Fix @SupportedLanguage directive for articles

The `@SupportedLanguage` directive allows specifying a language that an
article is available in. It can be used within the `@Metadata` directive
like in the below example, where the article is made available in both
Swift and Objective-C:

```

@metadata {
  @SupportedLanguage(swift)
  @SupportedLanguage(objc)
}
```

This directive is processed when creating the topic graph node for the
article. In cases where the article is the root page or a standalone
article (when no technology root is defined), the directive is correctly
processed as a part of registering the article in the documentation
cache. However, in the case of a non-root article in a catalog
containing a root article, this step was being skipped. This patch fixes
this to ensure that the directive is processed for all articles in a
catalog.

rdar://160284853

Author: snprajwal

Sources/SwiftDocC/Infrastructure/DocumentationContext.swift

@@ -829,7 +829,13 @@ public class DocumentationContext {
 
                 insertLandmarks(tutorialArticle.landmarks, from: topicGraphNode, source: url)
             } else if let article = analyzed as? Article {
-                                
+                // If the article contains any `@SupportedLanguage` directives in the metadata,
+                // include those languages in the set of source languages for the reference.
+                let reference = if let supportedLanguages = article.supportedLanguages {
+                    reference.withSourceLanguages(supportedLanguages)
+                } else {
+                    reference
+                }
                 // Here we create a topic graph node with the prepared data but we don't add it to the topic graph just yet
                 // because we don't know where in the hierarchy the article belongs, we will add it later when crawling the manual curation via Topics task groups.
                 let topicGraphNode = TopicGraph.Node(reference: reference, kind: .article, source: .file(url: url), title: article.title!.plainText)
@@ -1883,17 +1889,7 @@ public class DocumentationContext {
         let path = NodeURLGenerator.pathForSemantic(article.value, source: article.source, bundle: bundle)
 
         // Use the languages specified by the `@SupportedLanguage` directives if present.
-        let availableSourceLanguages = article.value
-            .metadata
-            .flatMap { metadata in
-                let languages = Set(
-                    metadata.supportedLanguages
-                        .map(\.language)
-                )
-                
-                return languages.isEmpty ? nil : languages
-            }
-        ?? availableSourceLanguages
+        let availableSourceLanguages = article.value.supportedLanguages ?? availableSourceLanguages
 
         // If available source languages are provided and it contains Swift, use Swift as the default language of
         // the article.

Sources/SwiftDocC/Model/Rendering/RenderNodeTranslator.swift

@@ -630,7 +630,7 @@ public struct RenderNodeTranslator: SemanticVisitor {
         node.hierarchyVariants = hierarchyVariants
 
         // Emit variants only if we're not compiling an article-only catalog to prevent renderers from
-        // advertising the page as "Swift", which is the language DocC assigns to pages in article only pages.
+        // advertising the page as "Swift", which is the language DocC assigns to pages in article only catalogs.
         // (github.com/swiftlang/swift-docc/issues/240).
         if let topLevelModule = context.soleRootModuleReference,
            try! context.entity(with: topLevelModule).kind.isSymbol

Sources/SwiftDocC/Semantics/Article/Article.swift

@@ -66,6 +66,18 @@ public final class Article: Semantic, MarkupConvertible, Abstracted, Redirected,
         return abstractSection?.paragraph
     }
 
+    /// The list of supported languages for the article, if present.
+    ///
+    /// This information is available via `@SupportedLanguage` in the `@Metadata` directive.
+    public var supportedLanguages: Set<SourceLanguage>? {
+        guard let metadata = self.metadata else {
+            return nil
+        }
+
+        let langs = metadata.supportedLanguages.map(\.language)
+        return langs.isEmpty ? nil : Set(langs)
+    }
+
     /// An optional custom deprecation summary for a deprecated symbol.
     private(set) public var deprecationSummary: MarkupContainer?
 

Sources/SwiftDocC/Semantics/Metadata/Metadata.swift

@@ -105,7 +105,7 @@ public final class Metadata: Semantic, AutomaticDirectiveConvertible {
 
     func validate(source: URL?, problems: inout [Problem]) -> Bool {
         // Check that something is configured in the metadata block
-        if documentationOptions == nil && technologyRoot == nil && displayName == nil && pageImages.isEmpty && customMetadata.isEmpty && callToAction == nil && availability.isEmpty && pageKind == nil && pageColor == nil && titleHeading == nil && redirects == nil && alternateRepresentations.isEmpty {
+        if documentationOptions == nil && technologyRoot == nil && displayName == nil && pageImages.isEmpty && customMetadata.isEmpty && callToAction == nil && availability.isEmpty && pageKind == nil && pageColor == nil && supportedLanguages.isEmpty && titleHeading == nil && redirects == nil && alternateRepresentations.isEmpty {
             let diagnostic = Diagnostic(
                 source: source,
                 severity: .information,

Tests/SwiftDocCTests/Infrastructure/DocumentationContext/DocumentationContextTests.swift

@@ -5773,6 +5773,45 @@ let expected = """
         XCTAssertEqual(solution.replacements.first?.replacement, "")
     }
 
+    func testSupportedLanguageDirectiveInNonRootArticleWithSymbols() async throws {
+        let catalog = Folder(name: "unit-test.docc", content: [
+            TextFile(name: "Root.md", utf8Content: """
+            # Root
+
+            @Metadata {
+              @TechnologyRoot
+              @SupportedLanguage(objc)
+              @SupportedLanguage(data)
+            }
+
+            ## Topics
+
+            - <doc:Article>
+            """),
+            
+            TextFile(name: "Article.md", utf8Content: """
+            # Article
+            
+            @Metadata {
+              @SupportedLanguage(objc)
+              @SupportedLanguage(data)
+            }
+            """),
+        ])
+        
+        let (bundle, context) = try await loadBundle(catalog: catalog)
+        
+        XCTAssert(context.problems.isEmpty, "Unexpected problems:\n\(context.problems.map(\.diagnostic.summary).joined(separator: "\n"))")
+
+        do {
+            let reference = ResolvedTopicReference(bundleID: bundle.id, path: "/documentation/unit-test/Article", sourceLanguage: .swift)
+            let node = try context.entity(with: reference)
+            
+            let supportedLanguages = try XCTUnwrap((node.semantic as? Article)?.metadata?.supportedLanguages)
+            XCTAssertEqual(supportedLanguages.map(\.language), [.objectiveC, .data])
+        }
+    }
+
 }
 
 func assertEqualDumps(_ lhs: String, _ rhs: String, file: StaticString = #filePath, line: UInt = #line) {

Tests/SwiftDocCTests/Semantics/ArticleTests.swift

@@ -249,4 +249,26 @@ class ArticleTests: XCTestCase {
         XCTAssertNil(semantic.metadata?.pageKind)
         XCTAssertNil(semantic.metadata?.titleHeading)
     }
+
+    func testSupportedLanguageDirective() async throws {
+        let source = """
+        # Root
+
+        @Metadata {
+          @SupportedLanguage(swift)
+          @SupportedLanguage(objc)
+          @SupportedLanguage(data)
+        }
+        """
+        let document = Document(parsing: source, options: [.parseBlockDirectives])
+        let (bundle, _) = try await testBundleAndContext()
+        var problems = [Problem]()
+        let article = Article(from: document, source: nil, for: bundle, problems: &problems)
+
+        XCTAssert(problems.isEmpty, "Unexpectedly found problems: \(DiagnosticConsoleWriter.formattedDescription(for: problems))")
+
+        XCTAssertNotNil(article)
+        XCTAssertNotNil(article?.metadata, "Article should have a metadata container since the markup has a @Metadata directive")
+        XCTAssertEqual(article?.metadata?.supportedLanguages.map(\.language), [.swift, .objectiveC, .data])
+    }
 }