Allow processing symbol graphs of unknown languages

DocC currently has a set of 'known' programming languages for which it
has display names, and loading symbol graphs currently requires the
symbols' language to be known by DocC. There's no reason to not process
allow processing symbols of unknown language though, and this fix makes
DocC more flexible in that respect. The end-to-end compilation flow
works fine with the change.

Author: franklinsch

Sources/SwiftDocC/Infrastructure/Symbol Graph/SymbolReference.swift

@@ -182,8 +182,9 @@ extension UnifiedSymbolGraph.Symbol {
         // Adding a dedicated SymbolKit API for this purpose is tracked
         // with github.com/apple/swift-docc-symbolkit/issues/32 and rdar://85982095.
         return Set(
-            pathComponents.keys.compactMap { selector in
-                return SourceLanguage(knownLanguageIdentifier: selector.interfaceLanguage)
+            pathComponents.keys.map { selector in
+                SourceLanguage(knownLanguageIdentifier: selector.interfaceLanguage)
+                    ?? SourceLanguage(id: selector.interfaceLanguage)
             }
         )
     }

Tests/SwiftDocCTests/Infrastructure/SymbolReferenceTests.swift

@@ -216,4 +216,52 @@ class SymbolReferenceTests: XCTestCase {
             ]
         )
     }
+    
+    func testKnownSourceLanguagesOfUnifiedGraphSymbol() {
+        let module = SymbolGraph.Module(
+            name: "os",
+            platform: SymbolGraph.Platform()
+        )
+        
+        let unifiedSymbol = UnifiedSymbolGraph.Symbol(
+            fromSingleSymbol: makeSymbol(interfaceLanguage: "swift"),
+            module: module,
+            isMainGraph: true
+        )
+        
+        unifiedSymbol.mergeSymbol(
+            symbol: makeSymbol(interfaceLanguage: "c"),
+            module: module,
+            isMainGraph: true
+        )
+        
+        XCTAssertEqual(unifiedSymbol.sourceLanguages.map(\.id).sorted(), ["occ", "swift"])
+    }
+    
+    func testUnknownSourceLanguagesOfUnifiedGraphSymbol() {
+        let module = SymbolGraph.Module(
+            name: "os",
+            platform: SymbolGraph.Platform()
+        )
+        
+        let unifiedSymbol = UnifiedSymbolGraph.Symbol(
+            fromSingleSymbol: makeSymbol(interfaceLanguage: "unknown-language"),
+            module: module,
+            isMainGraph: true
+        )
+        
+        XCTAssertEqual(unifiedSymbol.sourceLanguages.map(\.id).sorted(), ["unknown-language"])
+    }
+    
+    private func makeSymbol(interfaceLanguage: String) -> SymbolGraph.Symbol {
+        SymbolGraph.Symbol(
+            identifier: .init(precise: "abcd", interfaceLanguage: interfaceLanguage),
+            names: .init(title: "abcd", navigator: nil, subHeading: nil, prose: nil),
+            pathComponents: ["test", "abcd"],
+            docComment: nil,
+            accessLevel: .init(rawValue: "public"),
+            kind: .init(parsedIdentifier: .module, displayName: "Framework"),
+            mixins: [:]
+        )
+    }
 }