bun:test: don't abort when formatting a test.each title value throws

src/jsc/ConsoleObject.rs

@@ -1810,6 +1810,34 @@
         }
     }
 
+    impl ZigFormatter<'_, '_> {
+        /// Format into a byte sink, propagating the real `JsError` (a throwing
+        /// `[util.inspect.custom]`, termination, OOM). The `Display` impl below
+        /// collapses that to `fmt::Error`, which `std::io::Write::write_fmt`
+        /// turns into a panic when the sink itself never errored; use this
+        /// wherever the caller can `?`-propagate instead.
+        pub fn write_to(&self, writer: &mut dyn bun_io::Write) -> JsResult<()> {
+            let formatter: &mut Formatter<'_> = self
+                .formatter
+                .take()
+                .expect("ZigFormatter::write_to re-entered or used after consumption");
+
+            formatter.stack_check.update();
+            let one = [self.value];
+            formatter.remaining_values = bun_ptr::RawSlice::new(&one);
+
+            let result = (|| {
+                let tag = Tag::get(self.value, formatter.global_this)?;
+                let global = formatter.global_this;
+                formatter.format::<false>(tag, writer, self.value, global)
+            })();
+
+            formatter.remaining_values = bun_ptr::RawSlice::EMPTY;
+            self.formatter.set(Some(formatter));
+            result
+        }
+    }
+
     impl core::fmt::Display for ZigFormatter<'_, '_> {
         fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
             // Move the unique `&mut Formatter` out of the cell for the body;

src/runtime/test_runner/jest.rs

@@ -709,8 +709,10 @@ pub(crate) fn format_label(
                         list.extend_from_slice(owned_slice.slice());
                     } else {
                         let mut formatter = crate::test_runner::expect::make_formatter(global_this);
-                        // formatter cleanup handled by Drop.
-                        write!(&mut list, "{}", value.to_fmt(&mut formatter)).unwrap();
+                        // formatter cleanup handled by Drop. `write_to` (not
+                        // `write!`) so a throwing custom formatter surfaces as
+                        // a JS error instead of a panic in `write_fmt`.
+                        value.to_fmt(&mut formatter).write_to(&mut list)?;
                     }
                     idx = var_end;
                     continue;
@@ -787,8 +789,7 @@ pub(crate) fn format_label(
                 }
                 b'p' => {
                     let mut formatter = crate::test_runner::expect::make_formatter(global_this);
-                    let value_fmt = current_arg.to_fmt(&mut formatter);
-                    write!(&mut list, "{}", value_fmt).unwrap();
+                    current_arg.to_fmt(&mut formatter).write_to(&mut list)?;
                     idx += 1;
                     args_idx += 1;
                 }

test/cli/test/bun-test.test.ts

@@ -1218,6 +1218,21 @@
         expect(stderr).toContain("First user: Alice with tag: admin");
       });
 
+      test("surfaces a throwing custom formatter in the interpolated value as a test error", () => {
+        const stderr = runTest({
+          args: [],
+          expectExitCode: 1,
+          input: `
+            import { test } from "bun:test";
+
+            test.each([{ a: { b: { [Symbol.for("nodejs.util.inspect.custom")]() { throw new Error("boom from inspect.custom"); } } } }])("case $a.b", () => {});
+            test.each([[{ [Symbol.for("nodejs.util.inspect.custom")]() { throw new Error("boom from inspect.custom"); } }]])("case %p", () => {});
+          `,
+        });
+
+        expect(stderr).toContain("boom from inspect.custom");
+      });
+
       test("handles missing properties gracefully", () => {
         const cases = [{ a: 1 }];