Caching not working as intended?

[laniakea64]

IIUC, our CI is supposed to cache

1. Rust crate dependencies downloaded into ~/.cargo,
2. The last test runner build (target directory).

Looking at CI logs from latest push, (2) seems either not working or useless: although the cache was correctly restored and no test runner changes were made since previous push, CI looks to have rebuilt the test runner from scratch.

If it's going to recompile everything anyway, should we make this the defined behavior? i.e. remove the attempt to cache target, and have the cache key based only on Cargo.lock, not on our Rust code. This would also save cache space and result in more cache reuse: no longer would a from-scratch cache be necessary for Rust code changes that do not change dependencies.

Alternatively, do we want to keep the current caching scheme, investigate why CI is compiling more than needed and adjust it so it stops doing that?

Not going to unilaterally go ahead try something different in this area, since not sure if messing with this in the wrong way might unintentionally result in incurring additional bills on @NoahTheDuke's Github account? 😨