[Fusion] Added pre-merge validation rule "InvalidFieldSharingRule" (#8738)

Author: glen-84

src/HotChocolate/AspNetCore/benchmarks/k6/performance-data.json

@@ -1,20 +1,20 @@
 {
-  "timestamp": "2025-10-28T10:35:15Z",
+  "timestamp": "2025-10-28T10:49:37Z",
   "tests": {
     "single-fetch": {
       "name": "Single Fetch (50 products, names only)",
       "response_time": {
-        "min": 1.367661,
-        "p50": 2.180789,
-        "max": 41.619313,
-        "avg": 2.428901219293327,
-        "p90": 3.533614900000001,
-        "p95": 4.113232549999999,
-        "p99": 6.6563870399999745
+        "min": 1.350863,
+        "p50": 2.2166075000000003,
+        "max": 47.099152,
+        "avg": 2.5468350569105658,
+        "p90": 3.8372352,
+        "p95": 4.785266199999996,
+        "p99": 7.70086295
       },
       "throughput": {
-        "requests_per_second": 78.71140999724626,
-        "total_iterations": 7163
+        "requests_per_second": 78.75998155086224,
+        "total_iterations": 7165
       },
       "reliability": {
         "error_rate": 0
@@ -23,17 +23,17 @@
     "dataloader": {
       "name": "DataLoader (50 products with brands)",
       "response_time": {
-        "min": 2.652555,
-        "p50": 4.058844000000001,
-        "max": 19.402633,
-        "avg": 4.470873844347582,
-        "p90": 6.1823778,
-        "p95": 7.565768349999999,
-        "p99": 11.076760349999999
+        "min": 2.633183,
+        "p50": 3.997002,
+        "max": 18.020958,
+        "avg": 4.356490303162331,
+        "p90": 5.9975004,
+        "p95": 7.313034499999997,
+        "p99": 11.124407259999987
       },
       "throughput": {
-        "requests_per_second": 78.4977656947644,
-        "total_iterations": 7143
+        "requests_per_second": 78.52036814348355,
+        "total_iterations": 7146
       },
       "reliability": {
         "error_rate": 0

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Events/GroupEvents.cs

@@ -23,6 +23,11 @@ internal record InputTypeGroupEvent(
     string InputTypeName,
     ImmutableArray<InputTypeInfo> InputTypeGroup) : IEvent;
 
+internal record ObjectFieldGroupEvent(
+    string FieldName,
+    ImmutableArray<ObjectFieldInfo> FieldGroup,
+    string TypeName) : IEvent;
+
 internal record OutputFieldGroupEvent(
     string FieldName,
     ImmutableArray<OutputFieldInfo> FieldGroup,

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Extensions/MutableComplexTypeDefinitionExtensions.cs

@@ -24,4 +24,9 @@ public static void ApplyKeyDirective(this MutableComplexTypeDefinition type, str
                     new ArgumentAssignment(ArgumentNames.Fields, keyFields)));
         }
     }
+
+    public static IEnumerable<Directive> GetKeyDirectives(this MutableComplexTypeDefinition type)
+    {
+        return type.Directives.AsEnumerable().Where(d => d.Name == DirectiveNames.Key);
+    }
 }

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Extensions/MutableOutputFieldDefinitionExtensions.cs

@@ -96,6 +96,14 @@ public static string GetKeyFields(
         return mergedSelectionSet.ToString(indented: false).AsSpan()[2..^2].ToString();
     }
 
+    public static string? GetOverrideFrom(this MutableOutputFieldDefinition field)
+    {
+        var overrideDirective =
+            field.Directives.AsEnumerable().SingleOrDefault(d => d.Name == DirectiveNames.Override);
+
+        return (string?)overrideDirective?.Arguments[ArgumentNames.From].Value;
+    }
+
     public static bool HasInternalDirective(this MutableOutputFieldDefinition field)
     {
         return field.Directives.ContainsName(DirectiveNames.Internal);

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Extensions/OutputFieldDefinitionExtensions.cs

@@ -1,4 +1,5 @@
 using System.Collections.Immutable;
+using HotChocolate.Features;
 using HotChocolate.Fusion.Features;
 using HotChocolate.Types;
 using static HotChocolate.Fusion.WellKnownArgumentNames;
@@ -26,8 +27,9 @@ public static ImmutableArray<string> GetSchemaNames(
         return [.. schemaNames];
     }
 
-    public static SourceFieldMetadata? GetSourceFieldMetadata(this IOutputFieldDefinition field)
+    public static SourceFieldMetadata GetRequiredSourceFieldMetadata(
+        this IOutputFieldDefinition field)
     {
-        return field.Features.Get<SourceFieldMetadata>();
+        return field.Features.GetRequired<SourceFieldMetadata>();
     }
 }

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Extensions/OutputFieldInfoExtensions.cs

@@ -2,7 +2,15 @@ namespace HotChocolate.Fusion.Features;
 
 internal sealed class SourceFieldMetadata
 {
+    public bool HasShareableDirective { get; set; }
+
+    public bool IsExternal { get; set; }
+
+    public bool IsInternal { get; set; }
+
     public bool IsKeyField { get; set; }
 
+    public bool IsOverridden { get; set; }
+
     public bool IsShareable { get; set; }
 }

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Features/SourceFieldMetadata.cs

@@ -0,0 +1,8 @@
+using HotChocolate.Types.Mutable;
+
+namespace HotChocolate.Fusion.Info;
+
+internal record ObjectFieldInfo(
+    MutableOutputFieldDefinition Field,
+    MutableObjectTypeDefinition Type,
+    MutableSchemaDefinition Schema);

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Info/ObjectFieldInfo.cs

@@ -28,6 +28,7 @@ public static class LogEntryCodes
     public const string InputFieldTypesNotMergeable = "INPUT_FIELD_TYPES_NOT_MERGEABLE";
     public const string InputWithMissingRequiredFields = "INPUT_WITH_MISSING_REQUIRED_FIELDS";
     public const string InterfaceFieldNoImplementation = "INTERFACE_FIELD_NO_IMPLEMENTATION";
+    public const string InvalidFieldSharing = "INVALID_FIELD_SHARING";
     public const string InvalidGraphQL = "INVALID_GRAPHQL";
     public const string InvalidShareableUsage = "INVALID_SHAREABLE_USAGE";
     public const string IsInvalidFields = "IS_INVALID_FIELDS";

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

@@ -502,6 +502,22 @@ public static LogEntry InterfaceFieldNoImplementation(
             .Build();
     }
 
+    public static LogEntry InvalidFieldSharing(
+        MutableOutputFieldDefinition field,
+        MutableSchemaDefinition schema)
+    {
+        return LogEntryBuilder.New()
+            .SetMessage(
+                LogEntryHelper_InvalidFieldSharing,
+                field.Coordinate.ToString(),
+                schema.Name)
+            .SetCode(LogEntryCodes.InvalidFieldSharing)
+            .SetSeverity(LogSeverity.Error)
+            .SetTypeSystemMember(field)
+            .SetSchema(schema)
+            .Build();
+    }
+
     public static LogEntry InvalidGraphQL(string exceptionMessage, MutableSchemaDefinition schema)
     {
         return LogEntryBuilder.New()