ref(sveltekit): Replace recast + @babel/parser with acorn (#19533)

Replaces `recast` and `@babel/parser` with `acorn` and
`@sveltejs/acorn-typescript` for AST parsing in the sveltekit
auto-instrumentation plugin.

- [x] If you've added code that should be tested, please add tests.
- [x] Ensure your code lints and the test suite passes (`yarn lint`) &
(`yarn test`).
- [ ] Link an issue if there is one related to your pull request.

Ref #19447

## What this does

The `autoInstrument` Vite plugin parses SvelteKit `+page.ts` /
`+layout.server.ts` files to detect `export const load` or `export
function load` declarations. It only **reads** the AST — never
transforms or prints it — so recast's source-preserving round-trip
feature is entirely unused.

This replaces the full recast + @babel/parser pipeline with acorn
(Node.js's own parser) and `@sveltejs/acorn-typescript` (the actively
maintained TypeScript plugin used by SvelteKit itself).

### Changes

- **Delete** `recastTypescriptParser.ts` — the 91-line Babel parser
config with 20+ plugin declarations is no longer needed
- **Replace** `recast.parse()` with
`acorn.Parser.extend(tsPlugin()).parse()` in `autoInstrument.ts`
- **Fix** AST node type: `StringLiteral` (Babel-specific) → `Literal`
(ESTree standard) for string literal export detection
- **Fix** AST structure: `ast.program` (recast's File wrapper) → direct
`ast` (acorn returns Program)
- **Improve** error handling: `try/catch` around parse (acorn throws
SyntaxError) replaces dubious null check
- **Improve** type safety: remove unsafe double-cast
`(specifier.exported as unknown as t.StringLiteral)` in favor of proper
discriminant narrowing

### Dependency reduction

**Removed:** `recast`, `ast-types`, `esprima`, `source-map`, `tslib`,
`tiny-invariant`, `@babel/parser`, `@babel/types`,
`@babel/helper-string-parser`, `@babel/helper-validator-identifier`

**Added:** `acorn` (already in dep tree via Vite/Rollup),
`@sveltejs/acorn-typescript` (already in dep tree via `@sveltejs/kit`)

Net effect: ~8 fewer transitive dependencies, zero new packages for end
users.

### Why @sveltejs/acorn-typescript?

The original `acorn-typescript` package by TyrealHu hasn't been updated
since January 2024 and has known unpatched bugs. The Svelte team's fork
(`@sveltejs/acorn-typescript`) is actively maintained and is already a
transitive dependency of every SvelteKit project.

All 69 existing tests pass without modification.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: JPeer264 <jan.peer@sentry.io>
Co-authored-by: Lukas Stracke <lukas.stracke@sentry.io>

Author: 4 people

packages/sveltekit/package.json

@@ -47,18 +47,17 @@
     }
   },
   "dependencies": {
-    "@babel/parser": "7.26.9",
     "@sentry/cloudflare": "10.45.0",
     "@sentry/core": "10.45.0",
     "@sentry/node": "10.45.0",
     "@sentry/svelte": "10.45.0",
     "@sentry/vite-plugin": "^5.1.0",
+    "@sveltejs/acorn-typescript": "^1.0.9",
+    "acorn": "^8.14.0",
     "magic-string": "~0.30.0",
-    "recast": "0.23.11",
     "sorcery": "1.0.0"
   },
   "devDependencies": {
-    "@babel/types": "^7.26.3",
     "@sveltejs/kit": "^2.53.3",
     "@sveltejs/vite-plugin-svelte": "^3.0.0",
     "svelte": "^4.2.8",

packages/sveltekit/src/vite/autoInstrument.ts

@@ -1,10 +1,11 @@
+import { tsPlugin } from '@sveltejs/acorn-typescript';
+import * as acorn from 'acorn';
 import * as fs from 'fs';
 import * as path from 'path';
-import * as recast from 'recast';
 import type { Plugin } from 'vite';
 import { WRAPPED_MODULE_SUFFIX } from '../common/utils';
-import { parser } from './recastTypescriptParser';
-import t = recast.types.namedTypes;
+
+const AcornParser = acorn.Parser.extend(tsPlugin());
 
 export type AutoInstrumentSelection = {
   /**
@@ -123,23 +124,21 @@ export async function canWrapLoad(id: string, debug: boolean): Promise<boolean>
 
   const code = (await fs.promises.readFile(id, 'utf8')).toString();
 
-  const ast = recast.parse(code, {
-    parser,
-  });
-
-  const program = (ast as { program?: t.Program }).program;
-
-  if (!program) {
+  let program: acorn.Program;
+  try {
+    program = AcornParser.parse(code, {
+      sourceType: 'module',
+      ecmaVersion: 'latest',
+      locations: true,
+    });
+  } catch {
     // eslint-disable-next-line no-console
     debug && console.log(`Skipping wrapping ${id} because it doesn't contain valid JavaScript or TypeScript`);
     return false;
   }
 
   const hasLoadDeclaration = program.body
-    .filter(
-      (statement): statement is recast.types.namedTypes.ExportNamedDeclaration =>
-        statement.type === 'ExportNamedDeclaration',
-    )
+    .filter((statement): statement is acorn.ExportNamedDeclaration => statement.type === 'ExportNamedDeclaration')
     .find(exportDecl => {
       // find `export const load = ...`
       if (exportDecl.declaration?.type === 'VariableDeclaration') {
@@ -160,11 +159,8 @@ export async function canWrapLoad(id: string, debug: boolean): Promise<boolean>
         return exportDecl.specifiers.find(specifier => {
           return (
             (specifier.exported.type === 'Identifier' && specifier.exported.name === 'load') ||
-            // Type casting here because somehow the 'exportExtensions' plugin isn't reflected in the possible types
-            // This plugin adds support for exporting something as a string literal (see comment above)
-            // Doing this to avoid adding another babel plugin dependency
-            ((specifier.exported.type as 'StringLiteral' | '') === 'StringLiteral' &&
-              (specifier.exported as unknown as t.StringLiteral).value === 'load')
+            // ESTree/acorn represents `export { x as "load" }` with a Literal node (not Babel's StringLiteral)
+            (specifier.exported.type === 'Literal' && specifier.exported.value === 'load')
           );
         });
       }

packages/sveltekit/src/vite/recastTypescriptParser.ts

@@ -1914,13 +1914,6 @@
     "@babel/template" "^7.28.6"
     "@babel/types" "^7.28.6"
 
-"@babel/parser@7.26.9":
-  version "7.26.9"
-  resolved "https://registry.yarnpkg.com/@babel/parser/-/parser-7.26.9.tgz#d9e78bee6dc80f9efd8f2349dcfbbcdace280fd5"
-  integrity sha512-81NWa1njQblgZbQHxWHpxxCzNsa3ZwvFqpUg7P+NNUU6f3UU2jBEg4OlF/J6rl8+PQGh1q6/zWScd001YwcA5A==
-  dependencies:
-    "@babel/types" "^7.26.9"
-
 "@babel/parser@^7.1.0", "@babel/parser@^7.14.7", "@babel/parser@^7.16.4", "@babel/parser@^7.18.10", "@babel/parser@^7.20.7", "@babel/parser@^7.22.10", "@babel/parser@^7.22.16", "@babel/parser@^7.23.5", "@babel/parser@^7.23.6", "@babel/parser@^7.25.4", "@babel/parser@^7.26.7", "@babel/parser@^7.27.7", "@babel/parser@^7.28.0", "@babel/parser@^7.28.4", "@babel/parser@^7.28.5", "@babel/parser@^7.28.6", "@babel/parser@^7.29.0", "@babel/parser@^7.4.5", "@babel/parser@^7.7.0":
   version "7.29.0"
   resolved "https://registry.yarnpkg.com/@babel/parser/-/parser-7.29.0.tgz#669ef345add7d057e92b7ed15f0bac07611831b6"
@@ -2981,7 +2974,7 @@
     "@babel/types" "^7.29.0"
     debug "^4.3.1"
 
-"@babel/types@^7.0.0", "@babel/types@^7.18.10", "@babel/types@^7.18.6", "@babel/types@^7.20.7", "@babel/types@^7.22.10", "@babel/types@^7.22.15", "@babel/types@^7.22.17", "@babel/types@^7.22.19", "@babel/types@^7.23.6", "@babel/types@^7.24.7", "@babel/types@^7.25.4", "@babel/types@^7.26.3", "@babel/types@^7.26.8", "@babel/types@^7.26.9", "@babel/types@^7.27.1", "@babel/types@^7.27.3", "@babel/types@^7.27.7", "@babel/types@^7.28.2", "@babel/types@^7.28.5", "@babel/types@^7.28.6", "@babel/types@^7.29.0", "@babel/types@^7.3.0", "@babel/types@^7.4.4", "@babel/types@^7.7.0", "@babel/types@^7.7.2":
+"@babel/types@^7.0.0", "@babel/types@^7.18.10", "@babel/types@^7.18.6", "@babel/types@^7.20.7", "@babel/types@^7.22.10", "@babel/types@^7.22.15", "@babel/types@^7.22.17", "@babel/types@^7.22.19", "@babel/types@^7.23.6", "@babel/types@^7.24.7", "@babel/types@^7.25.4", "@babel/types@^7.26.8", "@babel/types@^7.27.1", "@babel/types@^7.27.3", "@babel/types@^7.27.7", "@babel/types@^7.28.2", "@babel/types@^7.28.5", "@babel/types@^7.28.6", "@babel/types@^7.29.0", "@babel/types@^7.3.0", "@babel/types@^7.4.4", "@babel/types@^7.7.0", "@babel/types@^7.7.2":
   version "7.29.0"
   resolved "https://registry.yarnpkg.com/@babel/types/-/types-7.29.0.tgz#9f5b1e838c446e72cf3cd4b918152b8c605e37c7"
   integrity sha512-LwdZHpScM4Qz8Xw2iKSzS+cfglZzJGvofQICy7W7v4caru4EaAmyUuO6BGrbyQ2mYV11W0U8j5mBhd14dd3B0A==
@@ -8652,9 +8645,14 @@
     "@supabase/storage-js" "2.7.1"
 
 "@sveltejs/acorn-typescript@^1.0.5":
-  version "1.0.8"
-  resolved "https://registry.yarnpkg.com/@sveltejs/acorn-typescript/-/acorn-typescript-1.0.8.tgz#69c746a7c232094c117c50dedbd1279fc64887b7"
-  integrity sha512-esgN+54+q0NjB0Y/4BomT9samII7jGwNy/2a3wNZbT2A2RpmXsXwUt24LvLhx6jUq2gVk4cWEvcRO6MFQbOfNA==
+  version "1.0.9"
+  resolved "https://registry.yarnpkg.com/@sveltejs/acorn-typescript/-/acorn-typescript-1.0.9.tgz#ac0bde368d6623727b0e0bc568cf6b4e5d5c4baa"
+  integrity sha512-lVJX6qEgs/4DOcRTpo56tmKzVPtoWAaVbL4hfO7t7NVwl9AAXzQR6cihesW1BmNMPl+bK6dreu2sOKBP2Q9CIA==
+
+"@sveltejs/acorn-typescript@^1.0.9":
+  version "1.0.9"
+  resolved "https://registry.yarnpkg.com/@sveltejs/acorn-typescript/-/acorn-typescript-1.0.9.tgz#ac0bde368d6623727b0e0bc568cf6b4e5d5c4baa"
+  integrity sha512-lVJX6qEgs/4DOcRTpo56tmKzVPtoWAaVbL4hfO7t7NVwl9AAXzQR6cihesW1BmNMPl+bK6dreu2sOKBP2Q9CIA==
 
 "@sveltejs/kit@^2.53.3":
   version "2.53.3"
@@ -25831,17 +25829,6 @@ real-require@^0.2.0:
   resolved "https://registry.yarnpkg.com/real-require/-/real-require-0.2.0.tgz#209632dea1810be2ae063a6ac084fee7e33fba78"
   integrity sha512-57frrGM/OCTLqLOAh0mhVA9VBMHd+9U7Zb2THMGdBUoZVOtGbJzjxsYGDJ3A9AYYCP4hn6y1TVbaOfzWtm5GFg==
 
-recast@0.23.11, recast@^0.23.4:
-  version "0.23.11"
-  resolved "https://registry.yarnpkg.com/recast/-/recast-0.23.11.tgz#8885570bb28cf773ba1dc600da7f502f7883f73f"
-  integrity sha512-YTUo+Flmw4ZXiWfQKGcwwc11KnoRAYgzAE2E7mXKCjSviTKShtxBsN6YUUBB2gtaBzKzeKunxhUwNHQuRryhWA==
-  dependencies:
-    ast-types "^0.16.1"
-    esprima "~4.0.0"
-    source-map "~0.6.1"
-    tiny-invariant "^1.3.3"
-    tslib "^2.0.1"
-
 recast@^0.18.1:
   version "0.18.10"
   resolved "https://registry.yarnpkg.com/recast/-/recast-0.18.10.tgz#605ebbe621511eb89b6356a7e224bff66ed91478"
@@ -25862,6 +25849,17 @@ recast@^0.20.5:
     source-map "~0.6.1"
     tslib "^2.0.1"
 
+recast@^0.23.4:
+  version "0.23.11"
+  resolved "https://registry.yarnpkg.com/recast/-/recast-0.23.11.tgz#8885570bb28cf773ba1dc600da7f502f7883f73f"
+  integrity sha512-YTUo+Flmw4ZXiWfQKGcwwc11KnoRAYgzAE2E7mXKCjSviTKShtxBsN6YUUBB2gtaBzKzeKunxhUwNHQuRryhWA==
+  dependencies:
+    ast-types "^0.16.1"
+    esprima "~4.0.0"
+    source-map "~0.6.1"
+    tiny-invariant "^1.3.3"
+    tslib "^2.0.1"
+
 rechoir@^0.6.2:
   version "0.6.2"
   resolved "https://registry.yarnpkg.com/rechoir/-/rechoir-0.6.2.tgz#85204b54dba82d5742e28c96756ef43af50e3384"