Handle imported @page directives.

- Prior to this imported `@page` directives would flow through the Razor system without error. This resulted in inconsistent behavior between MVC and Razor. Now, imported `@page` directives result in diagnostics on the page directive node.
- Added two tests to verify that we still treat views with imported page directives as Razor pages BUT we also log a diagnostic on the page directive node.
- Renamed the `ViewComponentDiagnosticFactory` class to `RazorExtensionsDiagnosticFactory` so it can be used for more than just view component diagnostics. This way we can ensure that our diagnostics don't end up overlapping.

#1503

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/Properties/Resources.Designer.cs

@@ -94,6 +94,20 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
         internal static string FormatMvcRazorParser_InvalidPropertyType(object p0, object p1, object p2)
             => string.Format(CultureInfo.CurrentCulture, GetString("MvcRazorParser_InvalidPropertyType"), p0, p1, p2);
 
+        /// <summary>
+        /// The '@{0}' directive specified in {1} file will not be imported. The directive must appear at the top of each Razor cshtml file.
+        /// </summary>
+        internal static string PageDirectiveCannotBeImported
+        {
+            get => GetString("PageDirectiveCannotBeImported");
+        }
+
+        /// <summary>
+        /// The '@{0}' directive specified in {1} file will not be imported. The directive must appear at the top of each Razor cshtml file.
+        /// </summary>
+        internal static string FormatPageDirectiveCannotBeImported(object p0, object p1)
+            => string.Format(CultureInfo.CurrentCulture, GetString("PageDirectiveCannotBeImported"), p0, p1);
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

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

@@ -1,12 +1,13 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System.IO;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Razor.Language;
 
 namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 {
-    internal class ViewComponentDiagnosticFactory
+    internal class RazorExtensionsDiagnosticFactory
     {
         private const string DiagnosticPrefix = "RZ";
 
@@ -98,5 +99,19 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
             return diagnostic;
         }
+
+        public static readonly RazorDiagnosticDescriptor PageDirective_CannotBeImported =
+            new RazorDiagnosticDescriptor(
+                $"{DiagnosticPrefix}3905",
+                () => Resources.PageDirectiveCannotBeImported,
+                RazorDiagnosticSeverity.Error);
+
+        public static RazorDiagnostic CreatePageDirective_CannotBeImported(SourceSpan source)
+        {
+            var fileName = Path.GetFileName(source.FilePath);
+            var diagnostic = RazorDiagnostic.Create(PageDirective_CannotBeImported, source, PageDirective.Directive.Directive, fileName);
+
+            return diagnostic;
+        }
     }
 }

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

@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
 using Microsoft.AspNetCore.Razor.Language;
 using Microsoft.AspNetCore.Razor.Language.Intermediate;
 
@@ -41,6 +42,34 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
             method.Modifiers.Add("async");
             method.Modifiers.Add("override");
             method.ReturnType = $"global::{typeof(System.Threading.Tasks.Task).FullName}";
+
+            EnsureValidPageDirective(codeDocument);
+        }
+
+        private void EnsureValidPageDirective(RazorCodeDocument codeDocument)
+        {
+            var document = codeDocument.GetDocumentIntermediateNode();
+            var visitor = new Visitor();
+            visitor.VisitDocument(document);
+
+            if (visitor.DirectiveNode.IsImported())
+            {
+                visitor.DirectiveNode.Diagnostics.Add(
+                    RazorExtensionsDiagnosticFactory.CreatePageDirective_CannotBeImported(visitor.DirectiveNode.Source.Value));
+            }
+        }
+
+        private class Visitor : IntermediateNodeWalker
+        {
+            public DirectiveIntermediateNode DirectiveNode { get; private set; }
+
+            public override void VisitDirective(DirectiveIntermediateNode node)
+            {
+                if (node.Descriptor == PageDirective.Directive)
+                {
+                    DirectiveNode = node;
+                }
+            }
         }
     }
 }

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/Resources.resx

@@ -135,4 +135,7 @@
   <data name="MvcRazorParser_InvalidPropertyType" xml:space="preserve">
     <value>Invalid tag helper property '{0}.{1}'. Dictionary values must not be of type '{2}'.</value>
   </data>
+  <data name="PageDirectiveCannotBeImported" xml:space="preserve">
+    <value>The '@{0}' directive specified in {1} file will not be imported. The directive must appear at the top of each Razor cshtml file.</value>
+  </data>
 </root>

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

@@ -94,13 +94,13 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
             if (methods.Length == 0)
             {
-                diagnostic =  ViewComponentDiagnosticFactory.CreateViewComponent_CannotFindMethod(type.ToDisplayString(FullNameTypeDisplayFormat));
+                diagnostic =  RazorExtensionsDiagnosticFactory.CreateViewComponent_CannotFindMethod(type.ToDisplayString(FullNameTypeDisplayFormat));
                 method = null;
                 return false;
             }
             else if (methods.Length > 1)
             {
-                diagnostic = ViewComponentDiagnosticFactory.CreateViewComponent_AmbiguousMethods(type.ToDisplayString(FullNameTypeDisplayFormat));
+                diagnostic = RazorExtensionsDiagnosticFactory.CreateViewComponent_AmbiguousMethods(type.ToDisplayString(FullNameTypeDisplayFormat));
                 method = null;
                 return false;
             }
@@ -120,7 +120,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
                 }
                 else
                 {
-                    diagnostic = ViewComponentDiagnosticFactory.CreateViewComponent_AsyncMethod_ShouldReturnTask(type.ToDisplayString(FullNameTypeDisplayFormat));
+                    diagnostic = RazorExtensionsDiagnosticFactory.CreateViewComponent_AsyncMethod_ShouldReturnTask(type.ToDisplayString(FullNameTypeDisplayFormat));
                     method = null;
                     return false;
                 }
@@ -130,19 +130,19 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
                 // Will invoke synchronously. Method must not return void, Task or Task<T>.
                 if (returnType.SpecialType == SpecialType.System_Void)
                 {
-                    diagnostic = ViewComponentDiagnosticFactory.CreateViewComponent_SyncMethod_ShouldReturnValue(type.ToDisplayString(FullNameTypeDisplayFormat));
+                    diagnostic = RazorExtensionsDiagnosticFactory.CreateViewComponent_SyncMethod_ShouldReturnValue(type.ToDisplayString(FullNameTypeDisplayFormat));
                     method = null;
                     return false;
                 }
                 else if (returnType == _taskSymbol)
                 {
-                    diagnostic = ViewComponentDiagnosticFactory.CreateViewComponent_SyncMethod_CannotReturnTask(type.ToDisplayString(FullNameTypeDisplayFormat));
+                    diagnostic = RazorExtensionsDiagnosticFactory.CreateViewComponent_SyncMethod_CannotReturnTask(type.ToDisplayString(FullNameTypeDisplayFormat));
                     method = null;
                     return false;
                 }
                 else if (returnType.IsGenericType && returnType.ConstructedFrom == _genericTaskSymbol)
                 {
-                    diagnostic = ViewComponentDiagnosticFactory.CreateViewComponent_SyncMethod_CannotReturnTask(type.ToDisplayString(FullNameTypeDisplayFormat));
+                    diagnostic = RazorExtensionsDiagnosticFactory.CreateViewComponent_SyncMethod_CannotReturnTask(type.ToDisplayString(FullNameTypeDisplayFormat));
                     method = null;
                     return false;
                 }

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

@@ -109,9 +109,8 @@ namespace Microsoft.AspNetCore.Razor.Language
                 var directive = (DirectiveIntermediateNode)reference.Node;
                 var descriptor = directive.Descriptor;
                 var seenDirective = !seenDirectives.Add(descriptor);
-                var imported = ReferenceEquals(directive.Annotations[CommonAnnotations.Imported], CommonAnnotations.Imported);
 
-                if (!imported)
+                if (!directive.IsImported())
                 {
                     continue;
                 }

src/Microsoft.AspNetCore.Razor.Language/Intermediate/IntermediateNodeExtensions.cs

@@ -11,6 +11,11 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
     {
         private static readonly IReadOnlyList<RazorDiagnostic> EmptyDiagnostics = Array.Empty<RazorDiagnostic>();
 
+        public static bool IsImported(this IntermediateNode node)
+        {
+            return ReferenceEquals(node.Annotations[CommonAnnotations.Imported], CommonAnnotations.Imported);
+        }
+
         public static IReadOnlyList<RazorDiagnostic> GetAllDiagnostics(this IntermediateNode node)
         {
             if (node == null)

test/Microsoft.AspNetCore.Mvc.Razor.Extensions.Test/PageDirectiveTest.cs

@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
 using Microsoft.AspNetCore.Razor.Language;
 using Microsoft.AspNetCore.Razor.Language.Intermediate;
 using Xunit;
@@ -9,6 +10,25 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 {
     public class PageDirectiveTest
     {
+        [Fact]
+        public void TryGetPageDirective_ReturnsTrue_IfPageIsImported()
+        {
+            // Arrange
+            var content = "Hello world";
+            var sourceDocument = RazorSourceDocument.Create(content, "file");
+            var importDocument = RazorSourceDocument.Create("@page", "imports.cshtml");
+            var codeDocument = RazorCodeDocument.Create(sourceDocument, new[] { importDocument });
+            var engine = CreateEngine();
+            var irDocument = CreateIRDocument(engine, codeDocument);
+
+            // Act
+            var result = PageDirective.TryGetPageDirective(irDocument, out var pageDirective);
+
+            // Assert
+            Assert.True(result);
+            Assert.Null(pageDirective.RouteTemplate);
+        }
+
         [Fact]
         public void TryGetPageDirective_ReturnsFalse_IfPageDoesNotHaveDirective()
         {

test/Microsoft.AspNetCore.Mvc.Razor.Extensions.Test/RazorPageDocumentClassifierPassTest.cs

@@ -10,6 +10,32 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 {
     public class RazorPageDocumentClassifierPassTest
     {
+        [Fact]
+        public void RazorPageDocumentClassifierPass_LogsErrorForImportedPageDirectives()
+        {
+            // Arrange
+            var sourceSpan = new SourceSpan("import.cshtml", 0, 0, 0, 5);
+            var expectedDiagnostic = RazorExtensionsDiagnosticFactory.CreatePageDirective_CannotBeImported(sourceSpan);
+            var importDocument = RazorSourceDocument.Create("@page", "import.cshtml");
+            var sourceDocument = RazorSourceDocument.Create("<p>Hello World</p>", "main.cshtml");
+            var codeDocument = RazorCodeDocument.Create(sourceDocument, new[] { importDocument });
+            var engine = CreateEngine();
+            var irDocument = CreateIRDocument(engine, codeDocument);
+            var pass = new RazorPageDocumentClassifierPass
+            {
+                Engine = engine
+            };
+
+            // Act
+            pass.Execute(codeDocument, irDocument);
+
+            // Assert
+            var pageDirectives = irDocument.FindDirectiveReferences(PageDirective.Directive);
+            var directive = Assert.Single(pageDirectives);
+            var diagnostic = Assert.Single(directive.Node.Diagnostics);
+            Assert.Equal(expectedDiagnostic, diagnostic);
+        }
+
         [Fact]
         public void RazorPageDocumentClassifierPass_SetsDocumentKind()
         {

test/Microsoft.AspNetCore.Mvc.Razor.Extensions.Test/ViewComponentTagHelperDescriptorFactoryTest.cs

@@ -159,7 +159,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
             // Assert
             var diagnostic = Assert.Single(descriptor.GetAllDiagnostics());
-            Assert.Equal(ViewComponentDiagnosticFactory.ViewComponent_CannotFindMethod.Id, diagnostic.Id);
+            Assert.Equal(RazorExtensionsDiagnosticFactory.ViewComponent_CannotFindMethod.Id, diagnostic.Id);
         }
 
         [Fact]
@@ -208,7 +208,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
             // Assert
             var diagnostic = Assert.Single(descriptor.GetAllDiagnostics());
-            Assert.Equal(ViewComponentDiagnosticFactory.ViewComponent_AsyncMethod_ShouldReturnTask.Id, diagnostic.Id);
+            Assert.Equal(RazorExtensionsDiagnosticFactory.ViewComponent_AsyncMethod_ShouldReturnTask.Id, diagnostic.Id);
         }
 
         [Fact]
@@ -225,7 +225,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
             // Assert
             var diagnostic = Assert.Single(descriptor.GetAllDiagnostics());
-            Assert.Equal(ViewComponentDiagnosticFactory.ViewComponent_AsyncMethod_ShouldReturnTask.Id, diagnostic.Id);
+            Assert.Equal(RazorExtensionsDiagnosticFactory.ViewComponent_AsyncMethod_ShouldReturnTask.Id, diagnostic.Id);
         }
 
         [Fact]
@@ -242,7 +242,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
             // Assert
             var diagnostic = Assert.Single(descriptor.GetAllDiagnostics());
-            Assert.Equal(ViewComponentDiagnosticFactory.ViewComponent_SyncMethod_ShouldReturnValue.Id, diagnostic.Id);
+            Assert.Equal(RazorExtensionsDiagnosticFactory.ViewComponent_SyncMethod_ShouldReturnValue.Id, diagnostic.Id);
         }
 
         [Fact]
@@ -259,7 +259,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
             // Assert
             var diagnostic = Assert.Single(descriptor.GetAllDiagnostics());
-            Assert.Equal(ViewComponentDiagnosticFactory.ViewComponent_SyncMethod_CannotReturnTask.Id, diagnostic.Id);
+            Assert.Equal(RazorExtensionsDiagnosticFactory.ViewComponent_SyncMethod_CannotReturnTask.Id, diagnostic.Id);
         }
 
         [Fact]
@@ -276,7 +276,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
             // Assert
             var diagnostic = Assert.Single(descriptor.GetAllDiagnostics());
-            Assert.Equal(ViewComponentDiagnosticFactory.ViewComponent_SyncMethod_CannotReturnTask.Id, diagnostic.Id);
+            Assert.Equal(RazorExtensionsDiagnosticFactory.ViewComponent_SyncMethod_CannotReturnTask.Id, diagnostic.Id);
         }
     }