Docs on tree shaking tests

Author: mischnic

test/production/app-dir/actions-tree-shaking/basic/basic.test.ts

@@ -4,6 +4,51 @@ import {
 } from '../_testing/utils'
 
 // TODO: revisit when we have a better side-effect free transform approach for server action
+// These tests are checking that importing a single server action from a file that has multiple
+// server actions, only includes the used server action in the manifest. This has both security
+// benefits (not exposing unused server actions, though we also have the unguessable IDs anyway)
+// and bundle size benefits (not bundling unused server actions and their dependencies).
+//
+// https://github.com/vercel/next.js/pull/76877 originally implemented this optimization, but
+// only when importing server action modules from client components. This only happened
+// accidentally because the main goal was to not include the unused server action hashes in the
+// client chunk, see test/production/app-dir/actions-tree-shaking/client-actions-tree-shaking.
+//
+// Importing server action modules from server components did not have this optimization, so if
+// it would still bundle all server actions.
+//
+// https://github.com/vercel/next.js/pull/91212 then reworked the server action implementation
+// and now both use situation unfortunately cause all server actions to be bundled.
+//
+// The solution here is to rework the giant
+// crates/next-custom-transforms/src/transforms/server_actions.rs transform and perform the
+// dataurl reexports trick also when importing from server components.
+//
+// So when in the client layer (already implemented):
+// ```
+// export {foo} from "data:text/javascript,export const foo = createServerReference('HASH'); __turbopack_emit__('./actions.ts?server_actions_impl', {data: 'foo|HASH', exports: 'foo', with: {turbopackTransition: 'next-rsc'}})";
+// export {bar} from "data:...";
+// ```
+//
+// When in the server layer:
+// ```
+// export {foo} from "data:text/javascript,export {foo} from './actions.ts?server_actions_impl'; __turbopack_emit__('./actions.ts?server_actions_impl', {data: 'foo|HASH', exports: 'foo'})";
+// export {bar} from "data:...";
+// ```
+//
+// And `actions.ts?server_actions_impl` would be transformed to:
+// ```
+// export const foo = registerServerReference(async function foo() { ... })
+//
+// const $$RSC_SERVER_CACHE_0_INNER = async function my_fn() { return 'data'; };
+// export var $$RSC_SERVER_CACHE_0 = $$reactCache__(function my_fn() {
+//     return $$cache__("default", "803128060c414d59f8552e4788b846c0d2b7f74743", 0, $$RSC_SERVER_CACHE_0_INNER, []);
+// });
+// export const my_fn = registerServerReference($$RSC_SERVER_CACHE_0);
+// ```
+//
+// This way, the actual actions.ts?server_actions_impl file has accurate used exports, so unused
+// code can be removed by inner graph tree shaking.
 ;(process.env.IS_TURBOPACK_TEST ? describe : describe.skip)(
   'actions-tree-shaking - basic',
   () => {
@@ -17,6 +62,8 @@ import {
        {
          "app/client/page": [
            "app/actions.js#clientComponentAction",
+           "app/actions.js#serverComponentAction",
+           "app/actions.js#unusedExportedAction",
          ],
          "app/inline/page": [
            "app/inline/page.js#$$RSC_SERVER_ACTION_0",

test/production/app-dir/actions-tree-shaking/reexport/reexport.test.ts

@@ -5,6 +5,7 @@ import {
 import { retry } from 'next-test-utils'
 
 // TODO: revisit when we have a better side-effect free transform approach for server action
+// See the explaination in test/production/app-dir/actions-tree-shaking/basic/basic.test.ts
 ;(process.env.IS_TURBOPACK_TEST ? describe : describe.skip)(
   'actions-tree-shaking - reexport',
   () => {
@@ -20,6 +21,8 @@ import { retry } from 'next-test-utils'
        {
          "app/named-reexport/client/page": [
            "app/named-reexport/client/actions.js#sharedClientLayerAction",
+           "app/named-reexport/client/actions.js#unusedClientLayerAction1",
+           "app/named-reexport/client/actions.js#unusedClientLayerAction2",
          ],
          "app/named-reexport/server/page": [
            "app/named-reexport/server/actions.js#sharedServerLayerAction",
@@ -28,6 +31,7 @@ import { retry } from 'next-test-utils'
          ],
          "app/namespace-reexport-2/client/page": [
            "app/namespace-reexport-2/actions/action-modules.js#action",
+           "app/namespace-reexport-2/nested.js#foo",
            "app/namespace-reexport-2/nested.js#getFoo",
          ],
          "app/namespace-reexport-2/server/page": [
@@ -37,6 +41,8 @@ import { retry } from 'next-test-utils'
          ],
          "app/namespace-reexport/client/page": [
            "app/namespace-reexport/client/actions.js#sharedClientLayerAction",
+           "app/namespace-reexport/client/actions.js#unusedClientLayerAction1",
+           "app/namespace-reexport/client/actions.js#unusedClientLayerAction2",
          ],
          "app/namespace-reexport/server/page": [
            "app/namespace-reexport/server/actions.js#sharedServerLayerAction",

test/production/app-dir/actions-tree-shaking/shared-module-actions/shared-module-actions.test.ts

@@ -4,6 +4,7 @@ import {
 } from '../_testing/utils'
 
 // TODO: revisit when we have a better side-effect free transform approach for server action
+// See the explaination in test/production/app-dir/actions-tree-shaking/basic/basic.test.ts
 ;(process.env.IS_TURBOPACK_TEST ? describe : describe.skip)(
   'actions-tree-shaking - shared-module-actions',
   () => {
@@ -17,9 +18,13 @@ import {
        {
          "app/client/one/page": [
            "app/client/actions.js#sharedClientLayerAction",
+           "app/client/actions.js#unusedClientLayerAction1",
+           "app/client/actions.js#unusedClientLayerAction2",
          ],
          "app/client/two/page": [
            "app/client/actions.js#sharedClientLayerAction",
+           "app/client/actions.js#unusedClientLayerAction1",
+           "app/client/actions.js#unusedClientLayerAction2",
          ],
          "app/server/one/page": [
            "app/server/actions.js#sharedServerLayerAction",