Enable rich LSIF hover information.

src/EditorFeatures/Core/LanguageServer/Handlers/Hover/HoverHandler.cs

@@ -69,6 +69,7 @@ public HoverHandler(IGlobalOptionService globalOptions)
             int position,
             SymbolDescriptionOptions options,
             LanguageServices languageServices,
+            ClientCapabilities clientCapabilities,
             CancellationToken cancellationToken)
         {
             Debug.Assert(semanticModel.Language is LanguageNames.CSharp or LanguageNames.VisualBasic);
@@ -83,7 +84,7 @@ public HoverHandler(IGlobalOptionService globalOptions)
             }
 
             var text = await semanticModel.SyntaxTree.GetTextAsync(cancellationToken).ConfigureAwait(false);
-            return await GetHoverAsync(info, text, semanticModel.Language, document: null, classificationOptions: null, clientCapabilities: null, cancellationToken).ConfigureAwait(false);
+            return await GetHoverAsync(info, text, semanticModel.Language, document: null, classificationOptions: null, clientCapabilities, cancellationToken).ConfigureAwait(false);
         }
 
         private static async Task<Hover> GetHoverAsync(

src/Features/Lsif/Generator/Generator.cs

@@ -17,6 +17,7 @@
 using Microsoft.CodeAnalysis.LanguageService;
 using Microsoft.CodeAnalysis.Shared.Extensions;
 using Roslyn.Utilities;
+using LspProtocol = Microsoft.VisualStudio.LanguageServer.Protocol;
 using Methods = Microsoft.VisualStudio.LanguageServer.Protocol.Methods;
 
 namespace Microsoft.CodeAnalysis.LanguageServerIndexFormat.Generator
@@ -33,6 +34,20 @@ internal sealed class Generator
         private const bool FoldingRangeProvider = true;
         private const bool DiagnosticProvider = false;
 
+        private static readonly LspProtocol.ClientCapabilities LspClientCapabilities = new()
+        {
+            TextDocument = new LspProtocol.TextDocumentClientCapabilities()
+            {
+                Hover = new LspProtocol.HoverSetting()
+                {
+                    ContentFormat = new[]
+                    {
+                        LspProtocol.MarkupKind.PlainText,
+                        LspProtocol.MarkupKind.Markdown,
+                    }
+                }
+            }
+        };
         private readonly ILsifJsonWriter _lsifJsonWriter;
         private readonly IdFactory _idFactory = new IdFactory();
 
@@ -252,7 +267,7 @@ SymbolKind.RangeVariable or
                     // See /~https://github.com/Microsoft/language-server-protocol/blob/main/indexFormat/specification.md#resultset for an example.
                     if (symbolResultsTracker.ResultSetNeedsInformationalEdgeAdded(symbolForLinkedResultSet, Methods.TextDocumentHoverName))
                     {
-                        var hover = await HoverHandler.GetHoverAsync(semanticModel, syntaxToken.SpanStart, options.SymbolDescriptionOptions, languageServices, CancellationToken.None);
+                        var hover = await HoverHandler.GetHoverAsync(semanticModel, syntaxToken.SpanStart, options.SymbolDescriptionOptions, languageServices, LspClientCapabilities, CancellationToken.None);
                         if (hover != null)
                         {
                             var hoverResult = new HoverResult(hover, idFactory);

src/Features/Lsif/GeneratorTest/HoverTests.vb

@@ -42,28 +42,46 @@ class C
             Dim expectedHoverContents As String
             Select Case code
                 Case "class [|C|] { string s; }"
-                    expectedHoverContents = "class C"
+                    expectedHoverContents = "```csharp
+class C
+```
+  "
                 Case "class C { void [|M|]() { } }"
-                    expectedHoverContents = "void C.M()"
+                    expectedHoverContents = "```csharp
+void C.M()
+```
+  "
                 Case "class C { string [|s|]; }"
-                    expectedHoverContents = $"({FeaturesResources.field}) string C.s"
+                    expectedHoverContents = $"```csharp
+({FeaturesResources.field}) string C.s
+```
+  "
                 Case "class C { void M(string [|s|]) { M(s); } }"
-                    expectedHoverContents = $"({FeaturesResources.parameter}) string s"
+                    expectedHoverContents = $"```csharp
+({FeaturesResources.parameter}) string s
+```
+  "
                 Case "class C { void M(string s) { string [|local|] = """"; } }"
-                    expectedHoverContents = $"({FeaturesResources.local_variable}) string local"
+                    expectedHoverContents = $"```csharp
+({FeaturesResources.local_variable}) string local
+```
+  "
                 Case "
 class C
 {
     /// <summary>Doc Comment</summary>
     void [|M|]() { }
 }"
-                    expectedHoverContents = "void C.M()
-Doc Comment"
+                    expectedHoverContents = "```csharp
+void C.M()
+```
+
+Doc&nbsp;Comment  "
                 Case Else
                     Throw TestExceptionUtilities.UnexpectedValue(code)
             End Select
 
-            Assert.Equal(MarkupKind.PlainText, hoverMarkupContent.Kind)
+            Assert.Equal(MarkupKind.Markdown, hoverMarkupContent.Kind)
             Assert.Equal(expectedHoverContents + Environment.NewLine, hoverMarkupContent.Value)
         End Function
 
@@ -99,29 +117,50 @@ class C
             Dim expectedHoverContents As String
             Select Case code
                 Case "class C { [|string|] s; }"
-                    expectedHoverContents = "class System.String"
+                    expectedHoverContents = "```csharp
+class System.String
+```
+  "
                 Case "class C { void M() { [|M|](); } }"
-                    expectedHoverContents = "void C.M()"
+                    expectedHoverContents = "```csharp
+void C.M()
+```
+  "
                 Case "class C { void M(string s) { M([|s|]); } }"
-                    expectedHoverContents = $"({FeaturesResources.parameter}) string s"
+                    expectedHoverContents = $"```csharp
+({FeaturesResources.parameter}) string s
+```
+  "
                 Case "class C { void M(string s) { string local = """"; M([|local|]); } }"
-                    expectedHoverContents = $"({FeaturesResources.local_variable}) string local"
+                    expectedHoverContents = $"```csharp
+({FeaturesResources.local_variable}) string local
+```
+  "
                 Case "using [|S|] = System.String;"
-                    expectedHoverContents = "class System.String"
+                    expectedHoverContents = "```csharp
+class System.String
+```
+  "
                 Case "class C { [|global|]::System.String s; }"
-                    expectedHoverContents = "<global namespace>"
+                    expectedHoverContents = "```csharp
+<global namespace>
+```
+  "
                 Case "
 class C
 {
     /// <see cref=""C.[|M|]()"" />
     void M() { }
 }"
-                    expectedHoverContents = "void C.M()"
+                    expectedHoverContents = "```csharp
+void C.M()
+```
+  "
                 Case Else
                     Throw TestExceptionUtilities.UnexpectedValue(code)
             End Select
 
-            Assert.Equal(MarkupKind.PlainText, hoverMarkupContent.Kind)
+            Assert.Equal(MarkupKind.Markdown, hoverMarkupContent.Kind)
             Assert.Equal(expectedHoverContents + Environment.NewLine, hoverMarkupContent.Value)
         End Function
 
@@ -161,8 +200,11 @@ class C
             Next
 
             Dim hoverMarkupContent = DirectCast(hoverVertex.Result.Contents.Value.Fourth, MarkupContent)
-            Assert.Equal(MarkupKind.PlainText, hoverMarkupContent.Kind)
-            Assert.Equal("class System.String" + Environment.NewLine, hoverMarkupContent.Value)
+            Assert.Equal(MarkupKind.Markdown, hoverMarkupContent.Kind)
+            Assert.Equal("```csharp
+class System.String
+```
+  " + Environment.NewLine, hoverMarkupContent.Value)
         End Function
     End Class
 End Namespace