As far as I can tell from experimentation, the existing useResourceCacheWhen only helps with Sass transformations:

• We don't cache Babel transformations
• We don't cache Tailwind transformations
• We throw a "You need to install PostCSS" error even though we cache PostCSS transformations

It sounds like the resources.Vendor approach would handle all of the above, storing the cached transformations in the _vendor/resources directory.

Questions:

1. Would this mean that the current assets cache ([caches.assets]) is no longer required or used?
2. Would we still use the useResourceCacheWhen setting to control when we use _vendor/resources?
3. How will module and theme authors include transformed resources in source control? Today they can include the resources directory. Would they somehow include _vendor/resources instead?

@jmooring OK, so now I have slept a little on this. I haven't edited my text above, but I think it holds mostly water, but I think we'll:

• Drop the resource.Vendor construct. It mostly adds confusion/complexity, I think. I think ...
• We formalise the list of "vendorable resources" (Babel, Tailwind, PostCSS, js.Build, js.Batch and possibly Sass).
• We create a better ignoreVendorPaths config setup/default where you can say e.g. ignore vendoring for resources/tailwindcss/**.. or something; and I'm guessing we can have it default off for, say, Sass.
• [caches.assets] will still have its (important use) as a file cache (but maybe some of it will be adjusted, not used anymore). I'm probably abusing the term vendoring above, but it technically fits so nicely into what we have, so I think I'm going to ... live with it. A cache is something shorter lived and less formalised, and can be deleted at any time.
• useResourceCacheWhen would (eventually) go away.

A note to self: I think we need to be a little lenient with the key used to vendor these resources, e.g. from the example above:

{{ $opts := dict
   "inlineImports" true
   "optimize" (not hugo.IsDevelopment)
}}

If I run

hugo vendor -e production
hugo server

I want the vendored resource when running the server.

EDIT IN: OK, looking at the assets cache definition now, I would say that that cache would go away after this.

There's a very rough implementation of this in #13310

• Has a hugo vendor command.
• Only works for TailwindCSS and js.Build, but I have managed to build a site with TailwindCSS app with typography plugin + a fair amount of JS libraries via npm ... without any node_modules folder, so I will call this promising.

I'm closing this for now. I will come up with something very similar, but I will outline that more precisely in a different issue.