Comment by epage

A little of both. Incremental compilation cache is likely the single largest item in the target directory but it gets cleaned up on each invocation so it doesn't scale in size with time.

I believe the next release will have a cache GC but only for global caches (e.g. `.crate` files). I'd like us to at least cleanup the layout of the target directory so its easier to track stuff before GCing it. Work is underway for this. A cheap GC we could add earlier is for artifacts specific to older cargo versions.