Associate C# version with each Razor extension version.

- Roslyn now default to 8.0 C# so it's no longer specified in a users project file. Because of this restriction we can no longer see the projects `LangVersion` when generating Razor files. Meaning, we can't conditionally generate C# 8.0 aware code because our Razor default is not C# 8.0. Therefore, when we detect that the C# lang version hasn't been provided we don't set a C# version and instead rely on what the default configuration for the Razor project is. In practice this looks like:
  - MVC1.X = C# 7.3
  - MVC2.X = C# 7.3
  - MVC Latest = C# 8.0
- I thought about adding a feature flags variant for `RazorCodeGenerationOptions` but decided not to since C# features are all configurable options that are based on more than just the `RazorLangVersion`.
- Added integration tests to ensure that command line builds with implicit `LangVersion`s result in proper C# nullability handling.

aspnet/AspNetCoredotnet/aspnetcore-tooling#12594
\n\nCommit migrated from f039aa9354

src/Razor/Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X/src/RazorExtensions.cs

@@ -4,6 +4,7 @@
 using System;
 using Microsoft.AspNetCore.Razor.Language;
 using Microsoft.AspNetCore.Razor.Language.Extensions;
+using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.Razor;
 
 namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X
@@ -38,6 +39,9 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X
             builder.Features.Add(new MvcViewDocumentClassifierPass());
 
             builder.Features.Add(new MvcImportProjectFeature());
+
+            // The default C# language version for what this Razor configuration supports.
+            builder.SetCSharpLanguageVersion(LanguageVersion.CSharp7_3);
         }
 
         public static void RegisterViewComponentTagHelpers(RazorProjectEngineBuilder builder)

src/Razor/Microsoft.AspNetCore.Mvc.Razor.Extensions.Version2_X/src/RazorExtensions.cs

@@ -4,6 +4,7 @@
 using System;
 using Microsoft.AspNetCore.Razor.Language;
 using Microsoft.AspNetCore.Razor.Language.Extensions;
+using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.Razor;
 
 namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version2_X
@@ -44,6 +45,9 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions.Version2_X
             builder.Features.Add(new InstrumentationPass());
 
             builder.Features.Add(new MvcImportProjectFeature());
+
+            // The default C# language version for what this Razor configuration supports.
+            builder.SetCSharpLanguageVersion(LanguageVersion.CSharp7_3);
         }
     }
 }

src/Razor/Microsoft.AspNetCore.Mvc.Razor.Extensions/src/RazorExtensions.cs

@@ -4,6 +4,7 @@
 using System;
 using Microsoft.AspNetCore.Razor.Language;
 using Microsoft.AspNetCore.Razor.Language.Extensions;
+using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.Razor;
 
 namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
@@ -39,6 +40,9 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
             builder.Features.Add(new MvcViewDocumentClassifierPass());
 
             builder.Features.Add(new MvcImportProjectFeature());
+
+            // The default C# language version for what this Razor configuration supports.
+            builder.SetCSharpLanguageVersion(LanguageVersion.CSharp8);
         }
     }
 }

src/Razor/Microsoft.AspNetCore.Razor.Tools/src/GenerateCommand.cs

@@ -176,20 +176,6 @@ namespace Microsoft.AspNetCore.Razor.Tools
             });
 
             var success = true;
-            var csharpLanguageVersion = LanguageVersion.Default;
-            if (CSharpLanguageVersion.HasValue())
-            {
-                var rawLanguageVersion = CSharpLanguageVersion.Value();
-                if (!LanguageVersionFacts.TryParse(CSharpLanguageVersion.Value(), out var parsedLanguageVersion))
-                {
-                    success = false;
-                    Error.WriteLine($"Unknown C# language version {rawLanguageVersion}.");
-                }
-                else
-                {
-                    csharpLanguageVersion = parsedLanguageVersion;
-                }
-            }
 
             var engine = RazorProjectEngine.Create(configuration, compositeFileSystem, b =>
             {
@@ -206,7 +192,22 @@ namespace Microsoft.AspNetCore.Razor.Tools
                     b.SetRootNamespace(RootNamespace.Value());
                 }
 
-                b.SetCSharpLanguageVersion(csharpLanguageVersion);
+                if (CSharpLanguageVersion.HasValue())
+                {
+                    // Only set the C# language version if one was specified, otherwise it defaults to whatever
+                    // value was set in the corresponding RazorConfiguration's extensions.
+
+                    var rawLanguageVersion = CSharpLanguageVersion.Value();
+                    if (LanguageVersionFacts.TryParse(rawLanguageVersion, out var csharpLanguageVersion))
+                    {
+                        b.SetCSharpLanguageVersion(csharpLanguageVersion);
+                    }
+                    else
+                    {
+                        success = false;
+                        Error.WriteLine($"Unknown C# language version {rawLanguageVersion}.");
+                    }
+                }
             });
 
             var results = GenerateCode(engine, sourceItems);

src/Razor/Microsoft.CodeAnalysis.Razor/src/RazorProjectEngineBuilderExtensions.cs

@@ -60,7 +60,7 @@ namespace Microsoft.CodeAnalysis.Razor
                     throw new ArgumentNullException(nameof(options));
                 }
 
-                if (options.Configuration.LanguageVersion.Major < 3)
+                if (options.Configuration != null && options.Configuration.LanguageVersion.Major < 3)
                 {
                     // Prior to 3.0 there were no C# version specific controlled features. Suppress nullability enforcement.
                     options.SuppressNullabilityEnforcement = true;

src/Razor/Microsoft.NET.Sdk.Razor/test/IntegrationTests/BuildIntegrationTest.cs

@@ -641,7 +641,38 @@ namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
 
         [Fact]
         [InitializeTestProject("SimpleMvc")]
-        public async Task Build_CSharp8_NullableEnforcement_WarningsDuringBuild_BuildServer()
+        public async Task Build_ImplicitCSharp8_NullableEnforcement_WarningsDuringBuild_NoBuildServer()
+        {
+            var result = await DotnetMSBuild(
+                "Build",
+                "/p:Nullable=enable",
+                suppressBuildServer: true);
+            var indexFilePath = Path.Combine(RazorIntermediateOutputPath, "Views", "Home", "Index.cshtml.g.cs");
+
+            Assert.BuildPassed(result, allowWarnings: true);
+            Assert.BuildWarning(result, "CS8618");
+            Assert.FileContainsLine(result, indexFilePath, "#nullable restore");
+            Assert.FileContainsLine(result, indexFilePath, "#nullable disable");
+        }
+
+        [Fact]
+        [InitializeTestProject("SimpleMvc")]
+        public async Task Build_ExplicitCSharp73_NullableEnforcement_Disabled_NoNullableFeature_NoBuildServer()
+        {
+            var result = await DotnetMSBuild(
+                "Build",
+                "/p:LangVersion=7.3",
+                suppressBuildServer: true);
+            var indexFilePath = Path.Combine(RazorIntermediateOutputPath, "Views", "Home", "Index.cshtml.g.cs");
+
+            Assert.BuildPassed(result, allowWarnings: false);
+            Assert.FileDoesNotContainLine(result, indexFilePath, "#nullable restore");
+            Assert.FileDoesNotContainLine(result, indexFilePath, "#nullable disable");
+        }
+
+        [Fact]
+        [InitializeTestProject("SimpleMvc")]
+        public async Task Build_ExplicitCSharp8_NullableEnforcement_WarningsDuringBuild_BuildServer()
         {
             var result = await DotnetMSBuild(
                 "Build",