Suggested Workflows

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

Configuring rust-analyzer for rustc

rust-analyzer can help you check and format your code whenever you save a file. By default, rust-analyzer runs the cargo check and rustfmt commands, but you can override these commands to use more adapted versions of these tools when hacking on rustc. For example, for Visual Studio Code, you can write:

    "rust-analyzer.checkOnSave.overrideCommand": [
    "rust-analyzer.rustfmt.overrideCommand": [
    "editor.formatOnSave": true

in your .vscode/settings.json file. This will ask rust-analyzer to use check to check the sources, and the stage 0 rustfmt to format them.

If running check on save is inconvenient, in VS Code you can use a Build Task instead:

// .vscode/tasks.json
    "version": "2.0.0",
    "tasks": [
            "label": "./ check",
            "command": "./ check",
            "type": "shell",
            "problemMatcher": "$rustc",
            "presentation": { "clear": true },
            "group": { "kind": "build", "isDefault": true }

Check, check, and check again

When doing simple refactorings, it can be useful to run ./ check continuously. If you set up rust-analyzer as described above, this will be done for you every time you save a file. 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 ./ 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: ./ build -i library/std
    • As documented previously, this will build a functional stage1 compiler as part of running all stage0 commands (which include building a std compatible with the stage1 compiler) as well as the first few steps of the "stage 1 actions" up to "stage1 (sysroot stage1) builds std".
  • Subsequent builds: ./ build -i library/std --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: ./ test -i src/test/ui
  • Subsequent test run: ./ test -i src/test/ui --keep-stage 1

Fine-tuning optimizations

Setting optimize = false makes the compiler too slow for tests. However, to improve the test cycle, you can disable optimizations selectively only for the crates you'll have to rebuild (source). For example, when working on rustc_mir_build, the rustc_mir_build and rustc_driver crates take the most time to incrementally rebuild. You could therefore set the following in the root Cargo.toml:

opt-level = 0
opt-level = 0

Working on multiple branches at the same time

Working on multiple branches in parallel can be a little annoying, since building the compiler on one branch will cause the old build and the incremental compilation cache to be overwritten. One solution would be to have multiple clones of the repository, but that would mean storing the Git metadata multiple times, and having to update each clone individually.

Fortunately, Git has a better solution called worktrees. This lets you create multiple "working trees", which all share the same Git database. Moreover, because all of the worktrees share the same object database, if you update a branch (e.g. master) in any of them, you can use the new commits from any of the worktrees. One caveat, though, is that submodules do not get shared. They will still be cloned multiple times.

Given you are inside the root directory for your rust repository, you can create a "linked working tree" in a new "rust2" directory by running the following command:

git worktree add ../rust2

Creating a new worktree for a new branch based on master looks like:

git worktree add -b my-feature ../rust2 master

You can then use that rust2 folder as a separate workspace for modifying and building rustc!

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:

llvm-config = "/path/to/llvm/llvm-7.0.1/bin/llvm-config"

We have observed the following paths before, which may be different from your system:

  • /usr/bin/llvm-config-8
  • /usr/lib/llvm-8/bin/llvm-config

Note that you need to have the LLVM FileCheck tool installed, which is used for codegen tests. This tool is normally built with LLVM, but if you use your own preinstalled LLVM, you will need to provide FileCheck in some other way. On Debian-based systems, you can install the llvm-N-tools package (where N is the LLVM version number, e.g. llvm-8-tools). Alternately, you can specify the path to FileCheck with the llvm-filecheck config item in config.toml or you can disable codegen test with the codegen-tests item in config.toml.