There is an .early-rmeta file that is generated that contains the result of type checking the outer part of the functions (arguments, returns) that is then passed down to all the other dependencies that can use it and get a "headstart" on their work. -Zearly-metadata is the flag for Rustc here to turn that on. It also needed to learn how to swap the real metadata in for the early stuff that it was previously using, once the real stuff showed up.

Rustc needs to be able to pause and resume at certain points and Cargo needs to be know how to look for and react to that, which is what -Zheadstart does for Cargo. So there's some changes job_queue that happen to there. I haven't checked this directly yet, but my understanding is that it doesn't kill the process, it just has it sit and wait, which normally would occupy a slot, but doesn't any longer. ---

So not a cache, just being able to pause and start work and better utilize the available slots.