[WIP] Ensure DocumentationBuilder never has a null preferred MarkupKind

src/Analysis/Engine/Impl/Documentation/DocumentationBuilder.cs

@@ -38,6 +38,7 @@ internal abstract class DocumentationBuilder {
 
         public static DocumentationBuilder Create(InformationDisplayOptions displayOptions) {
             displayOptions = displayOptions ?? DefaultDisplayOptions;
+            displayOptions.preferredFormat = displayOptions.preferredFormat ?? MarkupKind.PlainText; // MarkupKind should never be null.
 
             if (displayOptions.preferredFormat == MarkupKind.Markdown) {
                 return new MarkdownDocumentationBuilder(displayOptions);

src/Analysis/Engine/Test/CompletionTests.cs

@@ -27,6 +27,7 @@
 using Microsoft.Python.LanguageServer.Implementation;
 using Microsoft.PythonTools;
 using Microsoft.PythonTools.Analysis;
+using Microsoft.PythonTools.Analysis.Documentation;
 using Microsoft.PythonTools.Analysis.FluentAssertions;
 using Microsoft.PythonTools.Analysis.Infrastructure;
 using Microsoft.PythonTools.Interpreter;
@@ -1012,6 +1013,22 @@ public async Task WithWhitespaceAroundDot() {
             }
         }
 
+        [TestMethod, Priority(0)]
+        public async Task MarkupKindValid() {
+            using (var s = await CreateServerAsync()) {
+                var u = await s.OpenDefaultDocumentAndGetUriAsync("import sys\nsys.\n");
+
+                await s.WaitForCompleteAnalysisAsync(CancellationToken.None);
+                var res = await s.Completion(new CompletionParams {
+                    textDocument = new TextDocumentIdentifier { uri = u },
+                    position = new SourceLocation(2, 5),
+                    context = new CompletionContext { triggerCharacter = ".", triggerKind = CompletionTriggerKind.TriggerCharacter },
+                }, CancellationToken.None);
+
+                res.items?.Select(i => i.documentation.kind).Should().NotBeEmpty().And.BeSubsetOf(new[] { MarkupKind.PlainText, MarkupKind.Markdown });
+            }
+        }
+
         private static async Task AssertCompletion(Server s, Uri uri, IReadOnlyCollection<string> contains, IReadOnlyCollection<string> excludes, Position? position = null, CompletionContext? context = null, Func<CompletionItem, string> cmpKey = null, string expr = null, Range? applicableSpan = null) {
             await s.WaitForCompleteAnalysisAsync(CancellationToken.None);
             var res = await s.Completion(new CompletionParams {

src/Analysis/Engine/Test/HoverTests.cs

@@ -25,6 +25,7 @@
 using Microsoft.Python.Tests.Utilities.FluentAssertions;
 using Microsoft.PythonTools;
 using Microsoft.PythonTools.Analysis;
+using Microsoft.PythonTools.Analysis.Documentation;
 using Microsoft.PythonTools.Analysis.FluentAssertions;
 using Microsoft.PythonTools.Interpreter;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
@@ -143,6 +144,21 @@ def fob_derived(self):
             }
         }
 
+        [TestMethod, Priority(0)]
+        public async Task MarkupKindValid() {
+            using (var s = await CreateServerAsync()) {
+                var u = await s.OpenDefaultDocumentAndGetUriAsync("123");
+
+                await s.WaitForCompleteAnalysisAsync(CancellationToken.None);
+                var hover = await s.Hover(new TextDocumentPositionParams {
+                    textDocument = new TextDocumentIdentifier { uri = u },
+                    position = new SourceLocation(1, 1),
+                }, CancellationToken.None);
+
+                hover.contents.kind.Should().BeOneOf(MarkupKind.PlainText, MarkupKind.Markdown);
+            }
+        }
+
         private static async Task AssertHover(Server s, Uri uri, SourceLocation position, string hoverText, IEnumerable<string> typeNames, SourceSpan? range = null, string expr = null) {
             await s.WaitForCompleteAnalysisAsync(CancellationToken.None);
             var hover = await s.Hover(new TextDocumentPositionParams {