From 9c5d5997c2577a43d5dbf0e63d17cb4ed20916f9 Mon Sep 17 00:00:00 2001
From: Sverre Johansen <sverre.johansen@gmail.com>
Date: Wed, 2 Sep 2026 12:16:53 +0200
Subject: [PATCH] fix(router-utils): clone cached nodes to release Babel's
 traversal cache

`extractModuleInfoFromAst` stored each `var` binding's initializer as the
`t.Expression` node from the parsed file. `StartCompiler.moduleCache` holds
that module info for as long as the module is known, so in a dev server the
node stays reachable for the life of the process.

`@babel/traverse` keys its `NodePath` and `Scope` caches on node identity
(`WeakMap<node, ...>`), which is what lets a file's traversal state be
collected once the AST is dropped. Holding one node from a file defeats that
for the whole file: the surviving cache entry holds `NodePath`s whose
`parentPath` chain reaches the Program path, and whose `scope` reaches every
binding in the file. In a heap snapshot of our dev server these retained
`NodePath` graphs survived a full GC.

Store a location-free deep clone instead. The clone has a fresh identity, so
it is not a key in either WeakMap and pins nothing, and dropping `loc` also
drops the `Position`/`SourceLocation` objects. The readers
(`resolveBindingKind`, `resolveExprKind`) only look at node types and
`callee`/`object`/`property`/`name`, so a clone is equivalent for them.

Measured on our app: 11.6% less peak dev-server memory over four paired cold
runs, with byte-identical build output.
---
 .changeset/start-compiler-module-info-ast-retention.md | 5 +++++
 packages/router-utils/src/compiler-helpers.ts          | 8 ++++++--
 2 files changed, 11 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/start-compiler-module-info-ast-retention.md

diff --git a/.changeset/start-compiler-module-info-ast-retention.md b/.changeset/start-compiler-module-info-ast-retention.md
new file mode 100644
index 00000000000..871e90b7f3f
--- /dev/null
+++ b/.changeset/start-compiler-module-info-ast-retention.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/router-utils': patch
+---
+
+Stop the Start compiler's module cache from retaining parsed ASTs. `extractModuleInfoFromAst` now stores a detached clone of each binding's initializer, so a cached module no longer keeps the `@babel/traverse` `NodePath` and `Scope` graph of the file it came from reachable.
diff --git a/packages/router-utils/src/compiler-helpers.ts b/packages/router-utils/src/compiler-helpers.ts
index 5460c349141..4b9bdd2641a 100644
--- a/packages/router-utils/src/compiler-helpers.ts
+++ b/packages/router-utils/src/compiler-helpers.ts
@@ -95,7 +95,9 @@ function addVariableDeclarationModuleInfo(
     for (const name of collectIdentifiersFromPattern(declarator.id)) {
       bindings.set(name, {
         type: 'var',
-        init: declarator.init ?? null,
+        // Detached: `@babel/traverse` keys its caches on node identity, so
+        // storing the original would pin this file's whole traversal graph.
+        init: declarator.init ? t.cloneNode(declarator.init, true, true) : null,
       })
       exportMap?.set(name, name)
     }
@@ -552,7 +554,9 @@ export function extractModuleInfoFromAst(ast: t.File): ExtractedModuleInfo {
         const synth = '__default_export__'
         bindings.set(synth, {
           type: 'var',
-          init: t.isExpression(declaration) ? declaration : null,
+          init: t.isExpression(declaration)
+            ? t.cloneNode(declaration, true, true)
+            : null,
         })
         exportMap.set('default', synth)
       }