[Fusion] Added pre-merge validation rule "ProvidesOnNonCompositeField…

…Rule" (#7883)

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

@@ -16,6 +16,7 @@ public static class LogEntryCodes
     public const string ProvidesDirectiveInFieldsArg = "PROVIDES_DIRECTIVE_IN_FIELDS_ARG";
     public const string ProvidesFieldsHasArgs = "PROVIDES_FIELDS_HAS_ARGS";
     public const string ProvidesFieldsMissingExternal = "PROVIDES_FIELDS_MISSING_EXTERNAL";
+    public const string ProvidesOnNonCompositeField = "PROVIDES_ON_NON_COMPOSITE_FIELD";
     public const string QueryRootTypeInaccessible = "QUERY_ROOT_TYPE_INACCESSIBLE";
     public const string RootMutationUsed = "ROOT_MUTATION_USED";
     public const string RootQueryUsed = "ROOT_QUERY_USED";

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

@@ -346,6 +346,25 @@ public static LogEntry ProvidesFieldsMissingExternal(
             schema);
     }
 
+    public static LogEntry ProvidesOnNonCompositeField(
+        OutputFieldDefinition field,
+        INamedTypeDefinition type,
+        SchemaDefinition schema)
+    {
+        var coordinate = new SchemaCoordinate(type.Name, field.Name);
+
+        return new LogEntry(
+            string.Format(
+                LogEntryHelper_ProvidesOnNonCompositeField,
+                coordinate,
+                schema.Name),
+            LogEntryCodes.ProvidesOnNonCompositeField,
+            LogSeverity.Error,
+            coordinate,
+            field,
+            schema);
+    }
+
     public static LogEntry QueryRootTypeInaccessible(
         INamedTypeDefinition type,
         SchemaDefinition schema)

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

@@ -0,0 +1,34 @@
+using HotChocolate.Fusion.Events;
+using HotChocolate.Fusion.Extensions;
+using HotChocolate.Skimmed;
+using static HotChocolate.Fusion.Logging.LogEntryHelper;
+
+namespace HotChocolate.Fusion.PreMergeValidation.Rules;
+
+/// <summary>
+/// The <c>@provides</c> directive allows a field to “provide” additional nested fields on the
+/// composite type it returns. If a field’s base type is not an object or interface type (e.g.,
+/// String, Int, Boolean, Enum, Union, or an Input type), it cannot hold nested fields for
+/// <c>@provides</c> to select. Consequently, attaching <c>@provides</c> to such a field is
+/// invalid and raises a PROVIDES_ON_NON_OBJECT_FIELD error.
+/// </summary>
+/// <seealso href="https://graphql.github.io/composite-schemas-spec/draft/#sec-Provides-on-Non-Composite-Field">
+/// Specification
+/// </seealso>
+internal sealed class ProvidesOnNonCompositeFieldRule : IEventHandler<OutputFieldEvent>
+{
+    public void Handle(OutputFieldEvent @event, CompositionContext context)
+    {
+        var (field, type, schema) = @event;
+
+        if (ValidationHelper.HasProvidesDirective(field))
+        {
+            var fieldType = field.Type.NamedType();
+
+            if (fieldType is not ComplexTypeDefinition)
+            {
+                context.Log.Write(ProvidesOnNonCompositeField(field, type, schema));
+            }
+        }
+    }
+}

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

@@ -75,6 +75,9 @@
   <data name="LogEntryHelper_ProvidesFieldsMissingExternal" xml:space="preserve">
     <value>The @provides directive on field '{0}' in schema '{1}' references field '{2}', which must be marked as external.</value>
   </data>
+  <data name="LogEntryHelper_ProvidesOnNonCompositeField" xml:space="preserve">
+    <value>The field '{0}' in schema '{1}' includes a @provides directive, but does not return a composite type.</value>
+  </data>
   <data name="LogEntryHelper_QueryRootTypeInaccessible" xml:space="preserve">
     <value>The root query type in schema '{0}' must be accessible.</value>
   </data>

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

@@ -60,6 +60,7 @@ private CompositionResult<SchemaDefinition> MergeSchemaDefinitions(CompositionCo
         new ProvidesDirectiveInFieldsArgumentRule(),
         new ProvidesFieldsHasArgumentsRule(),
         new ProvidesFieldsMissingExternalRule(),
+        new ProvidesOnNonCompositeFieldRule(),
         new QueryRootTypeInaccessibleRule(),
         new RootMutationUsedRule(),
         new RootQueryUsedRule(),

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

@@ -5,6 +5,11 @@ namespace HotChocolate.Fusion;
 
 internal sealed class ValidationHelper
 {
+    public static bool HasProvidesDirective(IDirectivesProvider type)
+    {
+        return type.Directives.ContainsName(WellKnownDirectiveNames.Provides);
+    }
+
     public static bool IsAccessible(IDirectivesProvider type)
     {
         return !type.Directives.ContainsName(WellKnownDirectiveNames.Inaccessible);