOptions(new CSharpParseOptions(langVersion))
                 .AddDocument("RegexGenerator.g.cs", SourceText.From(code, Encoding.UTF8)).Project;