Improve unsupported DisplayList content error diagnostics

Sources/DisplayListDescription/DisplayListDescriptionConverter.swift

@@ -415,8 +415,17 @@ private struct Renderer {
         let children = directLists(in: item)
         if children.contains(where: { $0.head == "content-seed" }) {
             guard let content = children.first(where: { ContentKind(head: $0.head) != nil }) else {
+                if let unsupportedHead = children.first(where: {
+                    $0.head != "frame" && $0.head != "content-seed"
+                })?.head {
+                    var message = "Item identity \(identity) contains unsupported DisplayList content “\(unsupportedHead)”."
+                    if let suggestion = ContentKind.suggestedHead(for: unsupportedHead) {
+                        message += " Did you mean “\(suggestion)”?"
+                    }
+                    throw DisplayListDescriptionError.unsupported(message: message)
+                }
                 throw DisplayListDescriptionError.unsupported(
-                    message: "This item contains content-seed but no supported content value."
+                    message: "Item identity \(identity) contains content-seed but no content value."
                 )
             }
             rendered.include(try renderContent(content))
@@ -647,6 +656,15 @@ private enum ContentKind {
     case view
     case placeholder
 
+    static func suggestedHead(for head: String) -> String? {
+        switch head {
+        case "chameleonColor": "chameleon-color"
+        case "platformView": "platform-view"
+        case "platformLayer": "platform-layer"
+        default: nil
+        }
+    }
+
     init?(head: String?) {
         switch head {
         case "backdrop": self = .backdrop

Tests/DisplayListDescriptionTests/DisplayListDescriptionConverterTests.swift

@@ -207,6 +207,25 @@ final class DisplayListDescriptionConverterTests: XCTestCase {
         }
     }
 
+    func testReportsUnsupportedContentNameAndSuggestedSpelling() {
+        let description = """
+        (display-list
+          (item #:identity 4 #:version 2
+            (frame {{12, 8.3333333333333339}, {164, 40}})
+            (content-seed 5)
+            (platformView DemoPlatformViewFactory())))
+        """
+
+        XCTAssertThrowsError(try DisplayListDescriptionConverter.convert(description)) { error in
+            XCTAssertEqual(
+                error as? DisplayListDescriptionError,
+                .unsupported(
+                    message: "Item identity 4 contains unsupported DisplayList content “platformView”. Did you mean “platform-view”?"
+                )
+            )
+        }
+    }
+
     private func utf16Slice(_ text: String, _ start: Int, _ end: Int) -> String {
         let codeUnits = Array(text.utf16)
         return String(decoding: codeUnits[start..<end], as: UTF16.self)