Suggested Workflows

The full bootstrapping process takes quite a while. Here are three suggestions to make your life easier.

Check, check, and check again

The first workflow, which is useful when doing simple refactorings, is to run ./x.py check continuously. Here you are just checking that the compiler can build, but often that is all you need (e.g., when renaming a method). You can then run ./x.py build when you actually need to run tests.

In fact, it is sometimes useful to put off tests even when you are not 100% sure the code will work. You can then keep building up refactoring commits and only run the tests at some later time. You can then use git bisect to track down precisely which commit caused the problem. A nice side-effect of this style is that you are left with a fairly fine-grained set of commits at the end, all of which build and pass tests. This often helps reviewing.

Incremental builds with --keep-stage.

Sometimes just checking whether the compiler builds is not enough. A common example is that you need to add a debug! statement to inspect the value of some state or better understand the problem. In that case, you really need a full build. By leveraging incremental, though, you can often get these builds to complete very fast (e.g., around 30 seconds). The only catch is this requires a bit of fudging and may produce compilers that don't work (but that is easily detected and fixed).

The sequence of commands you want is as follows:

  • Initial build: ./x.py build -i --stage 1 src/libstd
    • As documented above, this will build a functional stage1 compiler as part of running all stage0 commands (which include building a libstd compatible with the stage1 compiler) as well as the first few steps of the "stage 1 actions" up to "stage1 (sysroot stage1) builds libstd".
  • Subsequent builds: ./x.py build -i --stage 1 src/libstd --keep-stage 1
    • Note that we added the --keep-stage 1 flag here

As mentioned, the effect of --keep-stage 1 is that we just assume that the old standard library can be re-used. If you are editing the compiler, this is almost always true: you haven't changed the standard library, after all. But sometimes, it's not true: for example, if you are editing the "metadata" part of the compiler, which controls how the compiler encodes types and other states into the rlib files, or if you are editing things that wind up in the metadata (such as the definition of the MIR).

The TL;DR is that you might get weird behavior from a compile when using --keep-stage 1 -- for example, strange ICEs or other panics. In that case, you should simply remove the --keep-stage 1 from the command and rebuild. That ought to fix the problem.

You can also use --keep-stage 1 when running tests. Something like this:

  • Initial test run: ./x.py test -i --stage 1 src/test/ui
  • Subsequent test run: ./x.py test -i --stage 1 src/test/ui --keep-stage 1

Building with system LLVM

By default, LLVM is built from source, and that can take significant amount of time. An alternative is to use LLVM already installed on your computer.

This is specified in the target section of config.toml:

[target.x86_64-unknown-linux-gnu]
llvm-config = "/path/to/llvm/llvm-7.0.1/bin/llvm-config"

On my system, this path is /usr/bin/llvm-config-7, but this probably varies by installation.