Bootstrapping the Compiler

This subchapter is about the bootstrapping process.

What is bootstrapping? How does it work?

Bootstrapping is the process of using a compiler to compile itself. More accurately, it means using an older compiler to compile a newer version of the same compiler.

This raises a chicken-and-egg paradox: where did the first compiler come from? It must have been written in a different language. In Rust's case it was written in OCaml. However it was abandoned long ago and the only way to build a modern version of rustc is a slightly less modern version.

This is exactly how works: it downloads the current beta release of rustc, then uses it to compile the new compiler.

Stages of bootstrapping

Compiling rustc is done in stages:

  • Stage 0: the stage0 compiler is usually (you can configure to use something else) the current beta rustc compiler and its associated dynamic libraries (which will download for you). This stage0 compiler is then used only to compile rustbuild, std, and rustc. When compiling rustc, this stage0 compiler uses the freshly compiled std. There are two concepts at play here: a compiler (with its set of dependencies) and its 'target' or 'object' libraries (std and rustc). Both are staged, but in a staggered manner.
  • Stage 1: the code in your clone (for new version) is then compiled with the stage0 compiler to produce the stage1 compiler. However, it was built with an older compiler (stage0), so to optimize the stage1 compiler we go to next the stage.
    • In theory, the stage1 compiler is functionally identical to the stage2 compiler, but in practice there are subtle differences. In particular, the stage1 compiler itself was built by stage0 and hence not by the source in your working directory: this means that the symbol names used in the compiler source may not match the symbol names that would have been made by the stage1 compiler. This is important when using dynamic linking and the lack of ABI compatibility between versions. This primarily manifests when tests try to link with any of the rustc_* crates or use the (now deprecated) plugin infrastructure. These tests are marked with ignore-stage1.
  • Stage 2: we rebuild our stage1 compiler with itself to produce the stage2 compiler (i.e. it builds itself) to have all the latest optimizations. (By default, we copy the stage1 libraries for use by the stage2 compiler, since they ought to be identical.)
  • (Optional) Stage 3: to sanity check our new compiler, we can build the libraries with the stage2 compiler. The result ought to be identical to before, unless something has broken.

The stage2 compiler is the one distributed with rustup and all other install methods. However, it takes a very long time to build because one must first build the new compiler with an older compiler and then use that to build the new compiler with itself. For development, you usually only want the stage1 compiler: build library/std.

Default stages tries to be helpful and pick the stage you most likely meant for each subcommand. These defaults are as follows:

  • doc: --stage 0
  • build: --stage 1
  • test: --stage 1
  • dist: --stage 2
  • install: --stage 2
  • bench: --stage 2

You can always override the stage by passing --stage N explicitly.

Complications of bootstrapping

Since the build system uses the current beta compiler to build the stage-1 bootstrapping compiler, the compiler source code can't use some features until they reach beta (because otherwise the beta compiler doesn't support them). On the other hand, for compiler intrinsics and internal features, the features have to be used. Additionally, the compiler makes heavy use of nightly features (#![feature(...)]). How can we resolve this problem?

There are two methods used:

  1. The build system sets --cfg bootstrap when building with stage0, so we can use cfg(not(bootstrap)) to only use features when built with stage1. This is useful for e.g. features that were just stabilized, which require #![feature(...)] when built with stage0, but not for stage1.
  2. The build system sets RUSTC_BOOTSTRAP=1. This special variable means to break the stability guarantees of rust: Allow using #![feature(...)] with a compiler that's not nightly. This should never be used except when bootstrapping the compiler.

Contributing to bootstrap

When you use the bootstrap system, you'll call it through However, most of the code lives in src/bootstrap. bootstrap has a difficult problem: it is written in Rust, but yet it is run before the rust compiler is built! To work around this, there are two components of bootstrap: the main one written in rust, and is what gets run by It takes care of downloading the stage0 compiler, which will then build the bootstrap binary written in Rust.

Because there are two separate codebases behind, they need to be kept in sync. In particular, both and the bootstrap binary parse config.toml and read the same command line arguments. keeps these in sync by setting various environment variables, and the programs sometimes have to add arguments that are explicitly ignored, to be read by the other.

Adding a setting to config.toml

This section is a work in progress. In the meantime, you can see an example contribution here.

Understanding stages of bootstrap

This is a detailed look into the separate bootstrap stages. When running you will see output such as:

Building stage0 std artifacts
Copying stage0 std from stage0
Building stage0 compiler artifacts
Copying stage0 rustc from stage0
Building LLVM for x86_64-apple-darwin
Building stage0 codegen artifacts
Assembling stage1 compiler
Building stage1 std artifacts
Copying stage1 std from stage1
Building stage1 compiler artifacts
Copying stage1 rustc from stage1
Building stage1 codegen artifacts
Assembling stage2 compiler
Uplifting stage1 std
Copying stage2 std from stage1
Generating unstable book md files
Building stage0 tool unstable-book-gen
Building stage0 tool rustbook
Documenting standalone
Building rustdoc for stage2
Documenting book redirect pages
Documenting stage2 std
Building rustdoc for stage1
Documenting stage2 whitelisted compiler
Documenting stage2 compiler
Documenting stage2 rustdoc
Documenting error index
Uplifting stage1 rustc
Copying stage2 rustc from stage1
Building stage2 tool error_index_generator

A deeper look into's phases can be seen here:

A diagram of the rustc compilation phases

Keep in mind this diagram is a simplification, i.e. rustdoc can be built at different stages, the process is a bit different when passing flags such as --keep-stage, or if there are non-host targets.

The following tables indicate the outputs of various stage actions:

Stage 0 ActionOutput
beta extractedbuild/HOST/stage0
stage0 builds bootstrapbuild/bootstrap
stage0 builds test/stdbuild/HOST/stage0-std/TARGET
copy stage0-std (HOST only)build/HOST/stage0-sysroot/lib/rustlib/HOST
stage0 builds rustc with stage0-sysrootbuild/HOST/stage0-rustc/HOST
copy stage0-rustc (except executable)build/HOST/stage0-sysroot/lib/rustlib/HOST
build llvmbuild/HOST/llvm
stage0 builds codegen with stage0-sysrootbuild/HOST/stage0-codegen/HOST
stage0 builds rustdoc with stage0-sysrootbuild/HOST/stage0-tools/HOST

--stage=0 stops here.

Stage 1 ActionOutput
copy (uplift) stage0-rustc executable to stage1build/HOST/stage1/bin
copy (uplift) stage0-codegen to stage1build/HOST/stage1/lib
copy (uplift) stage0-sysroot to stage1build/HOST/stage1/lib
stage1 builds test/stdbuild/HOST/stage1-std/TARGET
copy stage1-std (HOST only)build/HOST/stage1/lib/rustlib/HOST
stage1 builds rustcbuild/HOST/stage1-rustc/HOST
copy stage1-rustc (except executable)build/HOST/stage1/lib/rustlib/HOST
stage1 builds codegenbuild/HOST/stage1-codegen/HOST

--stage=1 stops here.

Stage 2 ActionOutput
copy (uplift) stage1-rustc executablebuild/HOST/stage2/bin
copy (uplift) stage1-sysrootbuild/HOST/stage2/lib and build/HOST/stage2/lib/rustlib/HOST
stage2 builds test/std (not HOST targets)build/HOST/stage2-std/TARGET
copy stage2-std (not HOST targets)build/HOST/stage2/lib/rustlib/TARGET
stage2 builds rustdocbuild/HOST/stage2-tools/HOST
copy rustdocbuild/HOST/stage2/bin

--stage=2 stops here.

Note that the convention uses is that:

  • A "stage N artifact" is an artifact that is produced by the stage N compiler.
  • The "stage (N+1) compiler" is assembled from "stage N artifacts".
  • A --stage N flag means build with stage N.

In short, stage 0 uses the stage0 compiler to create stage0 artifacts which will later be uplifted to stage1.

Every time any of the main artifacts (std and rustc) are compiled, two steps are performed. When std is compiled by a stage N compiler, that std will be linked to programs built by the stage N compiler (including rustc built later on). It will also be used by the stage (N+1) compiler to link against itself. This is somewhat intuitive if one thinks of the stage (N+1) compiler as "just" another program we are building with the stage N compiler. In some ways, rustc (the binary, not the rustbuild step) could be thought of as one of the few no_core binaries out there.

So "stage0 std artifacts" are in fact the output of the downloaded stage0 compiler, and are going to be used for anything built by the stage0 compiler: e.g. rustc artifacts. When it announces that it is "building stage1 std artifacts" it has moved on to the next bootstrapping phase. This pattern continues in latter stages.

Also note that building host std and target std are different based on the stage (e.g. see in the table how stage2 only builds non-host std targets. This is because during stage2, the host std is uplifted from the "stage 1" std -- specifically, when "Building stage 1 artifacts" is announced, it is later copied into stage2 as well (both the compiler's libdir and the sysroot).

This std is pretty much necessary for any useful work with the compiler. Specifically, it's used as the std for programs compiled by the newly compiled compiler (so when you compile fn main() { } it is linked to the last std compiled with build library/std).

The rustc generated by the stage0 compiler is linked to the freshly-built std, which means that for the most part only std needs to be cfg-gated, so that rustc can use featured added to std immediately after their addition, without need for them to get into the downloaded beta. The std built by the stage1/bin/rustc compiler, also known as "stage1 std artifacts", is not necessarily ABI-compatible with that compiler. That is, the rustc binary most likely could not use this std itself. It is however ABI-compatible with any programs that the stage1/bin/rustc binary builds (including itself), so in that sense they're paired.

This is also where --keep-stage 1 library/std comes into play. Since most changes to the compiler don't actually change the ABI, once you've produced a std in stage 1, you can probably just reuse it with a different compiler. If the ABI hasn't changed, you're good to go, no need to spend the time recompiling that std. --keep-stage simply assumes the previous compile is fine and copies those artifacts into the appropriate place, skipping the cargo invocation.

The reason we first build std, then rustc, is largely just because we want to minimize cfg(stage0) in the code for rustc. Currently rustc is always linked against a "new" std so it doesn't ever need to be concerned with differences in std; it can assume that the std is as fresh as possible.

The reason we need to build it twice is because of ABI compatibility. The beta compiler has it's own ABI, and then the stage1/bin/rustc compiler will produce programs/libraries with the new ABI. We used to build three times, but because we assume that the ABI is constant within a codebase, we presume that the libraries produced by the "stage2" compiler (produced by the stage1/bin/rustc compiler) is ABI-compatible with the stage1/bin/rustc compiler's produced libraries. What this means is that we can skip that final compilation -- and simply use the same libraries as the stage2/bin/rustc compiler uses itself for programs it links against.

This stage2/bin/rustc compiler is shipped to end-users, along with the stage 1 {std,rustc} artifacts.

Passing stage-specific flags to rustc allows you to pass stage-specific flags to rustc when bootstrapping. The RUSTFLAGS_STAGE_0, RUSTFLAGS_STAGE_1 and RUSTFLAGS_STAGE_2 environment variables pass the given flags when building stage 0, 1, and 2 artifacts respectively.

Additionally, the RUSTFLAGS_STAGE_NOT_0 variable, as its name suggests, pass the given arguments if the stage is not 0.

Environment Variables

During bootstrapping, there are a bunch of compiler-internal environment variables that are used. If you are trying to run an intermediate version of rustc, sometimes you may need to set some of these environment variables manually. Otherwise, you get an error like the following:

thread 'main' panicked at 'RUSTC_STAGE was not set: NotPresent', library/core/src/

If ./stageN/bin/rustc gives an error about environment variables, that usually means something is quite wrong -- or you're trying to compile e.g. rustc or std or something that depends on environment variables. In the unlikely case that you actually need to invoke rustc in such a situation, you can find the environment variable values by adding the following flag to your command: --on-fail=print-env.