frontend-plugin-api: remove type support for multiple attachment points

This is a follow-up to the deprecation in #32521, completely removing
the Array variant from the ExtensionDefinitionAttachTo type.

The runtime still supports multiple attachment points for backward
compatibility, but new code will receive type errors.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/frontend-plugin-api/report.api.md

@@ -1205,26 +1205,7 @@ export type ExtensionDefinitionAttachTo<
       input: string;
       id?: never;
     }
-  | ExtensionInput<UParentInputs>
-  /**
-   * @deprecated Multiple attachment points are deprecated and will be removed in a future release. Use a Utility API instead to share functionality across multiple locations. See https://backstage.io/docs/frontend-system/architecture/27-sharing-extensions for migration guidance.
-   */
-  | Array<
-      | {
-          id: string;
-          input: string;
-          relative?: never;
-        }
-      | {
-          relative: {
-            kind?: string;
-            name?: string;
-          };
-          input: string;
-          id?: never;
-        }
-      | ExtensionInput<UParentInputs>
-    >;
+  | ExtensionInput<UParentInputs>;
 
 // @public (undocumented)
 export type ExtensionDefinitionParameters = {

packages/frontend-plugin-api/src/blueprints/AnalyticsImplementationBlueprint.test.ts

@@ -32,12 +32,10 @@ describe('AnalyticsBlueprint', () => {
       {
         "$$type": "@backstage/ExtensionDefinition",
         "T": undefined,
-        "attachTo": [
-          {
-            "id": "api:app/analytics",
-            "input": "implementations",
-          },
-        ],
+        "attachTo": {
+          "id": "api:app/analytics",
+          "input": "implementations",
+        },
         "configSchema": undefined,
         "disabled": false,
         "factory": [Function],

packages/frontend-plugin-api/src/blueprints/AnalyticsImplementationBlueprint.ts

@@ -41,7 +41,7 @@ const factoryDataRef =
  */
 export const AnalyticsImplementationBlueprint = createExtensionBlueprint({
   kind: 'analytics',
-  attachTo: [{ id: 'api:app/analytics', input: 'implementations' }],
+  attachTo: { id: 'api:app/analytics', input: 'implementations' },
   output: [factoryDataRef],
   dataRefs: {
     factory: factoryDataRef,

packages/frontend-plugin-api/src/wiring/createExtension.test.ts

@@ -202,12 +202,14 @@ describe('createExtension', () => {
     });
   });
 
-  it('should create an extension with multiple attachment points', () => {
+  // Tests for backward compatibility - runtime still supports multiple attachment points
+  // but the TypeScript types no longer allow them
+  it('should create an extension with multiple attachment points (backward compat)', () => {
     const extension = createExtension({
       attachTo: [
         { id: 'root', input: 'default' },
         { id: 'other', input: 'default' },
-      ],
+      ] as any,
       output: [stringDataRef, numberDataRef.optional()],
       factory: () => [stringDataRef('bar')],
     });
@@ -216,14 +218,14 @@ describe('createExtension', () => {
     );
   });
 
-  it('should create an extension with relative attachment points', () => {
+  it('should create an extension with relative attachment points (backward compat)', () => {
     const extension = createExtension({
       attachTo: [
         { relative: {}, input: 'tabs' },
         { relative: { kind: 'page' }, input: 'tabs' },
         { relative: { name: 'index' }, input: 'tabs' },
         { relative: { kind: 'page', name: 'index' }, input: 'tabs' },
-      ],
+      ] as any,
       output: [stringDataRef],
       factory: () => [stringDataRef('bar')],
     });
@@ -232,7 +234,7 @@ describe('createExtension', () => {
     );
   });
 
-  it('should create an extension with relative attachment points by reference', () => {
+  it('should create an extension with relative attachment points by reference (backward compat)', () => {
     const baseOpts = {
       attachTo: { id: 'root', input: 'children' },
       inputs: {
@@ -269,7 +271,7 @@ describe('createExtension', () => {
         parent2.inputs.tabs,
         parent3.inputs.tabs,
         parent4.inputs.otherTabs,
-      ],
+      ] as any,
       output: [stringDataRef],
       factory: () => [stringDataRef('bar')],
     });
@@ -281,7 +283,7 @@ describe('createExtension', () => {
         parent2.inputs.tabs,
         parent3.inputs.tabs,
         parent4.inputs.otherTabs,
-      ],
+      ] as any,
     });
     expect(String(overrdeExtension)).toBe(
       'ExtensionDefinition{attachTo=page:<plugin>@tabs+<plugin>/index@tabs+page:<plugin>/index@otherTabs}',

packages/frontend-plugin-api/src/wiring/createExtension.ts

@@ -134,11 +134,10 @@ export type VerifyExtensionAttachTo<
  *
  * A standard attachment point declaration will specify the ID of the parent extension, as well as the name of the input to attach to.
  *
- * There are three more advanced forms that are available for more complex use-cases:
+ * There are two more advanced forms that are available for more complex use-cases:
  *
  * 1. Relative attachment points: using the `relative` property instead of `id`, the attachment point is resolved relative to the current plugin.
  * 2. Extension input references: using a reference in code to another extension's input in the same plugin. These references are always relative.
- * 3. Array of attachment points: an array of attachment points can be used to clone and attach to multiple extensions at once.
  *
  * @example
  * ```ts
@@ -151,12 +150,6 @@ export type VerifyExtensionAttachTo<
  * // Attach to a specific input of another extension
  * const page = ParentBlueprint.make({ ... });
  * const child = ChildBlueprint.make({ attachTo: page.inputs.children });
- *
- * // Attach to multiple parents at once (deprecated - use Utility APIs instead)
- * [
- *   { id: 'page/home', input: 'widgets' },
- *   { relative: { kind: 'page' }, input: 'widgets' },
- * ]
  * ```
  *
  * @public
@@ -166,19 +159,7 @@ export type ExtensionDefinitionAttachTo<
 > =
   | { id: string; input: string; relative?: never }
   | { relative: { kind?: string; name?: string }; input: string; id?: never }
-  | ExtensionInput<UParentInputs>
-  /**
-   * @deprecated Multiple attachment points are deprecated and will be removed in a future release. Use a Utility API instead to share functionality across multiple locations. See https://backstage.io/docs/frontend-system/architecture/27-sharing-extensions for migration guidance.
-   */
-  | Array<
-      | { id: string; input: string; relative?: never }
-      | {
-          relative: { kind?: string; name?: string };
-          input: string;
-          id?: never;
-        }
-      | ExtensionInput<UParentInputs>
-    >;
+  | ExtensionInput<UParentInputs>;
 
 /** @public */
 export type CreateExtensionOptions<

packages/frontend-plugin-api/src/wiring/createExtensionBlueprint.test.tsx

@@ -92,11 +92,7 @@ describe('createExtensionBlueprint', () => {
   it('should allow creation of extension blueprints with a generator', () => {
     const TestExtensionBlueprint = createExtensionBlueprint({
       kind: 'test-extension',
-      // Try multiple attachment points for this one
-      attachTo: [
-        { id: 'test-1', input: 'default' },
-        { id: 'test-2', input: 'default' },
-      ],
+      attachTo: { id: 'test-1', input: 'default' },
       output: [coreExtensionData.reactElement],
       *factory(params: { text: string }) {
         yield coreExtensionData.reactElement(<h1>{params.text}</h1>);
@@ -112,10 +108,7 @@ describe('createExtensionBlueprint', () => {
 
     expect(extension).toEqual({
       $$type: '@backstage/ExtensionDefinition',
-      attachTo: [
-        { id: 'test-1', input: 'default' },
-        { id: 'test-2', input: 'default' },
-      ],
+      attachTo: { id: 'test-1', input: 'default' },
       configSchema: undefined,
       disabled: false,
       inputs: {},
@@ -1345,7 +1338,9 @@ describe('createExtensionBlueprint', () => {
     ).toThrow('Refused to override params and factory at the same time');
   });
 
-  describe('with relative attachment points', () => {
+  // Tests for backward compatibility - runtime still supports multiple attachment points
+  // but the TypeScript types no longer allow them
+  describe('with relative attachment points (backward compat)', () => {
     const dataRef = createExtensionDataRef<string>().with({ id: 'test.data' });
 
     it('should create an extension with relative attachment points', () => {
@@ -1356,7 +1351,7 @@ describe('createExtensionBlueprint', () => {
           { relative: { kind: 'page' }, input: 'tabs' },
           { relative: { name: 'index' }, input: 'tabs' },
           { relative: { kind: 'page', name: 'index' }, input: 'tabs' },
-        ],
+        ] as any,
         output: [dataRef],
         factory: () => [dataRef('bar')],
       });
@@ -1371,7 +1366,7 @@ describe('createExtensionBlueprint', () => {
               { relative: { kind: 'page' }, input: 'tabs' },
               { relative: { name: 'index' }, input: 'tabs' },
               { relative: { kind: 'page', name: 'index' }, input: 'tabs' },
-            ],
+            ] as any,
             params: {},
           }),
         ),
@@ -1431,7 +1426,7 @@ describe('createExtensionBlueprint', () => {
           parent2.inputs.tabs,
           parent3.inputs.tabs,
           parent4.inputs.otherTabs,
-        ],
+        ] as any,
         output: [dataRef],
         factory: () => [dataRef('bar')],
       });
@@ -1445,7 +1440,7 @@ describe('createExtensionBlueprint', () => {
               parent2.inputs.tabs,
               parent3.inputs.tabs,
               parent4.inputs.otherTabs,
-            ],
+            ] as any,
             params: {},
           }),
         ),

packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.test.ts

@@ -140,6 +140,7 @@ describe('resolveExtensionDefinition', () => {
       input: 'children',
     });
 
+    // Test for backward compatibility - runtime still supports multiple attachment points
     expect(
       resolveExtensionDefinition(
         OpaqueExtensionDefinition.toInternal({
@@ -157,7 +158,7 @@ describe('resolveExtensionDefinition', () => {
               kind: 'k3',
               input: 'children',
             }),
-          ],
+          ] as any,
         }),
         { namespace: 'test' },
       ).attachTo,

packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts

@@ -149,11 +149,11 @@ function resolveExtensionId(
 }
 
 function resolveAttachTo(
-  attachTo: ExtensionDefinitionAttachTo,
+  attachTo: ExtensionDefinitionAttachTo | ExtensionDefinitionAttachTo[],
   namespace?: string,
 ): ExtensionAttachToSpec {
   const resolveSpec = (
-    spec: Exclude<ExtensionDefinitionAttachTo, Array<any>>,
+    spec: ExtensionDefinitionAttachTo,
   ): { id: string; input: string } => {
     if (OpaqueExtensionInput.isType(spec)) {
       const { context } = OpaqueExtensionInput.toInternal(spec);