From 58223df513c58350c912045129ac8c20f01dd32c Mon Sep 17 00:00:00 2001
From: robobun <117481402+robobun@users.noreply.github.com>
Date: Fri, 4 Sep 2026 11:19:16 +0000
Subject: [PATCH] bundler: keep the resolver's side effects when onResolve
 returns nothing

When every onResolve callback returns nothing, the bundler resolves the
import in run_resolver. That path dropped the resolver's side-effects
value in two places:

- enqueue_parse_task stored loader.side_effects() for the file. So an
  unused file from a package with "sideEffects": false stayed in the
  bundle. Also, a data file that an onLoad plugin turned into JavaScript
  lost its side effects.
- The early return for an external import did not set
  IS_EXTERNAL_WITHOUT_SIDE_EFFECTS. So an unused import of a Node.js
  builtin stayed in the output.

Both places now store the resolver's value, as the path without plugins
does.
---
 src/bundler/bundle_v2.rs            | 10 +++-
 test/bundler/bundler_plugin.test.ts | 89 +++++++++++++++++++++++++++++
 2 files changed, 98 insertions(+), 1 deletion(-)

diff --git a/src/bundler/bundle_v2.rs b/src/bundler/bundle_v2.rs
index 692acf621f68..32e0af52426e 100644
--- a/src/bundler/bundle_v2.rs
+++ b/src/bundler/bundle_v2.rs
@@ -2588,6 +2588,14 @@ pub mod bv2_impl {
             };
 
             if resolve_result.flags.is_external() {
+                let record: &mut ImportRecord = &mut self.graph.ast.items_import_records_mut()
+                    [import_record.importer_source_index as usize]
+                    .as_mut_slice()[import_record.import_record_index as usize];
+                record.flags.set(
+                    bun_ast::ImportRecordFlags::IS_EXTERNAL_WITHOUT_SIDE_EFFECTS,
+                    resolve_result.primary_side_effects_data
+                        != bun_ast::SideEffects::HasSideEffects,
+                );
                 return;
             }
 
@@ -3728,7 +3736,7 @@ pub mod bv2_impl {
             self.graph.input_files.append(crate::Graph::InputFile {
                 source: core::mem::take(source),
                 loader,
-                side_effects: loader.side_effects(),
+                side_effects: resolve_result.primary_side_effects_data,
                 ..Default::default()
             })?;
             // `ParseTask::init` takes `bun_ast::Index`; both Index newtypes
diff --git a/test/bundler/bundler_plugin.test.ts b/test/bundler/bundler_plugin.test.ts
index e9312694e2b2..2d0e83aeac46 100644
--- a/test/bundler/bundler_plugin.test.ts
+++ b/test/bundler/bundler_plugin.test.ts
@@ -189,6 +189,95 @@ describe("bundler", () => {
     });
   }
 
+  // A plugin that returns nothing hands the import back to the resolver. The
+  // bundle must keep the same side effects as a build without the plugin.
+  itBundled("plugin/ResolveReturnsUndefinedKeepsSideEffectsFalse", {
+    files: {
+      "/entry.js": /* js */ `
+        import { used } from "demo-pkg/used.js";
+        import { unused } from "demo-pkg/unused.js";
+        console.log(used);
+      `,
+      "/node_modules/demo-pkg/package.json": /* json */ `{ "sideEffects": false }`,
+      "/node_modules/demo-pkg/used.js": /* js */ `export const used = "used";`,
+      "/node_modules/demo-pkg/unused.js": /* js */ `
+        console.log("unused.js ran");
+        export const unused = 1;
+      `,
+    },
+    plugins(builder) {
+      builder.onResolve({ filter: /.*/ }, () => undefined);
+    },
+    run: {
+      stdout: "used",
+    },
+  });
+
+  itBundled("plugin/ResolveReturnsUndefinedKeepsSideEffectsArray", {
+    files: {
+      "/entry.js": /* js */ `
+        import "demo-pkg/register.js";
+        import { used } from "demo-pkg";
+        console.log(used);
+      `,
+      "/node_modules/demo-pkg/package.json": /* json */ `{ "sideEffects": ["./register.js"] }`,
+      "/node_modules/demo-pkg/index.js": /* js */ `
+        export { used } from "./used.js";
+        export { unused } from "./unused.js";
+      `,
+      "/node_modules/demo-pkg/register.js": /* js */ `console.log("register.js ran");`,
+      "/node_modules/demo-pkg/used.js": /* js */ `export const used = "used";`,
+      "/node_modules/demo-pkg/unused.js": /* js */ `
+        console.log("unused.js ran");
+        export const unused = 1;
+      `,
+    },
+    plugins(builder) {
+      builder.onResolve({ filter: /.*/ }, () => undefined);
+    },
+    run: {
+      stdout: "register.js ran\nused",
+    },
+  });
+
+  // The loader that onLoad returns replaces the loader from the file extension.
+  itBundled("plugin/ResolveReturnsUndefinedOnLoadToJsKeepsSideEffects", {
+    files: {
+      "/entry.js": /* js */ `
+        import "./setup.txt";
+        console.log("entry");
+      `,
+      "/setup.txt": `console.log("setup.txt ran");`,
+    },
+    plugins(builder) {
+      builder.onResolve({ filter: /.*/ }, () => undefined);
+      builder.onLoad({ filter: /\.txt$/ }, async args => ({
+        contents: await Bun.file(args.path).text(),
+        loader: "js",
+      }));
+    },
+    run: {
+      stdout: "setup.txt ran\nentry",
+    },
+  });
+
+  // An import of a Node.js builtin has no side effects, so an unused one is dropped.
+  itBundled("plugin/ResolveReturnsUndefinedDropsUnusedBuiltinImport", {
+    files: {
+      "/entry.js": /* js */ `
+        import fs from "node:fs";
+        console.log("entry");
+      `,
+    },
+    target: "node",
+    plugins(builder) {
+      builder.onResolve({ filter: /.*/ }, () => undefined);
+    },
+    onAfterBundle(api) {
+      api.expectFile("/out.js").not.toContain("node:fs");
+    },
+  });
+
   // Load Plugin Errors
   itBundled("plugin/ResolveThrow", {
     files: resolveFixture,