cleanup magic segment strings (#59552)

This uses the existing PAGE_SEGMENT constant in places where we had
`__PAGE__` and introduces a similar constant for `__DEFAULT__`.
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Closes NEXT-1860

packages/next/src/build/webpack/loaders/next-app-loader.ts

@@ -1,5 +1,9 @@
 import type webpack from 'next/dist/compiled/webpack/webpack'
-import type { ValueOf } from '../../../shared/lib/constants'
+import {
+  DEFAULT_SEGMENT_KEY,
+  PAGE_SEGMENT_KEY,
+  type ValueOf,
+} from '../../../shared/lib/constants'
 import type { ModuleReference, CollectedMetadata } from './metadata/types'
 
 import path from 'path'
@@ -274,7 +278,9 @@ async function createTreeCodeFromPath(
         if (resolvedPagePath) pages.push(resolvedPagePath)
 
         // Use '' for segment as it's the page. There can't be a segment called '' so this is the safest way to add it.
-        props[normalizeParallelKey(parallelKey)] = `['__PAGE__', {}, {
+        props[
+          normalizeParallelKey(parallelKey)
+        ] = `['${PAGE_SEGMENT_KEY}', {}, {
           page: [() => import(/* webpackMode: "eager" */ ${JSON.stringify(
             resolvedPagePath
           )}), ${JSON.stringify(resolvedPagePath)}],
@@ -379,7 +385,7 @@ async function createTreeCodeFromPath(
           definedFilePaths.find(([type]) => type === 'not-found')?.[1] ??
           defaultNotFoundPath
         subtreeCode = `{
-          children: ['__PAGE__', {}, {
+          children: ['${PAGE_SEGMENT_KEY}', {}, {
             page: [
               () => import(/* webpackMode: "eager" */ ${JSON.stringify(
                 notFoundPath
@@ -422,7 +428,7 @@ async function createTreeCodeFromPath(
           )) ?? 'next/dist/client/components/parallel-route-default'
 
         props[normalizeParallelKey(adjacentParallelSegment)] = `[
-          '__DEFAULT__',
+          '${DEFAULT_SEGMENT_KEY}',
           {},
           {
             defaultPage: [() => import(/* webpackMode: "eager" */ ${JSON.stringify(

packages/next/src/client/components/navigation.ts

@@ -12,6 +12,7 @@ import {
 } from '../../shared/lib/hooks-client-context.shared-runtime'
 import { clientHookInServerComponentError } from './client-hook-in-server-component-error'
 import { getSegmentValue } from './router-reducer/reducers/get-segment-value'
+import { PAGE_SEGMENT_KEY } from '../../shared/lib/constants'
 
 const INTERNAL_URLSEARCHPARAMS_INSTANCE = Symbol(
   'internal for urlsearchparams readonly'
@@ -141,7 +142,7 @@ function getSelectedParams(
     const segment = parallelRoute[0]
     const isDynamicParameter = Array.isArray(segment)
     const segmentValue = isDynamicParameter ? segment[1] : segment
-    if (!segmentValue || segmentValue.startsWith('__PAGE__')) continue
+    if (!segmentValue || segmentValue.startsWith(PAGE_SEGMENT_KEY)) continue
 
     // Ensure catchAll and optional catchall are turned into an array
     const isCatchAll =
@@ -203,7 +204,9 @@ function getSelectedLayoutSegmentPath(
   const segment = node[0]
 
   const segmentValue = getSegmentValue(segment)
-  if (!segmentValue || segmentValue.startsWith('__PAGE__')) return segmentPath
+  if (!segmentValue || segmentValue.startsWith(PAGE_SEGMENT_KEY)) {
+    return segmentPath
+  }
 
   segmentPath.push(segmentValue)
 

packages/next/src/client/components/router-reducer/apply-router-state-patch-to-tree.ts

@@ -2,6 +2,7 @@ import type {
   FlightRouterState,
   FlightSegmentPath,
 } from '../../../server/app-render/types'
+import { DEFAULT_SEGMENT_KEY } from '../../../shared/lib/constants'
 import { matchSegment } from '../match-segments'
 
 /**
@@ -16,7 +17,10 @@ function applyPatch(
 
   // if the applied patch segment is __DEFAULT__ then we can ignore it and return the initial tree
   // this is because the __DEFAULT__ segment is used as a placeholder on navigation
-  if (patchSegment === '__DEFAULT__' && initialSegment !== '__DEFAULT__') {
+  if (
+    patchSegment === DEFAULT_SEGMENT_KEY &&
+    initialSegment !== DEFAULT_SEGMENT_KEY
+  ) {
     return initialTree
   }
 

packages/next/src/client/components/router-reducer/compute-changed-path.ts

@@ -3,6 +3,10 @@ import type {
   Segment,
 } from '../../../server/app-render/types'
 import { INTERCEPTION_ROUTE_MARKERS } from '../../../server/future/helpers/interception-routes'
+import {
+  DEFAULT_SEGMENT_KEY,
+  PAGE_SEGMENT_KEY,
+} from '../../../shared/lib/constants'
 import { isGroupSegment } from '../../../shared/lib/segment'
 import { matchSegment } from '../match-segments'
 
@@ -39,12 +43,12 @@ export function extractPathFromFlightRouterState(
     : flightRouterState[0]
 
   if (
-    segment === '__DEFAULT__' ||
+    segment === DEFAULT_SEGMENT_KEY ||
     INTERCEPTION_ROUTE_MARKERS.some((m) => segment.startsWith(m))
   )
     return undefined
 
-  if (segment.startsWith('__PAGE__')) return ''
+  if (segment.startsWith(PAGE_SEGMENT_KEY)) return ''
 
   const segments = [segment]
   const parallelRoutes = flightRouterState[1] ?? {}

packages/next/src/client/components/router-reducer/create-router-cache-key.ts

@@ -1,12 +1,13 @@
 import type { Segment } from '../../../server/app-render/types'
+import { PAGE_SEGMENT_KEY } from '../../../shared/lib/constants'
 
 export function createRouterCacheKey(
   segment: Segment,
   withoutSearchParameters: boolean = false
 ) {
   return Array.isArray(segment)
     ? `${segment[0]}|${segment[1]}|${segment[2]}`.toLowerCase()
-    : withoutSearchParameters && segment.startsWith('__PAGE__')
-    ? '__PAGE__'
+    : withoutSearchParameters && segment.startsWith(PAGE_SEGMENT_KEY)
+    ? PAGE_SEGMENT_KEY
     : segment
 }

packages/next/src/client/components/router-reducer/reducers/navigate-reducer.ts

@@ -27,6 +27,7 @@ import {
 import { prunePrefetchCache } from './prune-prefetch-cache'
 import { prefetchQueue } from './prefetch-reducer'
 import { createEmptyCacheNode } from '../../app-router'
+import { DEFAULT_SEGMENT_KEY } from '../../../../shared/lib/constants'
 
 export function handleExternalUrl(
   state: ReadonlyReducerState,
@@ -465,7 +466,7 @@ function navigateReducer_PPR(
             // Filter out the __DEFAULT__ paths as they shouldn't be scrolled to in this case.
             if (
               scrollableSegmentPath[scrollableSegmentPath.length - 1] !==
-              '__DEFAULT__'
+              DEFAULT_SEGMENT_KEY
             ) {
               scrollableSegments.push(scrollableSegmentPath)
             }

packages/next/src/server/app-render/parse-loader-tree.ts

@@ -1,3 +1,4 @@
+import { DEFAULT_SEGMENT_KEY } from '../../shared/lib/constants'
 import type { LoaderTree } from '../lib/app-dir-module'
 
 export function parseLoaderTree(tree: LoaderTree) {
@@ -6,7 +7,7 @@ export function parseLoaderTree(tree: LoaderTree) {
   let { page } = components
   // a __DEFAULT__ segment means that this route didn't match any of the
   // segments in the route, so we should use the default page
-  page = segment === '__DEFAULT__' ? components.defaultPage : page
+  page = segment === DEFAULT_SEGMENT_KEY ? components.defaultPage : page
 
   const layoutOrPagePath = layout?.[1] || page?.[1]
 

packages/next/src/server/app-render/walk-tree-with-flight-router-state.tsx

@@ -21,6 +21,7 @@ import type { CreateSegmentPath, AppRenderContext } from './app-render'
 import { getLayerAssets } from './get-layer-assets'
 import { hasLoadingComponentInTree } from './has-loading-component-in-tree'
 import { createComponentTree } from './create-component-tree'
+import { DEFAULT_SEGMENT_KEY } from '../../shared/lib/constants'
 
 /**
  * Use router state to decide at what common layout to render the page.
@@ -236,7 +237,7 @@ export async function walkTreeWithFlightRouterState({
             // we don't need to send over default routes in the flight data
             // because they are always ignored by the client, unless it's a refetch
             if (
-              item[0] === '__DEFAULT__' &&
+              item[0] === DEFAULT_SEGMENT_KEY &&
               flightRouterState &&
               !!flightRouterState[1][parallelRouteKey][0] &&
               flightRouterState[1][parallelRouteKey][3] !== 'refetch'

packages/next/src/server/dev/on-demand-entry-handler.ts

@@ -33,6 +33,7 @@ import { PageNotFoundError, stringifyError } from '../../shared/lib/utils'
 import {
   COMPILER_INDEXES,
   COMPILER_NAMES,
+  PAGE_SEGMENT_KEY,
   RSC_MODULE_TYPES,
 } from '../../shared/lib/constants'
 import { HMR_ACTIONS_SENT_TO_BROWSER } from './hot-reloader-types'
@@ -130,7 +131,7 @@ function getEntrypointsFromTree(
     ? convertDynamicParamTypeToSyntax(segment[2], segment[0])
     : segment
 
-  const isPageSegment = currentSegment.startsWith('__PAGE__')
+  const isPageSegment = currentSegment.startsWith(PAGE_SEGMENT_KEY)
 
   const currentPath = [...parentPath, isPageSegment ? '' : currentSegment]
 

packages/next/src/server/lib/app-dir-module.ts

@@ -1,4 +1,5 @@
 import type { ComponentsType } from '../../build/webpack/loaders/next-app-loader'
+import { DEFAULT_SEGMENT_KEY } from '../../shared/lib/constants'
 
 /**
  * LoaderTree is generated in next-app-loader.
@@ -14,7 +15,7 @@ export async function getLayoutOrPageModule(loaderTree: LoaderTree) {
   const isLayout = typeof layout !== 'undefined'
   const isPage = typeof page !== 'undefined'
   const isDefaultPage =
-    typeof defaultPage !== 'undefined' && loaderTree[0] === '__DEFAULT__'
+    typeof defaultPage !== 'undefined' && loaderTree[0] === DEFAULT_SEGMENT_KEY
 
   let value = undefined
   let modType: 'layout' | 'page' | undefined = undefined

packages/next/src/shared/lib/constants.ts

@@ -98,6 +98,7 @@ export const EDGE_RUNTIME_WEBPACK = 'edge-runtime-webpack'
 export const STATIC_PROPS_ID = '__N_SSG'
 export const SERVER_PROPS_ID = '__N_SSP'
 export const PAGE_SEGMENT_KEY = '__PAGE__'
+export const DEFAULT_SEGMENT_KEY = '__DEFAULT__'
 export const GOOGLE_FONT_PROVIDER = 'https://fonts.googleapis.com/'
 export const OPTIMIZED_FONT_PROVIDERS = [
   { url: GOOGLE_FONT_PROVIDER, preconnect: 'https://fonts.gstatic.com' },