refactor(compiler)!: replace Babel with a Yuku-based compiler core for Router and Start

.changeset/compiler-core-yuku.md

@@ -0,0 +1,11 @@
+---
+'@tanstack/router-utils': minor
+---
+
+Add the compiler core shared by the router plugin and TanStack Start, built on Yuku (`@tanstack/router-utils/compiler`): one parse and one plan per module and environment, with every output (the module, its split chunks, Hydrate chunks, server function providers) emitted from the original source with exact source maps.
+
+The dev caches stay bounded by the live modules: deleted, renamed or no longer imported files leave no plans, raw facts or dependency-index rows behind, and publishing a plan updates only that plan's dependency rows. Each compile records its resolutions in its dependency ledger, and validation asks the bundler's resolver again once a file appeared or disappeared; while the file set is unchanged, the bundler is asked once per environment for each (importer, specifier) pair, so the `export *` chain of a barrel that a thousand modules import from is resolved once, not a thousand times. Emitted outputs are not cached: they are emitted from the frozen plan on request. `TSR_COMPILER_STATS=1` exposes the cache sizes (`compilerStats()`) of the compiler core, the router plugin and the route generator, and `TSR_COMPILER_NO_CACHE` (`plans`, `facts`, `targets`, `all`) disables caches for measurements.
+
+Removed the Babel helpers (`parseAst`, `generateFromAst`, `deadCodeElimination`, `findReferencedIdentifiers`, `stripTypeExports` and the binding helpers of `compiler-helpers`) and the `@babel/*` and `babel-dead-code-elimination` dependencies. Also removed the unused `logDiff`, `createIdentifier` and `decodeIdentifier` exports and the `ansis` and `diff` dependencies.
+
+The package requires Node `^20.19.0 || >=22.12.0`: its CommonJS build loads the ESM-only parser through `require(esm)`.

.changeset/compiler-one-mutation-model.md

@@ -0,0 +1,7 @@
+---
+'@tanstack/router-utils': patch
+'@tanstack/router-plugin': patch
+'@tanstack/start-plugin-core': patch
+---
+
+Route code splitting and Start's environment separation decide from one model of what writes a module's bindings. A value computed when the route module loads from a binding some function writes later (`const initial = [n]` next to `function inc() { n++ }`, a snapshot taken by an immediately invoked function, a call reading a member of an object a function mutates) keeps the value the source computed: it stays in the route module and the split chunks import it, instead of being recomputed when the chunk loads. An option value that reads such a binding is kept in place. The route's option values must be pure for declarations only the component reads to move into its chunk: an impure option (`validateSearch: makeSchema()`) now runs after the declarations before it, as in the source. A chain of components each writing a module binding the previous one reads no longer produces a chunk assigning a binding it imports (a `TypeError` at render): re-planning continues until every such option is kept in place. A server-only initialiser that a server function handler only assigns leaves the client with the handler. Nested server functions are no longer hoisted when they read a module binding something mutates. `TSR_COMPILER_VERIFY=1` re-parses every emitted output and fails the compile on a broken one (the test suites set it).

.changeset/dev-cold-start-and-invalidate-index.md

@@ -0,0 +1,7 @@
+---
+'@tanstack/start-plugin-core': patch
+'@tanstack/vue-start': patch
+'@tanstack/router-utils': patch
+---
+
+Vite dev: Start no longer pre-bundles `@tanstack/<framework>-router/hmr` on its own (the router package is excluded from optimization, so the subpath is served from source with it); pre-bundling it made a cold-start re-optimization strand the first page load (504 "Outdated Optimize Dep", no hydration, no reload). Vue Start declares `@tanstack/vue-router > @tanstack/vue-store` for the optimizer, like React Start does for `@tanstack/react-store`, so a cold start no longer reloads the page once. The compiler's plan cache finds a changed file's plans through a path index instead of scanning every plan on every hot update.

.changeset/dev-hmr-stable-updates.md

@@ -0,0 +1,7 @@
+---
+'@tanstack/router-utils': patch
+'@tanstack/router-plugin': patch
+'@tanstack/start-plugin-core': patch
+---
+
+Dev HMR: a route file that stops parsing keeps its last facts, so its importers (the route tree) no longer hot-update into a full reload onto the dev server's error page; a dependent whose compiled output did not change is not hot-propagated (a route file gaining an export no longer reloads the page); derived modules a file no longer produces (an extracted export after the export went, a split chunk after `codeSplitGroupings: []`) are dropped from the file's hot update instead of dead-ending into a reload; and a React route component moved between its chunk, the route module and its extracted-export module is swapped in (its React Refresh family changed) instead of kept stale.

.changeset/import-protection-bundler-witness.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/start-plugin-core': patch
+---
+
+Import protection no longer reads tsconfig to guess whether TypeScript erases an import whose bindings are only used as types: the bundler decides. Such an import of a denied module is reported (and mocked) exactly when Vite's or Rsbuild's TypeScript transform keeps it, in dev and in builds. This fixes the stock Vite template (a solution-style `tsconfig.json` with `verbatimModuleSyntax` in `tsconfig.app.json`), whose builds bundled the real server module unreported, and Rsbuild dev, which reported imports SWC erases.

.changeset/import-protection-lazy-locations.md

@@ -0,0 +1,6 @@
+---
+'@tanstack/router-utils': patch
+'@tanstack/start-plugin-core': patch
+---
+
+Faster Start builds: import protection locates an import through the bundler's source map only when it reports a violation (every checked module used to combine its input map), a dependency module an app imports is read but no longer parsed and summarised unless its text names something the compiler recognises or re-exports another module, and the compiler asks less of the parser per module. A violation's trace still locates each step in the user's files; a step through a module another plugin rewrote (React Compiler, MDX) is named without a line.

.changeset/react-start-rsc-compiler.md

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+---
+'@tanstack/react-start-rsc': minor
+---
+
+RSC CSS injection runs in Start's compiler. Server functions that only a `'use client'` component imports are in the production manifest (#7943). With RSC, server function provider modules load only what their handlers reach, so a route module's client-only code no longer runs under `react-server`.
+
+Removed `@tanstack/react-start-rsc/plugin/rscCssTransform` (the Babel transform).

.changeset/release-finished-environment-plans.md

@@ -0,0 +1,7 @@
+---
+'@tanstack/start-plugin-core': patch
+'@tanstack/router-plugin': patch
+'@tanstack/router-utils': patch
+---
+
+Production builds release an environment's compiler state as soon as that environment is built, instead of keeping every finished environment's state until the whole build ends. A build of several environments (client, SSR, then a server or RSC environment) no longer grows its heap with each one (#8794).

.changeset/route-generator-yuku.md

@@ -0,0 +1,17 @@
+---
+'@tanstack/router-generator': minor
+'@tanstack/router-cli': patch
+---
+
+Rebuild the route generator without Babel.
+
+**`routeTree.gen.ts` formatting changes once.** The route tree is printed directly in your `quoteStyle` and `semicolons`, and `enableRouteTreeFormatting` now defaults to `false`: Prettier is loaded only when you set it to `true`. A committed `routeTree.gen.ts` shows a one-time formatting diff after upgrading. Custom scaffolding templates are inserted as written.
+
+- Route files are recognised by binding: an aliased `createFileRoute` import gets its id fixed; a `createFileRoute` the app declares itself is left alone; a plain template literal path is reported, not rewritten (#7216).
+- The tree is rebuilt from the files on every change: deleting a layout keeps its children, a deleted `routeTree.gen.ts` is written again, edits of the virtual route config and of `__virtual.ts` files are picked up, and a moved route gets its new id without a content change.
+- Empty `.ts` route files get a scaffold without JSX (#8480); `.lazy.vue` files and `route.component.vue` pieces belong to their route; `importRoutesUsingAbsolutePaths` applies to lazy imports; unusual file names give valid trees.
+- A route file used for two routes, and two routes with one id, are errors naming the files.
+- Writes are atomic per write (also when `tmpDir` is on another filesystem); the generator's own writes no longer regenerate.
+- `tsr watch` and the Rspack/webpack plugins watch every route root, including `physical()` mounts.
+- `GeneratorPlugin.onRouteTreeChanged` receives the tree model (`{ model }`). The internal helpers `format`, `inferFullPath`, `checkRouteFullPathUniqueness` and `writeIfDifferent` are no longer exported.
+- `@tanstack/router-generator` and `@tanstack/router-cli` require Node `^20.19.0 || >=22.12.0`: the CommonJS build and the `tsr` binary load the ESM-only parser through `require(esm)`.

.changeset/route-hmr-runtime.md

@@ -0,0 +1,8 @@
+---
+'@tanstack/router-core': patch
+'@tanstack/react-router': patch
+'@tanstack/solid-router': patch
+'@tanstack/vue-router': patch
+---
+
+Add the development-only route HMR runtime used by the router plugin's output, on new `/hmr` subpaths (`@tanstack/router-core/hmr`, `@tanstack/{react,solid,vue}-router/hmr`). It updates the live route in place, keeps each split component's lazy wrapper across updates, and with Vite's bundled dev falls back to the original importer when Rolldown throws `MissingFactoryError`. An update patches only the route its module exported, so another router in the same window (module federation) is left alone (#7921). After the route tree is regenerated (a route added or renamed), updates keep reaching the route the rebuilt tree holds. In development, once a router error boundary has shown an error, later route edits reload the page, as without route HMR, so the edit that fixes the error shows (Solid and Vue: any router error boundary; React: only the global boundary, since React Refresh recovers route boundaries in place). Nothing of it reaches production bundles.

.changeset/router-plugin-library-build-externals.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/router-plugin': patch
+---
+
+Fix the Vite plugin failing a library build (`build.lib` with `external` dependencies) at the end of the build: the report of split exports that an entry chunk still holds walked the entry's imports, which also name the external packages, and read a chunk that does not exist (`Cannot read properties of undefined (reading 'modules')`).

.changeset/router-plugin-order-check-splitting-only.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/router-plugin': patch
+---
+
+Check the Vite plugin order (the router plugin before the framework's JSX plugin) only when `autoCodeSplitting` is on, as before: without splitting, route HMR works whatever the order, so a config listing `react()` or `solid()` first is not an error.

.changeset/router-plugin-yuku.md

@@ -0,0 +1,24 @@
+---
+'@tanstack/router-plugin': minor
+---
+
+Route code splitting and route HMR run on the new compiler core instead of Babel: faster dev startup, HMR and builds, with the same production output shape.
+
+**What changes for your app when you upgrade:**
+
+- **Vite 6 or later is required** (the `vite` peer range is now `>=6`): the plugin is tested on Vite 6, 7 and 8; Vite 5 is no longer supported.
+- **esbuild refuses `autoCodeSplitting: true`** with a clear error (route generation still works there); use Vite, Rspack or webpack for code splitting.
+- **CommonJS consumers need Node 20.19+ or 22.12+** (`engines`): a `webpack.config.js` / `rspack.config.js` that `require()`s the plugin loads the ESM-only parser through `require(esm)`, which Node 22.0–22.11 and 21 do not support.
+- **Routes the compiler cannot read statically stay unsplit with one warning per file per build** naming the file: route options built by a helper call (`createFileRoute('/x')(makeOptions())`), options that spread or compute, methods that read `this`, and route modules the compiler cannot parse. Main split some of these silently or crashed.
+- **`routeTree.gen.ts` formatting changes once** (`enableRouteTreeFormatting` now defaults to `false`; see the route generator's changeset).
+- **Removed:** `codeSplittingOptions.compilerPlugins` (the experimental Babel hook), and `plugin.hmr.style` with its `HmrOptions` type: the option was undocumented, and on Vite its only other value (`'webpack'`) turned route HMR off; each entry now emits its bundler's hot API.
+
+- Any route option value splits: identifiers, inline functions, method shorthands (#8623), object-form loaders (#8753), casts, `satisfies`, calls such as `memo(Page)`, and options held in a module-level constant. Module state the route module and its chunks share exists once (#8754, #8505).
+- **Exported route components and helpers are split too.** An exported pure function, class or component only split code needs leaves the route module: in a build it moves to a small side-effect-free module of its own that the route module re-exports (one instance for every other module importing it from the route file; the bundler drops it when nobody does), and each split chunk carries its own copy, so the chunk never depends on where the bundler places that module (Rolldown < 1.1.5 and a `manualChunks` group merging into the entry kept it eager). In dev the binding stays in the route module and the chunk copies it the same way. An importer of the route file and the route's chunk therefore hold two instances of such a stateless binding (as on main); state, effects and augmented bindings (`Page.displayName = …`) are never copied. Exported bindings that must stay in the route module (side effects, reassignment, read by the route module itself) and extracted modules an entry chunk loads anyway (another module imports the binding through the route file) are each reported once per file, in dev and in builds, on every bundler.
+- **A top-level declaration only split code needs that must stay in the route module is reported** once per declaration, in dev and in builds, on every bundler, positioned at it and naming what keeps it: a side effect written after it that stays (for example an inline `validateSearch: type({ … })`, named with its line), several split options reading it, a top-level change of it, or a value it reads as the file loads. `/* @__PURE__ */` on the call, or `/* #__NO_SIDE_EFFECTS__ */` on a function declared in the route file, lets it split. Calls of the router's own side-effect-free APIs with constant arguments (`getRouteApi`, `linkOptions`, `createLink`, `createRouteMask`, `lazyRouteComponent`, `lazyFn`, `retainSearchParams`, `stripSearchParams`, `parseSearchWith`, `stringifySearchWith`, `createRouterConfig`) are no such side effect when imported from the router package (also under another name or as a namespace member): `const { useSearch } = getRouteApi('/x')` splits with the component.
+- `createFileRoute` & co. are recognised by their import binding. Routes whose options cannot be read statically, and methods or function expressions that read `this` (#8688), stay in the route module with one warning naming the file.
+- Route HMR keeps split components' lazy wrappers across updates in React, Solid and Vue, refreshes edits of split loaders and other non-component options on Rspack and webpack, and needs no framework plugin for `.ts` route modules.
+- Production output is decided by the bundler's mode and is identical with `addHmr: false`.
+- Rspack and webpack use the plugin's own loader: source maps compose with earlier loaders and name the route file (#6667, #3828); project paths with `'`, `#` or spaces work (#7754). Rspack/webpack no longer rebuild every environment once after the first watch build.
+- esbuild refuses `autoCodeSplitting` with a clear error; the route generator keeps working there.
+- Removed: `codeSplittingOptions.compilerPlugins` (the experimental Babel hook), the `@babel/*` dependencies and the direct `chokidar` dependency (the route generator's watcher brings it). New entry `@tanstack/router-plugin/planners` exposes the route planners to TanStack Start.

.changeset/server-fn-registry-follows-edits.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/start-plugin-core': patch
+---
+
+On the Vite dev server, a server function an edit removes is no longer registered: calling it by its old id fails like any unknown id instead of reaching a module that no longer exports it.