Incremental compilation! And it's only going to incremental-er from here, as the compiler learns how to cache more and more sorts of interim artifacts to avoid redundant work. Though I think the wording in the OP is a bit off:

> Historically, the compiler has compiled your entire project, no matter how little you’ve changed the code.

This isn't quite correct. When you change the code in a given library, it has historically recompiled that entire library, and all libraries that depend on that library. But the compiler has never recompiled any libraries that were deeper in the dependency chain: if you have crates foo and bar, and bar imports foo, and you change the code in bar, foo was not recompiled.

Looking towards the future, I'm most excited about the prospect of using SIMD on stable Rust, which looks to be coming this year: https://github.com/rust-lang/rfcs/pull/2325

EDIT: Also worth mentioning is that Rayon (an official Rust library providing easy data parallelism) just reached 1.0 today, which is an important milestone in Rust's march towards ecosystem stability: https://github.com/rayon-rs/rayon

Have there been any thoughts around making the compiler something akin to a stateful server with an embedded datastore for the caching rather than using the filesystem?

What would be the benefit of this approach over using the filesystem?

I'd imagine it would be a benefit where, let's say, every project has `byteorder` in its dependency tree somewhere. If you had a local, stateful, build server that processes every build on your machine, you could re-use the object file from the last time you compiled that library regardless of if it was for this project or not. That's probably not a big gain for just `byteorder`, but if you multiply that by many libs or take into account some of the much bigger ones it could make quite a difference in build time. Even "first" build time for new projects you're building.

IIRC, some group in mozilla has already has something working that works like ccache that works (or rewrote ccache to work) with compiling rust. (Which I think is what steveklabnik is alluding to in his neighboring comment.) Although I guess that might only work within a single code base, not sure about that.