fix: leaked federation subgraph types on supergraph (#230)

Prevent subgraph-specific federation types and scalars being re-declared
within the subgraph leaking into the supergraph.

https://linear.app/the-guild/issue/CONSOLE-1643

Author: n1ru4l

.changeset/tangy-walls-decide.md

@@ -0,0 +1,5 @@
+---
+"@theguild/federation-composition": patch
+---
+
+Prevent subgraph-specific federation types and scalars being re-declared within the subgraph leaking into the supergraph.

__tests__/composition.spec.ts

@@ -529,7 +529,7 @@ testImplementations((api) => {
         typeDefs: parse(/* GraphQL */ `
           type User @key(fields: "id") {
             id: ID!
-            "it should not be in supergraph"
+
             name: String! @external
           }
 
@@ -8053,4 +8053,66 @@ testImplementations((api) => {
       }
     `);
   });
+
+  test("FieldSet does not leak as external type onto supergraph if defined in subgraph", () => {
+    const result = api.composeServices([
+      {
+        typeDefs: parse(/* GraphQL */ `
+          schema
+            @link(
+              url: "https://specs.apollo.dev/federation/v2.5"
+              import: ["@key", "FieldSet"]
+            ) {
+            query: Query
+            mutation: Mutation
+          }
+
+          interface Node {
+            id: ID!
+          }
+
+          type Mutation {
+            _: String
+          }
+
+          type Query {
+            node(id: ID!): Node
+            _service: _Service!
+            _entities(representations: [_Any!]!): [_Entity]!
+          }
+
+          type User implements Node
+            @key(fields: "id")
+            @key(fields: "username") {
+            id: ID!
+            username: String
+          }
+
+          type _Service {
+            sdl: String!
+          }
+
+          union _Entity = User
+
+          directive @key(
+            fields: FieldSet!
+            resolvable: Boolean = true
+          ) repeatable on OBJECT | INTERFACE
+
+          directive @link(url: String!, import: [String!]) repeatable on SCHEMA
+
+          scalar FieldSet
+
+          scalar Long
+
+          scalar _Any
+        `),
+        name: "a",
+        url: "a",
+      },
+    ]);
+
+    assertCompositionSuccess(result);
+    expect(result.supergraphSdl).not.includes("scalar FieldSet");
+  });
 });

src/supergraph/state.ts

@@ -211,7 +211,17 @@ export function createSupergraphStateBuilder() {
         }
       }
 
+      const allFederationImports = new Set<string>(
+        subgraphState.federation.imports.map((i) => i.alias ?? i.name),
+      );
+
       for (const [typeName, type] of subgraphState.types) {
+        // Federation Types that are imported, but also declared within th subgraph (most-likely for IDE support)
+        // should not be added to the supergraph.
+        if (allFederationImports.has(typeName)) {
+          continue;
+        }
+
         switch (type.kind) {
           case "OBJECT": {
             objectType.visitSubgraphState(