... the error message generation can take seconds to minutes.

Do you have a reproduction case that demonstrates it taking minutes? I find that extremely difficult to believe. Specifically, acorn is generally quite fast and there are hard practical limits already in place that would prevent it from taking "minutes". I think at best this would shave a couple hundred milliseconds off an error path, which is of questionable value given the added complexity.

Hey @jasnell, fair question. I'm working on putting together a proper reproduction case for the timing claim. Will update here once I have it.

Been a bit sidetracked dealing with Windows build issues on another PR, but this is on my list. Thanks for the patience.

@jasnell It's a valid known issue and merging would be highly appreciated.

There is a repro here: #52677 (comment)

Spent an hour trying to figure out why my console freezes for minutes while running my tests when I use assert.ok.

There are a few issues based on this:
privatenumber/tsx#548
#52677
#52962

I owed a repro since March and never delivered it. Here it is, with numbers.

The repro is the one @elemental-mind linked: https://gist.github.com/martinjlowm/aa7d02905f7c49935be0a3bf4819a5f3

Both builds are from b328bf7, same commit, same config. The file is a single 930KB line with the assert call inside that line. "sparse" is semicolons, "dense" is minified code with real identifiers and calls. Each run touches e.message, otherwise the message is never built and the slow path never runs, which is how I fooled myself twice while measuring this.

@jasnell your estimate is right for a single failure, it is 64ms here, not minutes. What I did not say in March is that the cost is paid per failure and is not cached. A suite with 1000 failing assertions on this file spends about 59s inside getFirstExpression, and it scales linearly with the line length. That is where the "minutes" reports come from.

Assert tests pass, including test-assert-long-line-perf.js.

I squashed the two commits into one and changed the prefix to lib:, which is what the history of error_source.js uses. The diff is unchanged.

Codecov Report

❌ Patch coverage is 87.91209% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.12%. Comparing base (b328bf7) to head (05010a4).
⚠️ Report is 1645 commits behind head on main.

@@            Coverage Diff             @@
##             main   #62338      +/-   ##
==========================================
+ Coverage   89.68%   90.12%   +0.44%     
==========================================
  Files         676      752      +76     
  Lines      206689   252250   +45561     
  Branches    39579    47458    +7879     
==========================================
+ Hits       185370   227343   +41973     
- Misses      13450    16220    +2770     
- Partials     7869     8687     +818     

... and 494 files with indirect coverage changes

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