There was an experimental PR that treats proc macros as idempotent with the corresponding colpike speed up. I don't know what happened with it, and stabilization required a lot of design work to not break backcompat. But this is something in the team's radar.

Define “fixing.” If you’re building on ephemeral containers, an external cache is necessary for files that don’t change.

managing deps as a separate project introduces a lot of day-to-day annoyances, especially if the files are actually changed decently often.

Does ccache fetch compiled code from a central server using checksums?

I will just say this: it is unfortunate that you have chosen not to engage with the content of the article.

Considering that Firefox mainly appeals to its own contributors, making the developer experience better would be important.

Obviously it'll make the developer more efficient to spend more time twiddling his thumbs waiting for his code to compile rather than creating a simple build performance win that allows him to, you know, spend more time improving Firefox. Not to mention all the other developers who stand to benefit from faster builds.