LOC is useful here not because it's a metric for output but because it's a metric for _understandability_. Reviewing 200 lines is a very different workload than reviewing 2000.
sccache caches at the per-rustc invocation level, whereas we cache at the package level. In practice, this means we get more cache hits and our cache hits take less overhead to load. You can see the benchmarks in our CI!
Monads are a generalization of Promises. Each type in Monad defines their own `.then` in a different way. For promises, `.then` is defined as "run this function once you have this deferred value from the last promise". For optionals (`Maybe`), `.then` is defined as "run this function if the last optional had an actual value". For Either, `.then` is defined as "run this function if the last Either returned Right, otherwise early-return with the value from Left" (this is functional early-return, basically).