[robobun]

Fixes #44880

Problem

• toEqual, toStrictEqual, toMatchObject, toHaveProperty, toHaveBeenCalledWith (and the Last / Nth variants) ignore Symbol.for("nodejs.util.inspect.custom") and Bun.inspect.custom when they print the failure diff. They walk every own property of the value. The issue's repro gives a 1.3 MB message for toEqual while toBe prints <Node> in 65 chars. A DOM node that reaches window can exhaust memory.
• The cause is Tag::get in src/runtime/test_runner/pretty_format.rs. It classifies by JSType only and never probes the hook. The console formatter (src/jsc/ConsoleObject.rs:2135) that toBe uses does probe it.

Fix

• Formatter::tag_of wraps Tag::get and, when FormatOptions.inspect_custom is set, probes the hook for Object, Array, Map, Set, Error, Function and Class tags. A callable hook gives the new Tag::CustomFormatted. print_custom_formatted calls the hook with (depth, options, inspect), writes a string result verbatim, formats any other result, and falls back to the default printer when the hook returns this.
• The diff path (diff_format.rs) sets inspect_custom: true. The snapshot path (mod.rs) sets it to false, so .snap output does not change and matches Jest.
• The hook call is shared with the console formatter through the new bun_jsc::console_object::call_custom_inspect. When both sides print alike, DiffFormatter now reports Received: serializes to the same string as toBe and Jest do.
• Verified: test/js/bun/test/expect-inspect-custom.test.ts (20 tests, 16 fail on bun 1.4.3). Also test/js/bun/test/snapshot-tests, printing, expect.test.js, expect/, mock-fn.test.js, spyMatchers.test.ts, expect-extend*, test/cli/test/bun-test.test.ts, test/js/bun/util/inspect.test.js. No .snap file changed.

Background

• JestPrettyFormat is a port of Jest's pretty-format. It prints both sides of a matcher diff and every snapshot. Tag is its value classification, computed once per value before the printer dispatches on it.
• The console formatter in ConsoleObject.rs is a separate printer used by console.log, Bun.inspect and the non-diff matchers (toBe, toBeNull). It gained the inspect.custom probe in 2024. The snapshot formatter never followed.
• Considered a probe inside the Tag::Object print arm only (about 60 lines). It runs after Tag::get classified the value by JSType, so a hooked class A extends Array or extends Error would still be walked while toBe prints <A>. The classifier-level probe covers those for the same per-value cost.
• Considered a byte cap on the diff text and a callToJSON port. The cap lets the walk happen and throws the text away. toJSON changes snapshot-adjacent output. Both left out.

Downsides

• A hook that prints a DOM node short form must exist. happy-dom 20 defines no inspect.custom, so a happy-dom element with no hook still prints its full graph. Users can define the hook on Node.prototype in a preload.
• Failing diff matchers now run user code (the hook). A throwing hook surfaces as a catchable error, same as toBe today. Passing matchers never call it (tested).
• Cost with the flag off (every toMatchSnapshot): one bool load and one tag compare per value, no allocation, no host call. Interleaved A/B of 2000 toMatchSnapshot calls on a 50-key nested object, debug build: main 60.5 s, PR 60.6 s, spread 2 s. Diff path on a hook-less 1000-key object (one extra fast_get per object): main 31.1 s, PR 31.5 s, spread 1.3 s. Debug binary text +25,688 bytes (size). No release toolchain, bloaty, valgrind or perf in this environment.

Notes

Repro message lengths (repro.test.ts from the issue), bun 1.4.3 vs this PR:

matcher	before	after
toBeNull	46	46
toBe	65	65
toEqual	1,337,893	68
toStrictEqual	1,497,935	121
toMatchObject	1,497,935	121
toHaveBeenCalledWith	1,497,929	115

Which values are probed: the probe runs after the JSType match and only for tags whose printer walks the value (Object, Array, Map, Set, Error, Function, Class), never for ProxyObject or a cell can_get() rejects. So TypedArray and Buffer keep the per-byte {"data": [...]} diff (Buffer.prototype has a hook), DOM wrappers stay on Tag::Private, Proxy and Date stay on the JSON arm.

Depth argument: indent already counts nesting levels, so the hook receives (8 - indent, {depth: 8}), the same pair the console formatter passes from its max_depth of 8. A nested hook sees a smaller depth than the root (tested).

Stash: tag_of stores (this, hook) on the formatter and the matching format call consumes it. If a peek site tagged a different value in between, print_custom_formatted looks the hook up again instead of calling a stale one.

Formatter grows by one bool (fits in padding) and one Option<(JSValue, JSValue)> (24 bytes). TagResult stays 2 bytes.

Pre-existing failures in this environment, reproduced on a main build too: snapshot-tests/snapshots/snapshot.test.ts "error snapshots" (expects ANSI colors, passes with FORCE_COLOR=1), the nested too deeply cases in bun-test.test.ts and pretty-format-overflow.test.ts (5 s timeouts under the debug ASAN build).

Left as is: expect.addSnapshotSerializer still throws "Not implemented". No depth or size cap on the diff printer. toJSON is still only called for FormData.

[robobun]

^{Updated 5:23 PM PT - Oct 19th, 2026}

@robobun, your commit 69534fa is building: #129172

[robobun]

Closing after a self-review. The formatter rewrite in #44268 (dylan-conway) deletes pretty_format.rs and sets disable_inspect_custom = true for the diff and snapshot formatters, which is also what Jest's pretty-format does. Merging this would flip toEqual failure text and then conflict with that rewrite. The happy-dom case that the reporters hit (nodes with no hook) is handled by #44066, #44833 and #44268, not by this change. The test file on this branch (test/js/bun/test/expect-inspect-custom.test.ts) stays available as a spec if honouring inspect.custom in diffs is wanted later.