Improve the message for empty_struct

Change-Id: I17b1fb652234016bbdb94f7c813e24ea8890cd85
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/216120
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson

pkg/analyzer/lib/src/dart/error/ffi_code.g.dart

@@ -46,12 +46,14 @@ class FfiCode extends AnalyzerErrorCode {
 
   /**
    * Parameters:
-   * 0: the name of the struct class
+   * 0: the name of the subclass
+   * 1: the name of the superclass
    */
   static const FfiCode EMPTY_STRUCT = FfiCode(
     'EMPTY_STRUCT',
-    "Struct '{0}' is empty. Empty structs are undefined behavior.",
-    correctionMessage: "Try adding a field to '{0}' or use a different Struct.",
+    "The class '{0}' can’t be empty because it's a subclass of '{1}'.",
+    correctionMessage:
+        "Try adding a field to '{0}' or use a different superclass.",
   );
 
   /**

pkg/analyzer/lib/src/generated/ffi_verifier.dart

@@ -78,7 +78,7 @@ class FfiVerifier extends RecursiveAstVisitor<void> {
           compound = node;
           if (node.declaredElement!.isEmptyStruct) {
             _errorReporter.reportErrorForNode(
-                FfiCode.EMPTY_STRUCT, node.name, [node.name.name]);
+                FfiCode.EMPTY_STRUCT, node.name, [node.name.name, className]);
           }
           if (className == _structClassName) {
             _validatePackedAnnotation(node.metadata);
@@ -722,10 +722,10 @@ class FfiVerifier extends RecursiveAstVisitor<void> {
       } else if (declaredType.isCompoundSubtype) {
         final clazz = (declaredType as InterfaceType).element;
         if (clazz.isEmptyStruct) {
-          // TODO(brianwilkerson) There are no tests for this branch. Ensure
-          //  that the diagnostic is correct and add tests.
-          _errorReporter
-              .reportErrorForNode(FfiCode.EMPTY_STRUCT, node, [clazz.name]);
+          _errorReporter.reportErrorForNode(FfiCode.EMPTY_STRUCT, node, [
+            clazz.name,
+            clazz.supertype!.getDisplayString(withNullability: false)
+          ]);
         }
         _validatePackingNesting(compound!.declaredElement!, clazz,
             errorNode: fieldType);

pkg/analyzer/messages.yaml

@@ -13746,11 +13746,12 @@ FfiCode:
     correctionMessage: "Try allocating it via allocation, or load from a 'Pointer'."
     comment: No parameters.
   EMPTY_STRUCT:
-    problemMessage: "Struct '{0}' is empty. Empty structs are undefined behavior."
-    correctionMessage: "Try adding a field to '{0}' or use a different Struct."
+    problemMessage: "The class '{0}' can’t be empty because it's a subclass of '{1}'."
+    correctionMessage: "Try adding a field to '{0}' or use a different superclass."
     comment: |-
       Parameters:
-      0: the name of the struct class
+      0: the name of the subclass
+      1: the name of the superclass
   EXTRA_ANNOTATION_ON_STRUCT_FIELD:
     problemMessage: Fields in a struct class must have exactly one annotation indicating the native type.
     correctionMessage: Try removing the extra annotation.