add error description to mixedGraphFormats error

Author: theMomax

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

@@ -126,7 +126,21 @@ struct SymbolGraphLoader {
         let foundGraphNotUsingExtensionSymbolFormat = loadedGraphs.values.map(\.usesExtensionSymbolFormat).contains(false)
 
         guard !foundGraphUsingExtensionSymbolFormat || !foundGraphNotUsingExtensionSymbolFormat else {
-            throw LoadingError.mixedGraphFormats
+            let usingExtensionSymbolFormat = loadedGraphs.filter { (_, value) in
+                guard let usesExtensionSymbolFormat = value.usesExtensionSymbolFormat else {
+                    return false
+                }
+                return usesExtensionSymbolFormat
+            }.map(\.key)
+            
+            let notUsingExtensionSymbolFormat = loadedGraphs.filter { (_, value) in
+                guard let usesExtensionSymbolFormat = value.usesExtensionSymbolFormat else {
+                    return false
+                }
+                return !usesExtensionSymbolFormat
+            }.map(\.key)
+            
+            throw LoadingError.mixedGraphFormats(usingExtensionSymbolFormat, notUsingExtensionSymbolFormat)
         }
 
         let usingExtensionSymbolFormat = foundGraphUsingExtensionSymbolFormat
@@ -148,8 +162,22 @@ struct SymbolGraphLoader {
         (self.unifiedGraphs, self.graphLocations) = graphLoader.finishLoading()
     }
 
-    private enum LoadingError: Error {
-        case mixedGraphFormats
+    private enum LoadingError: DescribedError {
+        case mixedGraphFormats([URL], [URL])
+        
+        var errorDescription: String {
+            switch self {
+            case let .mixedGraphFormats(usingExtensionSymbolFormat, notUsingExtensionSymbolFormat):
+                return """
+                Mixing symbol graph files with and without extension declarations is not supported.
+                
+                Symbol graph files with extension declarations:
+                \(usingExtensionSymbolFormat.map(\.absoluteString).joined(separator: "\n"))
+                Symbol graph files without extension declarations:
+                \(notUsingExtensionSymbolFormat.map(\.absoluteString).joined(separator: "\n"))
+                """
+            }
+        }
     }
 
     // Alias to declutter code

Tests/SwiftDocCTests/Infrastructure/SymbolGraph/SymbolGraphLoaderTests.swift

@@ -436,10 +436,19 @@ class SymbolGraphLoaderTests: XCTestCase {
         let invalid = allGraphs
         loader = try makeSymbolGraphLoader(symbolGraphURLs: invalid.map(\.url))
         // found non-empty extension graphs with and without extension block symbols -> should throw
-        do {
-            try loader.loadAll()
-            XCTFail("SymbolGraphLoader should throw when encountering a collection of symbol graph files, where some do and some don't use the extension block format")
-        } catch {}
+        XCTAssertThrowsError(try loader.loadAll(),
+                             "SymbolGraphLoader should throw when encountering a collection of symbol graph files, where some do and some don't use the extension block format") { error in
+            XCTAssertFalse(error.localizedDescription.contains(mainGraph.url.absoluteString))
+            XCTAssertFalse(error.localizedDescription.contains(emptyExtensionGraph.url.absoluteString))
+            XCTAssert(error.localizedDescription.contains("""
+            Symbol graph files with extension declarations:
+            \(extensionBlockFormatExtensionGraph.url.absoluteString)
+            """))
+            XCTAssert(error.localizedDescription.contains("""
+            Symbol graph files without extension declarations:
+            \(noExtensionBlockFormatExtensionGraph.url.absoluteString)
+            """))
+        }
     }
 
     // MARK: - Helpers