Support authoring a single anonymous topics section (#421)

The initial implementation of anonymous topics sections didn’t
account for the use-case of a single, anonymous topic section.

The current markup parsing logic expects to find at least one H3 heading
inside a valid topic section. So while the rest of DocC’s navigator
logic was ready to handle this markup, the initial check
for the existence and range of the task group in the markup
failed to return a value for this use case.

Resolves rdar://100994549

Author: ethan-kusters

Sources/SwiftDocC/Checker/Checkers/TopicsSectionWithoutSubheading.swift

@@ -400,7 +400,6 @@ public class DocumentationContext: DocumentationContextDataProviderDelegate {
             MissingAbstract(sourceFile: source).any(),
             NonOverviewHeadingChecker(sourceFile: source).any(),
             SeeAlsoInTopicsHeadingChecker(sourceFile: source).any(),
-            TopicsSectionWithoutSubheading(sourceFile: source).any(),
         ])
         checker.visit(document)
         diagnosticEngine.emit(checker.problems)

Sources/SwiftDocC/Infrastructure/DocumentationContext.swift

@@ -247,6 +247,14 @@ struct DocumentationMarkup {
                         topicsFirstTaskGroupIndex = index
                     }
                 }
+                // The first topic group in a topic section is allowed to be "anonymous", or without
+                // an H3 heading. We account for this by treating both UnorderedLists and Paragraphs as
+                // valid children indicating the start of a task group.
+                else if child is UnorderedList {
+                    topicsFirstTaskGroupIndex = index
+                } else if child is Paragraph {
+                    topicsFirstTaskGroupIndex = index
+                }
 
                 if topicsIndex == nil { topicsIndex = index }
 

Sources/SwiftDocC/Model/DocumentationMarkup.swift

@@ -3174,4 +3174,45 @@ Document
             ]
         )
     }
+    
+    func testTopicsSectionWithSingleAnonymousTopicGroup() throws {
+        let (_, bundle, context) = try testBundleAndContext(
+            copying: "TestBundle",
+            configureBundle: { url in
+                try """
+                # Article
+                
+                Abstract.
+                
+                ## Topics
+                
+                - ``MyKit/MyProtocol``
+                - ``MyKit/MyClass``
+                
+                """.write(to: url.appendingPathComponent("article.md"), atomically: true, encoding: .utf8)
+            }
+        )
+         
+        let articleReference = ResolvedTopicReference(
+            bundleIdentifier: bundle.identifier,
+            path: "/documentation/Test-Bundle/article",
+            sourceLanguage: .swift
+        )
+        
+        let articleNode = try context.entity(with: articleReference)
+        
+        var translator = RenderNodeTranslator(context: context, bundle: bundle, identifier: articleNode.reference, source: nil)
+        let articleRenderNode = try XCTUnwrap(translator.visit(articleNode.semantic) as? RenderNode)
+        
+        XCTAssertEqual(
+            articleRenderNode.topicSections.flatMap { taskGroup in
+                [taskGroup.title] + taskGroup.identifiers
+            },
+            [
+                nil,
+                "doc://org.swift.docc.example/documentation/MyKit/MyProtocol",
+                "doc://org.swift.docc.example/documentation/MyKit/MyClass",
+            ]
+        )
+    }
 }