Add experimental clientSegmentCache flag

This will be used to gate a rewrite of the client cache that we've just
started. Eventually it should merge with the PPR rollout, but that's
already in active experimention and we don't want to harm the PPR
rollout. This will also let us incrementally get to parity with the
existing implementation, instead of trying to reach parity in one
big step.

When enabled (and when a route is using PPR), prefetches will use a new
per-segment cache instead of the existing per-page cache.

packages/next/src/build/webpack/plugins/define-env-plugin.ts

@@ -217,6 +217,9 @@ export function getDefineEnv({
       clientRouterFilters?.staticFilter ?? false,
     'process.env.__NEXT_CLIENT_ROUTER_D_FILTER':
       clientRouterFilters?.dynamicFilter ?? false,
+    'process.env.__NEXT_CLIENT_SEGMENT_CACHE': Boolean(
+      config.experimental.clientSegmentCache
+    ),
     'process.env.__NEXT_OPTIMISTIC_CLIENT_CACHE':
       config.experimental.optimisticClientCache ?? true,
     'process.env.__NEXT_MIDDLEWARE_PREFETCH':

packages/next/src/server/config-schema.ts

@@ -288,6 +288,7 @@ export const configSchema: zod.ZodType<NextConfig> = z.lazy(() =>
         memoryBasedWorkersCount: z.boolean().optional(),
         craCompat: z.boolean().optional(),
         caseSensitiveRoutes: z.boolean().optional(),
+        clientSegmentCache: z.boolean().optional(),
         disableOptimizedLoading: z.boolean().optional(),
         disablePostcssPresetEnv: z.boolean().optional(),
         dynamicIO: z.boolean().optional(),

packages/next/src/server/config-shared.ts

@@ -249,6 +249,7 @@ export interface ExperimentalConfig {
   prerenderEarlyExit?: boolean
   linkNoTouchStart?: boolean
   caseSensitiveRoutes?: boolean
+  clientSegmentCache?: boolean
   appDocumentPreloading?: boolean
   preloadEntriesOnStart?: boolean
   /** @default true */
@@ -1085,6 +1086,7 @@ export const defaultConfig: NextConfig = {
     serverSourceMaps: false,
     linkNoTouchStart: false,
     caseSensitiveRoutes: false,
+    clientSegmentCache: false,
     appDocumentPreloading: undefined,
     preloadEntriesOnStart: true,
     clientRouterFilter: true,