Fix inaccurate HTML/CSS diagnostics on C#. (#4280)

* Fix inaccurate HTML/CSS diagnostics on C#.

- The CSS diagnostic system didn't recognize the C# portions of the document and was then warning about those portions of the document. It's expected that we'd get a diagnostic in this case; however, we should be filtering HTML diagnostics that map to C# literals because they can't see the C# literals properly.
- Added a test to capture this case.

### Before
![image](https://user-images.githubusercontent.com/2008729/135360938-4f7af400-5675-4c5f-93bc-e37aebd0dad7.png)

### After
![image](https://user-images.githubusercontent.com/2008729/135360655-79d2fb12-3c38-4d25-97c7-fc8a71f57394.png)

Fixes dotnet/aspnetcore#36321

* Update src/Razor/src/Microsoft.AspNetCore.Razor.LanguageServer/Diagnostics/RazorDiagnosticsEndpoint.cs

Co-authored-by: David Wengier <david.wengier@microsoft.com>

* Fix tests

Co-authored-by: David Wengier <david.wengier@microsoft.com>

src/Razor/src/Microsoft.AspNetCore.Razor.LanguageServer/Diagnostics/RazorDiagnosticsEndpoint.cs

@@ -171,6 +171,7 @@ namespace Microsoft.AspNetCore.Razor.LanguageServer.Diagnostics
 
             var filteredDiagnostics = unmappedDiagnostics
                 .Where(d =>
+                    !InCSharpLiteral(d, sourceText, syntaxTree) &&
                     !InAttributeContainingCSharp(d, sourceText, syntaxTree, processedAttributes) &&
                     !AppliesToTagHelperTagName(d, sourceText, syntaxTree) &&
                     !ShouldFilterHtmlDiagnosticBasedOnErrorCode(d, sourceText, syntaxTree))
@@ -179,6 +180,24 @@ namespace Microsoft.AspNetCore.Razor.LanguageServer.Diagnostics
             return filteredDiagnostics;
         }
 
+        private static bool InCSharpLiteral(OmniSharpVSDiagnostic d, SourceText sourceText, RazorSyntaxTree syntaxTree)
+        {
+            if (d.Range is null)
+            {
+                return false;
+            }
+
+            var owner = syntaxTree.GetOwner(sourceText, d.Range.End);
+            if (owner is null)
+            {
+                return false;
+            }
+
+            var isCSharp = owner.Kind is SyntaxKind.CSharpExpressionLiteral or SyntaxKind.CSharpStatementLiteral or SyntaxKind.CSharpEphemeralTextLiteral;
+
+            return isCSharp;
+        }
+
         private static bool AppliesToTagHelperTagName(OmniSharpVSDiagnostic diagnostic, SourceText sourceText, RazorSyntaxTree syntaxTree)
         {
             // Goal of this method is to filter diagnostics that touch TagHelper tag names. Reason being is TagHelpers can output anything. Meaning

src/Razor/test/Microsoft.AspNetCore.Razor.LanguageServer.Test/Diagnostics/RazorDiagnosticsEndpointTest.cs

@@ -270,6 +270,41 @@ namespace Microsoft.AspNetCore.Razor.LanguageServer.Diagnostics
             Assert.Empty(response.Diagnostics);
         }
 
+        [Fact]
+        public async Task Handle_FilterDiagnostics_CSharpInsideStylePropertySpace()
+        {
+            // Arrange
+            var documentPath = "C:/path/to/document.cshtml";
+            var codeDocument = CreateCodeDocumentWithCSharpProjection(
+                "<style> body { overflow: @DateTime.Now; } </style>",
+                "var __o = DateTime.Now",
+                new[] {
+                    new SourceMapping(
+                        new SourceSpan(26, 12),
+                        new SourceSpan(10, 12))
+                });
+            var documentResolver = CreateDocumentResolver(documentPath, codeDocument);
+            var diagnosticsEndpoint = new RazorDiagnosticsEndpoint(Dispatcher, documentResolver, DocumentVersionCache, MappingService, LoggerFactory);
+            var request = new RazorDiagnosticsParams()
+            {
+                Kind = RazorLanguageKind.Html,
+                Diagnostics = new[] {
+                    new OmniSharpVSDiagnostic() {
+                        Range = new Range(new Position(0, 25), new Position(0, 38)),
+                        Code = "CSS123456",
+                        Severity = DiagnosticSeverity.Warning
+                    }
+                },
+                RazorDocumentUri = new Uri(documentPath),
+            };
+
+            // Act
+            var response = await Task.Run(() => diagnosticsEndpoint.Handle(request, default));
+
+            // Assert
+            Assert.Empty(response.Diagnostics);
+        }
+
         [Fact]
         public async Task Handle_FilterDiagnostics_CSharpInsideStyleBlock()
         {
@@ -620,7 +655,7 @@ namespace Microsoft.AspNetCore.Razor.LanguageServer.Diagnostics
             var request = new RazorDiagnosticsParams()
             {
                 Kind = RazorLanguageKind.Razor,
-                Diagnostics = new[] { new OmniSharpVSDiagnostic() { Range = new Range(new Position(0, 3), new Position(0, 4)) } },
+                Diagnostics = new[] { new OmniSharpVSDiagnostic() { Range = new Range(new Position(0, 1), new Position(0, 3)) } },
                 RazorDocumentUri = new Uri(documentPath),
             };