fix(bundler): reject splitting with non-esm formats instead of crashing

When `splitting: true` was combined with `format: "iife"` (or "cjs"),
the bundler would crash with a null pointer dereference in
`sortedCrossChunkImports` because `exports_to_other_chunks` is only
populated for ESM format, while `imports_from_other_chunks` is populated
for all formats. This matches esbuild's behavior of rejecting this
combination with a clear error message.

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

src/bun.js/api/JSBundler.zig

@@ -1045,6 +1045,10 @@ pub const JSBundler = struct {
                 }
             }
 
+            if (this.code_splitting and this.format != .esm) {
+                return globalThis.throwInvalidArguments("Splitting currently only works with the \"esm\" format", .{});
+            }
+
             // ESM bytecode requires compile because module_info (import/export metadata)
             // is only available in compiled binaries. Without it, JSC must parse the file
             // twice (once for module analysis, once for bytecode), which is a deopt.

src/cli/build_command.zig

@@ -91,6 +91,12 @@ pub const BuildCommand = struct {
         this_transpiler.options.output_dir = ctx.bundler_options.outdir;
         this_transpiler.options.output_format = ctx.bundler_options.output_format;
 
+        if (ctx.bundler_options.code_splitting and ctx.bundler_options.output_format != .esm) {
+            Output.prettyErrorln("<r><red>error<r><d>:<r> Splitting currently only works with the \"esm\" format", .{});
+            Global.exit(1);
+            return;
+        }
+
         if (ctx.bundler_options.output_format == .internal_bake_dev) {
             this_transpiler.options.tree_shaking = false;
         }

test/bundler/bun-build-api.test.ts

@@ -134,6 +134,23 @@ describe("Bun.build", () => {
     ).toThrow();
   });
 
+  test("splitting with non-esm format throws", async () => {
+    expect(() =>
+      Bun.build({
+        entrypoints: ["hello.js"],
+        splitting: true,
+        format: "iife",
+      }),
+    ).toThrow('Splitting currently only works with the "esm" format');
+    expect(() =>
+      Bun.build({
+        entrypoints: ["hello.js"],
+        splitting: true,
+        format: "cjs",
+      }),
+    ).toThrow('Splitting currently only works with the "esm" format');
+  });
+
   test("returns errors properly", async () => {
     Bun.gc(true);
     const build = await buildNoThrow({