[mark-simulacrum]

Depending on what you're doing, there's often a few different approaches to reducing compile times. First of all, if you're going to be running (small) tests, then it may be beneficial to enable incremental with `--incremental` or `-i` to x.py. This does make the resulting compiler slower, but does make the build itself faster.

If you're making type-level changes that aren't going to alter functionality, or want to just make sure the whole compiler will build, you can run `x.py check -i` to get the equivalent of `cargo check` outside of the compiler tree.

If you know ahead of time that you're not changing the code generation/ABI of the compiler (metadata, mangling, that sort of thing) then you can also do something like `x.py --keep-stage 0` and work entirely in stage 2 (you wouldn't pass --stage 1 with this option). Alternatively, you can pass `--keep-stage 1` and run the stage 1 tests; this will save you a rebuild of std and test which is often unnecessary after a compiler change.

However, even with these suggestions/hints, building the Rust compiler can be rather slow.