Avoid using file in generated regexes pre-C# 11 (#81726)

Author: stephentoub

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

@@ -72,12 +72,22 @@ private static void EmitRegexPartialMethod(RegexMethod regexMethod, IndentedText
 
         /// <summary>Emits the Regex-derived type for a method where we're unable to generate custom code.</summary>
         private static void EmitRegexLimitedBoilerplate(
-            IndentedTextWriter writer, RegexMethod rm, string reason)
+            IndentedTextWriter writer, RegexMethod rm, string reason, LanguageVersion langVer)
         {
-            writer.WriteLine($"/// <summary>Caches a <see cref=\"Regex\"/> instance for the {rm.MethodName} method.</summary>");
+            string visibility;
+            if (langVer >= LanguageVersion.CSharp11)
+            {
+                visibility = "file";
+                writer.WriteLine($"/// <summary>Caches a <see cref=\"Regex\"/> instance for the {rm.MethodName} method.</summary>");
+            }
+            else
+            {
+                visibility = "internal";
+                writer.WriteLine($"/// <summary>This class supports generated regexes and should not be used by other code directly.</summary>");
+            }
             writer.WriteLine($"/// <remarks>A custom Regex-derived type could not be generated because {reason}.</remarks>");
             writer.WriteLine($"[{s_generatedCodeAttribute}]");
-            writer.WriteLine($"file sealed class {rm.GeneratedName} : Regex");
+            writer.WriteLine($"{visibility} sealed class {rm.GeneratedName} : Regex");
             writer.WriteLine($"{{");
             writer.WriteLine($"    /// <summary>Cached, thread-safe singleton instance.</summary>");
             writer.Write($"    internal static readonly Regex Instance = ");

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

@@ -80,7 +80,7 @@ x.Options is CSharpCompilationOptions options ?
                     // We'll still output a limited implementation that just caches a new Regex(...).
                     if (!SupportsCodeGeneration(regexMethod, methodOrDiagnosticAndCompilationData.Right.LanguageVersion, out string? reason))
                     {
-                        return (regexMethod, reason, Diagnostic.Create(DiagnosticDescriptors.LimitedSourceGeneration, regexMethod.MethodSyntax.GetLocation()));
+                        return (regexMethod, reason, Diagnostic.Create(DiagnosticDescriptors.LimitedSourceGeneration, regexMethod.MethodSyntax.GetLocation()), methodOrDiagnosticAndCompilationData.Right);
                     }
 
                     // Generate the core logic for the regex.
@@ -150,7 +150,7 @@ x.Options is CSharpCompilationOptions options ?
                 foreach (object? result in results)
                 {
                     RegexMethod? regexMethod = null;
-                    if (result is ValueTuple<RegexMethod, string, Diagnostic> limitedSupportResult)
+                    if (result is ValueTuple<RegexMethod, string, Diagnostic, CompilationData> limitedSupportResult)
                     {
                         context.ReportDiagnostic(limitedSupportResult.Item3);
                         regexMethod = limitedSupportResult.Item1;
@@ -212,11 +212,11 @@ x.Options is CSharpCompilationOptions options ?
                 writer.Indent++;
                 foreach (object? result in results)
                 {
-                    if (result is ValueTuple<RegexMethod, string, Diagnostic> limitedSupportResult)
+                    if (result is ValueTuple<RegexMethod, string, Diagnostic, CompilationData> limitedSupportResult)
                     {
                         if (!limitedSupportResult.Item1.IsDuplicate)
                         {
-                            EmitRegexLimitedBoilerplate(writer, limitedSupportResult.Item1, limitedSupportResult.Item2);
+                            EmitRegexLimitedBoilerplate(writer, limitedSupportResult.Item1, limitedSupportResult.Item2, limitedSupportResult.Item4.LanguageVersion);
                             writer.WriteLine();
                         }
                     }
@@ -273,7 +273,7 @@ x.Options is CSharpCompilationOptions options ?
         /// </remarks>
         private static bool SupportsCodeGeneration(RegexMethod method, LanguageVersion languageVersion, [NotNullWhen(false)] out string? reason)
         {
-            if (languageVersion <= LanguageVersion.CSharp10)
+            if (languageVersion < LanguageVersion.CSharp11)
             {
                 reason = "the language version must be C# 11 or higher.";
                 return false;

src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/RegexGeneratorHelper.netcoreapp.cs

@@ -103,7 +103,7 @@ internal static async Task<IReadOnlyList<Diagnostic>> RunGenerator(
             EmitResult results = comp.Emit(Stream.Null, cancellationToken: cancellationToken);
             ImmutableArray<Diagnostic> generatorDiagnostics = generatorResults.Diagnostics.RemoveAll(d => d.Severity <= DiagnosticSeverity.Hidden);
             ImmutableArray<Diagnostic> resultsDiagnostics = results.Diagnostics.RemoveAll(d => d.Severity <= DiagnosticSeverity.Hidden);
-            if (!results.Success || resultsDiagnostics.Length != 0 || generatorDiagnostics.Length != 0)
+            if (!results.Success || resultsDiagnostics.Length != 0)
             {
                 throw new ArgumentException(
                     string.Join(Environment.NewLine, resultsDiagnostics.Concat(generatorDiagnostics)) + Environment.NewLine +

src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/RegexGeneratorParserTests.cs

@@ -252,7 +252,7 @@ partial class C
                     [GeneratedRegex(""ab"")]
                     private static partial Regex InvalidLangVersion();
                 }
-            ", langVersion: version);
+            ", langVersion: version, compile: true);
 
             Assert.Equal("SYSLIB1044", Assert.Single(diagnostics).Id);
         }