Preserve pure comments

[edoardocavazza]

Right now @__PURE__ comments are drop from the final file, also if the comment option is all. In addition to being an inconsistent behavior, this strategy can be dangerous if terser is applied to an ES module, since those files can be consumed by browsers AND bundlers like Rollup which handle the pure annotation.

This PR also prevent terser from adding empty comments as described by #526.

Please consider this PR as a simple suggestion, I know that it could be a potential breaking change and that it can implement an unwanted behavior 😅

[fabiosantoscode]

Hey there!

What is your specific use case? I think that if rollup had a problem with the removal of pure comments @TrySound or someone else working on rollup would've brought it up at this point. Either to Terser or UglifyJS, which does the same thing. So please let me know, what is the use case?

[TrySound]

I think the point is distributing preminified esm which could be consumed by bundlers. Though probably not only lack of pure annotation may prevent rollup optimisations. IMO better to run terser or any other minifier in the end of app build process.

[edoardocavazza]

Hello! IMO, in the near future the usage of import/export in the browsers will grow a lot and distributing a single module entry point for bundlers and browsers (for example, using unpkg.com) simplifies a lot the workflow for maintainers and developers. As @TrySound pointed out, Rollup can not optimize final bundles if the pure annotation is missing. Furthermore, at the moment we are not able to handle @__PURE__ as any other comment because they are always stripped out. Do you think that introducing a new option for this case can make sense?

[edoardocavazza]

Please note that this PR does not make terser to ALWAYS preserve pure annotation, but to treat them as any other comment. You can still remove any comment or filter them out via option.

[fabiosantoscode]

You are both making a lot of sense here, thanks for your input :) but let's add a preserve_annotations output option (which defaults to false).

This is going to make the project harder to maintain, but it seems like there's a real need for this.

And thanks for removing those awkward empty comments :)

If you don't mind, please update the typescript typings to reflect this new option. If you don't I can always do that later so don't worry.

[edoardocavazza]

Hi @fabiosantoscode , I added the option, can you please review my commits?

[fabiosantoscode]

Hey there! Everything looks great, just that one change I pointed out in the review. Thanks for your contribution!

[fabiosantoscode]

Super cool! I'll release a version with this today or tomorrow. Thank you very much!

[danieltroger]

Hi, how can I enable this feature? cat module.js|terser -cm --comments "/__PURE__/" does not produce the desired result (with pure comments preserved)