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.

Correct builds != never running garbage cleanup. I would settle for it evicting older variants of a build (I also dislike the random hash that’s impossible to determine what specifically is different between two hashes / which one is newer).

Automatic garbage collection of old build artifacts* is coming in Rust 1.88 (currently on the beta channel, will become the new stable release in two weeks):

https://github.com/rust-lang/cargo/issues/12633

*EDIT: For now this only deletes old cached downloads, not build artifacts. Thanks epage for the correction below.

That is not for build artifacts but global caches like for `.crate` files but its a stepping stone.

Oops, thanks for the correction.

That’s as an additional flag and not the default?

In versions earlier than 1.88, garbage collection required the unstable -Zgc flag (and a nightly toolchain). But in 1.88 and later, automatic garbage collection is enabled by default.