Add support for C# 8 using variable declarations and add more C# 8 coverage.

- When adding additional C# 8 tests found that we didn't fully support this.
- Updated the C# 8.0 test to fully encompass everything C# 8.0.
- Added a feature flag to control using variable declaration errors when not top level.
- Added using variable declaration specific tests.

aspnet/AspNetCoredotnet/aspnetcore-tooling#5092
\n\nCommit migrated from ac08ad3659

src/Razor/Microsoft.AspNetCore.Razor.Language/src/Legacy/CSharpCodeParser.cs

@@ -2001,9 +2001,17 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
                 // using Identifier ==> Using Declaration
                 if (!topLevel)
                 {
-                    Context.ErrorSink.OnError(
+                    // using Variable Declaration
-                        RazorDiagnosticFactory.CreateParsing_NamespaceImportAndTypeAliasCannotExistWithinCodeBlock(
+
-                            new SourceSpan(block.Start, block.Name.Length)));
+                    if (!Context.FeatureFlags.AllowUsingVariableDeclarations)
+                    {
+                        Context.ErrorSink.OnError(
+                            RazorDiagnosticFactory.CreateParsing_NamespaceImportAndTypeAliasCannotExistWithinCodeBlock(
+                                new SourceSpan(block.Start, block.Name.Length)));
+                    }
+
+                    // There are cases when a user will do @using var x = 123; At which point we let C# notify the user
+                    // of their error like we do any other invalid expression.
                     if (transition != null)
                     {
                         builder.Add(transition);

src/Razor/Microsoft.AspNetCore.Razor.Language/src/RazorParserFeatureFlags.cs

@@ -11,6 +11,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             var allowHtmlCommentsInTagHelpers = false;
             var allowComponentFileKind = false;
             var allowRazorInAllCodeBlocks = false;
+            var allowUsingVariableDeclarations = false;
             var experimental_AllowConditionalDataDashAttributes = false;
 
             if (version.CompareTo(RazorLanguageVersion.Version_2_1) >= 0)
@@ -25,6 +26,7 @@ namespace Microsoft.AspNetCore.Razor.Language
                 // Added in 3.0
                 allowComponentFileKind = true;
                 allowRazorInAllCodeBlocks = true;
+                allowUsingVariableDeclarations = true;
             }
 
             if (version.CompareTo(RazorLanguageVersion.Experimental) >= 0)
@@ -37,6 +39,7 @@ namespace Microsoft.AspNetCore.Razor.Language
                 allowHtmlCommentsInTagHelpers,
                 allowComponentFileKind,
                 allowRazorInAllCodeBlocks,
+                allowUsingVariableDeclarations,
                 experimental_AllowConditionalDataDashAttributes);
         }
 
@@ -48,6 +51,8 @@ namespace Microsoft.AspNetCore.Razor.Language
 
         public abstract bool AllowRazorInAllCodeBlocks { get; }
 
+        public abstract bool AllowUsingVariableDeclarations { get; }
+
         public abstract bool EXPERIMENTAL_AllowConditionalDataDashAttributes { get; }
 
         private class DefaultRazorParserFeatureFlags : RazorParserFeatureFlags
@@ -57,12 +62,14 @@ namespace Microsoft.AspNetCore.Razor.Language
                 bool allowHtmlCommentsInTagHelpers,
                 bool allowComponentFileKind,
                 bool allowRazorInAllCodeBlocks,
+                bool allowUsingVariableDeclarations,
                 bool experimental_AllowConditionalDataDashAttributes)
             {
                 AllowMinimizedBooleanTagHelperAttributes = allowMinimizedBooleanTagHelperAttributes;
                 AllowHtmlCommentsInTagHelpers = allowHtmlCommentsInTagHelpers;
                 AllowComponentFileKind = allowComponentFileKind;
                 AllowRazorInAllCodeBlocks = allowRazorInAllCodeBlocks;
+                AllowUsingVariableDeclarations = allowUsingVariableDeclarations;
                 EXPERIMENTAL_AllowConditionalDataDashAttributes = experimental_AllowConditionalDataDashAttributes;
             }
 
@@ -74,6 +81,8 @@ namespace Microsoft.AspNetCore.Razor.Language
 
             public override bool AllowRazorInAllCodeBlocks { get; }
 
+            public override bool AllowUsingVariableDeclarations { get; }
+
             public override bool EXPERIMENTAL_AllowConditionalDataDashAttributes { get; }
         }
     }