bundler: fix dev server crash when a CSS rebuild fails import resolution

src/bundler/bundle_v2.rs

@@ -5720,19 +5720,33 @@ pub mod bv2_impl {
                 // `graph.ast.items(.import_records)[importer_source_index]` when
                 // they complete. Without this, the graph entry stays at
                 // JSAst.empty and the deferred plugin callback index-out-of-
-                // bounds crashes in BundleV2.onResolve / runResolver. The linker
-                // never runs because `transpiler.log.errors > 0` aborts the
-                // build before link time, so saving the AST is safe.
+                // bounds crashes in BundleV2.onResolve / runResolver. In a
+                // non-dev build the linker never runs after this error
+                // (`transpiler.log.errors > 0` aborts before link time). The
+                // dev server does link with failed files, but it filters them
+                // out by their empty `parts` list and never reads a failed
+                // file's import_records, so saving them is safe — unlike the
+                // `css` slot below.
                 let result_heap = *result.ast.import_records.allocator();
                 this.graph.ast.items_import_records_mut()[source_index.0 as usize] =
                     core::mem::replace(
                         &mut result.ast.import_records,
                         bun_alloc::ArenaVec::new_in(result_heap),
                     );
 
-                // Move the CSS stylesheet onto the graph row so teardown can find
-                // and drop it — the `Success` arm that would normally do this is skipped.
-                this.graph.ast.items_css_mut()[source_index.0 as usize] = result.ast.css.take();
+                // Drop the parsed stylesheet now — the `Success` arm that would
+                // normally move it onto the graph row is skipped. It must not be
+                // parked on the graph row either: the dev server proceeds with
+                // failed files and treats a populated `css` slot as a
+                // successfully parsed CSS file (CSS entry point discovery and
+                // import ordering in `finish_from_bake_dev_server`), so a parked
+                // stylesheet would produce a CSS chunk for a failed file while
+                // `graph.css_file_count` stays 0.
+                if let Some(css_ref) = result.ast.css.take() {
+                    // SAFETY: live arena pointer, uniquely owned here (the graph
+                    // row for this file stays `None`); dropped exactly once.
+                    unsafe { core::ptr::drop_in_place(css_ref.as_ptr()) };
+                }
 
                 parse_result.value = parse_task::ResultValue::Err(parse_task::ResultError {
                     err,

test/bake/dev/css.test.ts

@@ -206,6 +206,53 @@ devTest("syntax error crash", {
     expect((await dev.fetch("/")).status).toBe(500);
   },
 });
+devTest("css url resolve error on hot reload is recoverable", {
+  files: {
+    "styles.css": `
+      body {
+        color: red;
+      }
+    `,
+    "index.html": emptyHtmlFile({
+      styles: ["styles.css"],
+      body: `hello world`,
+    }),
+  },
+  async test(dev) {
+    {
+      await using c = await dev.client("/");
+      await c.style("body").color.expect.toBe("red");
+      // A CSS file that parses but fails import resolution must fail the
+      // rebuild with an error instead of being treated as a valid CSS chunk.
+      // previously: panic: assertion failed: !chunk.content.is_css()
+      await dev.write(
+        "styles.css",
+        `
+          body {
+            background-image: url(./missing.png);
+          }
+        `,
+        {
+          errors: ['styles.css:2:21: error: Could not resolve: "./missing.png"'],
+        },
+      );
+      expect((await dev.fetch("/")).status).toBe(500);
+    }
+    // Recovery is checked without a connected client: when a failed CSS root
+    // recovers, the patch currently ships the HTML route as a JS module
+    // without the route-reload flag, which trips a client-side debug assert
+    // (tracked in https://github.com/oven-sh/bun/issues/31908).
+    await dev.write(
+      "styles.css",
+      `
+        body {
+          color: blue;
+        }
+      `,
+    );
+    expect((await dev.fetch("/")).status).toBe(200);
+  },
+});
 devTest("circular css imports handle hot reload", {
   files: {
     "index.html": emptyHtmlFile({