Babel plugin runs on v2 addon code where it shouldn't

[simonihmig]

Context:

• app is using Embroider and has ember-cached-decorator-polyfill (it wouldn't need to actually, but also it shouldn't do any harm, right?)
• ember-headless-form uses @cached, but only supports Ember >= 4.4, so has no need for ember-cached-decorator-polyfill and no reference to it whatsoever
• reported also in https://github.com/CrowdStrike/ember-headless-form/issues/146
• app is using Embroider 2.x, but I have seen this also happen for 3.0! It is not happening for a classic build!

Reproduction:

• clone https://github.com/CrowdStrike/ember-toucan-core
• checkout branch cached-decorator-polyfill-reproduction (main was fixed by https://github.com/CrowdStrike/ember-toucan-core/pull/168)
• run pnpm i + pnpm start:docs

While debugging, I found:

• the polyfill's babel plugin is only added once to the "parent" here, and parent is the app (as expected)
• putting a breakpoint here in the babel plugin shows that the plugin also runs for the v2 addons use of @cached here. I believe this is unexpected, as the polyfill should only run on the app's code, shouldn't it?
• you can clearly see that the addon's code is transformed afterwards, applying the stuff the polyfill does (... importSync('ember-cached-decorator-polyfill').cached)
• when the module resolver run, it throws with ember-headless-form is trying to import from ember-cached-decorator-polyfill but that is not one of its explicit dependencies

[ef4]

Under embroider there is only one babel config during the final stage app build. So it's true that a transform added to the app is also going to run over the addons. In general babel plugins should try not to break code that doesn't need the feature they add.

We offer skipBabel to disable all babel transpilation of a given package, but that is rather extreme as you also lose preset-env, etc.

Perhaps the cached decorator polyfill could behave more like @babel/runtime, which has the option to emit an absolute or relative path in the import, so that it's resolvable regardless of which package it appear in.