Experiment with incremental builds

[jdm]

<nmatsakis> 	pcwalton: I won't get to that testing just now, but you can basically do `mkdir ~/incr; RUSTFLAGS=-Zincremental=$HOME/incr` and give it a spin. At some point there was an ICE with the script crate but that has been fixed, still a rustup may be needed :/

[jdm]

rust-lang/rust#39208

[SimonSapin]

Apparently we can now use CARGO_INCREMENTAL=1, and Cargo will take care of managing the cache directory somewhere in target/.

[jdm]

Further updates in https://internals.rust-lang.org/t/incremental-compilation-beta/4721/17.

[SimonSapin]

I’m waiting for the next nightly to try that last fix (rust-lang/rust#39424).

[SimonSapin]

Well, I didn’t wait. With Rust master, incremental compilation cuts build times after touch components/style/lib.rs by ~30% compared to -C codegen-units=4. Note though that this is the ideal case for incremental compilation since nothing other than a source file timestamp changes. The time for a real rebuild (with few but non-zero changes) would be somewhere in-between.

As a first step, let’s add it opt-in: #15390

[nox]

Experimenting was done, given we even have a flag to use it in mach now.