[Fusion] Added pre-merge validation rule "ExternalOnInterfaceRule" (#…

…7882)

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Logging/LogEntryCodes.cs

@@ -5,6 +5,7 @@ public static class LogEntryCodes
     public const string DisallowedInaccessible = "DISALLOWED_INACCESSIBLE";
     public const string ExternalArgumentDefaultMismatch = "EXTERNAL_ARGUMENT_DEFAULT_MISMATCH";
     public const string ExternalMissingOnBase = "EXTERNAL_MISSING_ON_BASE";
+    public const string ExternalOnInterface = "EXTERNAL_ON_INTERFACE";
     public const string ExternalUnused = "EXTERNAL_UNUSED";
     public const string KeyDirectiveInFieldsArg = "KEY_DIRECTIVE_IN_FIELDS_ARG";
     public const string KeyFieldsHasArgs = "KEY_FIELDS_HAS_ARGS";

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Logging/LogEntryHelper.cs

@@ -129,6 +129,22 @@ public static LogEntry ExternalMissingOnBase(
             schema);
     }
 
+    public static LogEntry ExternalOnInterface(
+        OutputFieldDefinition externalField,
+        INamedTypeDefinition type,
+        SchemaDefinition schema)
+    {
+        var coordinate = new SchemaCoordinate(type.Name, externalField.Name);
+
+        return new LogEntry(
+            string.Format(LogEntryHelper_ExternalOnInterface, coordinate, schema.Name),
+            LogEntryCodes.ExternalOnInterface,
+            LogSeverity.Error,
+            coordinate,
+            externalField,
+            schema);
+    }
+
     public static LogEntry ExternalUnused(
         OutputFieldDefinition externalField,
         INamedTypeDefinition type,

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/PreMergeValidation/Rules/ExternalOnInterfaceRule.cs

@@ -0,0 +1,29 @@
+using HotChocolate.Fusion.Events;
+using HotChocolate.Skimmed;
+using static HotChocolate.Fusion.Logging.LogEntryHelper;
+
+namespace HotChocolate.Fusion.PreMergeValidation.Rules;
+
+/// <summary>
+/// The <c>@external</c> directive indicates that a field is defined and resolved elsewhere, not in
+/// the current schema. In the case of an interface type, fields are abstract - they do not have
+/// direct resolutions at the interface level. Instead, each implementing object type provides the
+/// concrete field implementations. Marking an interface field with <c>@external</c> is therefore
+/// nonsensical, as there is no actual field resolution in the interface itself to “borrow” from
+/// another schema. Such usage raises an <c>EXTERNAL_ON_INTERFACE</c> error.
+/// </summary>
+/// <seealso href="https://graphql.github.io/composite-schemas-spec/draft/#sec-External-on-Interface">
+/// Specification
+/// </seealso>
+internal sealed class ExternalOnInterfaceRule : IEventHandler<OutputFieldEvent>
+{
+    public void Handle(OutputFieldEvent @event, CompositionContext context)
+    {
+        var (field, type, schema) = @event;
+
+        if (type is InterfaceTypeDefinition && ValidationHelper.IsExternal(field))
+        {
+            context.Log.Write(ExternalOnInterface(field, type, schema));
+        }
+    }
+}

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Properties/CompositionResources.resx

@@ -42,6 +42,9 @@
   <data name="LogEntryHelper_ExternalMissingOnBase" xml:space="preserve">
     <value>External field '{0}' in schema '{1}' is not defined (non-external) in any other schema.</value>
   </data>
+  <data name="LogEntryHelper_ExternalOnInterface" xml:space="preserve">
+    <value>Interface field '{0}' in schema '{1}' must not be marked as external.</value>
+  </data>
   <data name="LogEntryHelper_ExternalUnused" xml:space="preserve">
     <value>External field '{0}' in schema '{1}' is not referenced by a @provides directive in the schema.</value>
   </data>

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/SourceSchemaMerger.cs

@@ -49,6 +49,7 @@ private CompositionResult<SchemaDefinition> MergeSchemaDefinitions(CompositionCo
         new DisallowedInaccessibleElementsRule(),
         new ExternalArgumentDefaultMismatchRule(),
         new ExternalMissingOnBaseRule(),
+        new ExternalOnInterfaceRule(),
         new ExternalUnusedRule(),
         new KeyDirectiveInFieldsArgumentRule(),
         new KeyFieldsHasArgumentsRule(),