Use tempDirWithFiles harness utility in test

- Remove manual mkdtempSync/writeFileSync usage
- Use tempDirWithFiles for automatic cleanup
- Follows harness testing guidelines

src/bun.js/ModuleLoader.zig

@@ -2554,12 +2554,18 @@ pub const RuntimeTranspilerStore = struct {
                 }
 
                 if (strings.hasPrefixComptime(import_record.path.text, "bun:")) {
-                    import_record.path = Fs.Path.init(import_record.path.text["bun:".len..]);
-                    import_record.path.namespace = "bun";
-                    import_record.is_external_without_side_effects = true;
+                    // Validate and strip the bun: prefix for builtin modules.
+                    // Invalid modules (e.g., "bun:invalid") are left unchanged so the
+                    // resolver can report an error with the full "bun:invalid" name instead
+                    // of the confusing "Cannot find package 'invalid'".
+                    if (HardcodedModule.map.has(import_record.path.text)) {
+                        import_record.path = Fs.Path.init(import_record.path.text["bun:".len..]);
+                        import_record.path.namespace = "bun";
+                        import_record.is_external_without_side_effects = true;
 
-                    if (strings.eqlComptime(import_record.path.text, "test")) {
-                        import_record.tag = .bun_test;
+                        if (strings.eqlComptime(import_record.path.text, "test")) {
+                            import_record.tag = .bun_test;
+                        }
                     }
                 }
             }

src/bundler/bundle_v2.zig

@@ -3175,17 +3175,21 @@ pub const BundleV2 = struct {
                 }
 
                 if (strings.hasPrefixComptime(import_record.path.text, "bun:")) {
-                    import_record.path = Fs.Path.init(import_record.path.text["bun:".len..]);
-                    import_record.path.namespace = "bun";
-                    import_record.source_index = Index.invalid;
-                    import_record.is_external_without_side_effects = true;
+                    // Validate and strip the bun: prefix for builtin modules.
+                    // Invalid modules are left unchanged for better error messages.
+                    if (jsc.ModuleLoader.HardcodedModule.map.has(import_record.path.text)) {
+                        import_record.path = Fs.Path.init(import_record.path.text["bun:".len..]);
+                        import_record.path.namespace = "bun";
+                        import_record.source_index = Index.invalid;
+                        import_record.is_external_without_side_effects = true;
 
-                    if (strings.eqlComptime(import_record.path.text, "test")) {
-                        import_record.tag = .bun_test;
+                        if (strings.eqlComptime(import_record.path.text, "test")) {
+                            import_record.tag = .bun_test;
+                        }
+
+                        // don't link bun
+                        continue;
                     }
-
-                    // don't link bun
-                    continue;
                 }
             }
 

src/linker.zig

@@ -176,15 +176,19 @@ pub const Linker = struct {
                         }
 
                         if (strings.hasPrefixComptime(import_record.path.text, "bun:")) {
-                            import_record.path = Fs.Path.init(import_record.path.text["bun:".len..]);
-                            import_record.path.namespace = "bun";
+                            // Validate and strip the bun: prefix for builtin modules.
+                            // Invalid modules are left unchanged for better error messages.
+                            if (jsc.ModuleLoader.HardcodedModule.map.has(import_record.path.text)) {
+                                import_record.path = Fs.Path.init(import_record.path.text["bun:".len..]);
+                                import_record.path.namespace = "bun";
 
-                            if (strings.eqlComptime(import_record.path.text, "test")) {
-                                import_record.tag = .bun_test;
+                                if (strings.eqlComptime(import_record.path.text, "test")) {
+                                    import_record.tag = .bun_test;
+                                }
+
+                                // don't link bun
+                                continue;
                             }
-
-                            // don't link bun
-                            continue;
                         }
 
                         // Resolve dynamic imports lazily for perf

test/regression/issue/invalid-bun-module-prefix.test.ts

@@ -0,0 +1,22 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDirWithFiles } from "harness";
+
+test("invalid bun: module should preserve prefix in error", async () => {
+  const dir = tempDirWithFiles("bun-invalid-module", {
+    "test.js": 'import foo from "bun:apskdaposkdpok"',
+  });
+
+  const { stderr } = Bun.spawnSync({
+    cmd: [bunExe(), "test.js"],
+    cwd: dir,
+    env: bunEnv,
+    stderr: "pipe",
+  });
+
+  const output = stderr.toString();
+  // Error should mention "bun:apskdaposkdpok" not just "apskdaposkdpok"
+  expect(output).toContain("bun:apskdaposkdpok");
+  // The error could be "Cannot find package" or "Cannot resolve invalid URL"
+  // The key is that it shows the full "bun:..." prefix
+  expect(output).toMatch(/Cannot (find package|resolve invalid URL)/);
+});