fix(router-plugin): handle partial declarator exports + add edge case tests

Based on comprehensive code review feedback, this commit addresses:

**Bug Fix: Partial Declarators**
When multiple variables are declared in same statement (const a = 1, b = 2),
and only some are shared between split/non-split parts, we now correctly:
- Export only the shared variables
- Keep non-shared variables local in their respective files
- Split the declaration into separate statements when needed

Before: `export const a = 1, shared = new Map()` (both exported)
After: `const a = 1; export const shared = new Map()` (only shared exported)

**New Edge Case Tests (72 total across React/Solid):**
1. Split-only variable stays local (validates main correctness guard)
2. Partial declarators - only shared ones exported
3. let with reassignment (live binding behavior)
4. Destructuring precision (promotes correct binding)
5. Alias chains (tracks through variable aliases)
6. Nested closures (finds references in function bodies)

These tests ensure the implementation correctly handles complex variable
sharing scenarios and only promotes/imports variables that are genuinely
shared between split and non-split parts.

All 360 tests passing.

Reviewer feedback incorporated from PR TanStack#5767

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: KyleAMathews

packages/router-plugin/src/core/code-splitter/compilers.ts

@@ -529,6 +529,10 @@ export function compileCodeSplitReferenceRoute(
          */
         if (sharedModuleLevelIdents.size > 0) {
           modified = true
+
+          // Track which variable declarations we've already processed to avoid duplicate processing
+          const processedVarDecls = new Set<babel.NodePath<t.VariableDeclaration>>()
+
           sharedModuleLevelIdents.forEach((identName) => {
             const binding = programPath.scope.getBinding(identName)
             if (!binding) return
@@ -541,19 +545,57 @@ export function compileCodeSplitReferenceRoute(
               bindingPath.parentPath?.isVariableDeclaration() &&
               bindingPath.parentPath.parentPath?.isProgram()
             ) {
-              const varDecl = bindingPath.parentPath
+              const varDecl = bindingPath.parentPath as babel.NodePath<t.VariableDeclaration>
 
               // Only export const/let declarations (not imports or functions)
               if (
                 varDecl.node.kind === 'const' ||
                 varDecl.node.kind === 'let'
               ) {
-                // Convert to export declaration
-                const exportDecl = t.exportNamedDeclaration(varDecl.node, [])
-                varDecl.replaceWith(exportDecl)
-                knownExportedIdents.add(identName)
-                // Track in sharedExports for virtual compiler to use
-                opts.sharedExports?.add(identName)
+                // Skip if we've already processed this declaration
+                if (processedVarDecls.has(varDecl)) {
+                  return
+                }
+                processedVarDecls.add(varDecl)
+
+                // Check if this declaration has multiple declarators
+                const declarators = varDecl.node.declarations
+                if (declarators.length > 1) {
+                  // Split declarators: shared ones get exported, others stay local
+                  const sharedDeclarators: Array<t.VariableDeclarator> = []
+                  const localDeclarators: Array<t.VariableDeclarator> = []
+
+                  declarators.forEach((declarator) => {
+                    if (t.isIdentifier(declarator.id) && sharedModuleLevelIdents.has(declarator.id.name)) {
+                      sharedDeclarators.push(declarator)
+                      knownExportedIdents.add(declarator.id.name)
+                      opts.sharedExports?.add(declarator.id.name)
+                    } else {
+                      localDeclarators.push(declarator)
+                    }
+                  })
+
+                  // Replace with split declarations
+                  if (sharedDeclarators.length > 0 && localDeclarators.length > 0) {
+                    // Both shared and local declarators
+                    const localVarDecl = t.variableDeclaration(varDecl.node.kind, localDeclarators)
+                    const sharedVarDecl = t.variableDeclaration(varDecl.node.kind, sharedDeclarators)
+                    const exportDecl = t.exportNamedDeclaration(sharedVarDecl, [])
+
+                    varDecl.replaceWithMultiple([localVarDecl, exportDecl])
+                  } else if (sharedDeclarators.length > 0) {
+                    // All declarators are shared
+                    const sharedVarDecl = t.variableDeclaration(varDecl.node.kind, sharedDeclarators)
+                    const exportDecl = t.exportNamedDeclaration(sharedVarDecl, [])
+                    varDecl.replaceWith(exportDecl)
+                  }
+                } else {
+                  // Single declarator - export the whole thing
+                  const exportDecl = t.exportNamedDeclaration(varDecl.node, [])
+                  varDecl.replaceWith(exportDecl)
+                  knownExportedIdents.add(identName)
+                  opts.sharedExports?.add(identName)
+                }
               }
             }
           })

packages/router-plugin/tests/code-splitter/snapshots/react/1-default/shared-alias-chain.tsx

@@ -0,0 +1,17 @@
+console.warn("[tanstack-router] These exports from \"shared-alias-chain.tsx\" will not be code-split and will increase your bundle size:\n- alias\nFor the best optimization, these items should either have their export statements removed, or be imported from another location that is not a route file.");
+const $$splitComponentImporter = () => import('./shared-alias-chain.tsx?tsr-split=component');
+import { lazyRouteComponent } from '@tanstack/react-router';
+import { createFileRoute } from '@tanstack/react-router';
+
+// Alias chain - ensure we track through aliases
+const base = {
+  name: 'collection',
+  items: []
+};
+export const alias = base;
+export const Route = createFileRoute('/test')({
+  loader: async () => {
+    return alias.items;
+  },
+  component: lazyRouteComponent($$splitComponentImporter, 'component')
+});

packages/router-plugin/tests/code-splitter/snapshots/react/1-default/[email protected]

@@ -0,0 +1,5 @@
+import { alias } from "./shared-alias-chain.tsx"; // Alias chain - ensure we track through aliases
+function TestComponent() {
+  return <div>{alias.name}</div>;
+}
+export { TestComponent as component };

packages/router-plugin/tests/code-splitter/snapshots/react/1-default/[email protected]

@@ -0,0 +1,20 @@
+console.warn("[tanstack-router] These exports from \"shared-destructuring.tsx\" will not be code-split and will increase your bundle size:\n- apiUrl\nFor the best optimization, these items should either have their export statements removed, or be imported from another location that is not a route file.");
+const $$splitComponentImporter = () => import('./shared-destructuring.tsx?tsr-split=component');
+import { lazyRouteComponent } from '@tanstack/react-router';
+import { createFileRoute } from '@tanstack/react-router';
+
+// Destructuring - ensure we promote the right binding
+const cfg = {
+  apiUrl: 'http://api.com',
+  timeout: 5000
+};
+export const {
+  apiUrl
+} = cfg;
+export const Route = createFileRoute('/test')({
+  loader: async () => {
+    // Uses the destructured binding
+    return fetch(apiUrl).then(r => r.json());
+  },
+  component: lazyRouteComponent($$splitComponentImporter, 'component')
+});

packages/router-plugin/tests/code-splitter/snapshots/react/1-default/[email protected]

@@ -0,0 +1,13 @@
+// Destructuring - ensure we promote the right binding
+const cfg = {
+  apiUrl: 'http://api.com',
+  timeout: 5000
+};
+const {
+  apiUrl
+} = cfg;
+function TestComponent() {
+  // Also uses the destructured binding
+  return <div>API: {apiUrl}</div>;
+}
+export { TestComponent as component };

packages/router-plugin/tests/code-splitter/snapshots/react/1-default/shared-destructuring.tsx

@@ -0,0 +1,22 @@
+console.warn("[tanstack-router] These exports from \"shared-let-reassignment.tsx\" will not be code-split and will increase your bundle size:\n- store\nFor the best optimization, these items should either have their export statements removed, or be imported from another location that is not a route file.");
+const $$splitComponentImporter = () => import('./shared-let-reassignment.tsx?tsr-split=component');
+import { lazyRouteComponent } from '@tanstack/react-router';
+import { createFileRoute } from '@tanstack/react-router';
+
+// let with reassignment - tests live binding behavior
+export let store = {
+  count: 0
+};
+store = {
+  count: 1,
+  updated: true
+};
+export const Route = createFileRoute('/test')({
+  loader: async () => {
+    store.count++;
+    return {
+      data: 'loaded'
+    };
+  },
+  component: lazyRouteComponent($$splitComponentImporter, 'component')
+});