What Bootstrapping does

Note that this documentation mostly covers user-facing information. See bootstrap/README.md to read about bootstrap internals.

Stages of bootstrapping


  • Stage 0: the pre-compiled compiler
  • Stage 1: from current code, by an earlier compiler
  • Stage 2: the truly current compiler
  • Stage 3: the same-result test

Compiling rustc is done in stages. Here's a diagram, adapted from Jynn Nelson's talk on bootstrapping at RustConf 2022, with detailed explanations below.

The A, B, C, and D show the ordering of the stages of bootstrapping. Blue nodes are downloaded, yellow nodes are built with the stage0 compiler, and green nodes are built with the stage1 compiler.

graph TD
    s0c["stage0 compiler (1.63)"]:::downloaded -->|A| s0l("stage0 std (1.64)"):::with-s0c;
    s0c & s0l --- stepb[ ]:::empty;
    stepb -->|B| s0ca["stage0 compiler artifacts (1.64)"]:::with-s0c;
    s0ca -->|copy| s1c["stage1 compiler (1.64)"]:::with-s0c;
    s1c -->|C| s1l("stage1 std (1.64)"):::with-s1c;
    s1c & s1l --- stepd[ ]:::empty;
    stepd -->|D| s1ca["stage1 compiler artifacts (1.64)"]:::with-s1c;
    s1ca -->|copy| s2c["stage2 compiler"]:::with-s1c;

    classDef empty width:0px,height:0px;
    classDef downloaded fill: lightblue;
    classDef with-s0c fill: yellow;
    classDef with-s1c fill: lightgreen;

Stage 0: the pre-compiled compiler

The stage0 compiler is usually the current beta rustc compiler and its associated dynamic libraries, which x.py will download for you. (You can also configure x.py to use something else.)

The stage0 compiler is then used only to compile src/bootstrap, std, and rustc. When assembling the libraries and binaries that will become the stage1 rustc compiler, the freshly compiled std and rustc are used. 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: from current code, by an earlier compiler

The rustc source code is then compiled with the stage0 compiler to produce the stage1 compiler.

Stage 2: the truly current compiler

We then rebuild our stage1 compiler with itself to produce the stage2 compiler.

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 ABI generated by the stage0 compiler may not match the ABI that would have been made by the stage1 compiler, which can cause problems for dynamic libraries, tests, and tools using rustc_private.

Note that the proc_macro crate avoids this issue with a C FFI layer called proc_macro::bridge, allowing it to be used with stage 1.

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, which you can build with ./x build library. See Building the compiler.

Stage 3: the same-result test

Stage 3 is optional. 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.

Building the stages

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

  • check: --stage 0
  • 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.

For more information about stages, see below.

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.

Understanding stages of bootstrap


This is a detailed look into the separate bootstrap stages.

The convention x uses is that:

  • A --stage N flag means to run the stage N compiler (stageN/rustc).
  • A "stage N artifact" is a build artifact that is produced by the stage N compiler.
  • The stage N+1 compiler is assembled from stage N artifacts. This process is called uplifting.

Build artifacts

Anything you can build with x is a build artifact. Build artifacts include, but are not limited to:

  • binaries, like stage0-rustc/rustc-main
  • shared objects, like stage0-sysroot/rustlib/libstd-6fae108520cf72fe.so
  • rlib files, like stage0-sysroot/rustlib/libstd-6fae108520cf72fe.rlib
  • HTML files generated by rustdoc, like doc/std


  • ./x build --stage 0 means to build with the beta rustc.
  • ./x doc --stage 0 means to document using the beta rustdoc.
  • ./x test --stage 0 library/std means to run tests on the standard library without building rustc from source ('build with stage 0, then test the artifacts'). If you're working on the standard library, this is normally the test command you want.
  • ./x test tests/ui means to build the stage 1 compiler and run compiletest on it. If you're working on the compiler, this is normally the test command you want.

Examples of what not to do

  • ./x test --stage 0 tests/ui is not useful: it runs tests on the beta compiler and doesn't build rustc from source. Use test tests/ui instead, which builds stage 1 from source.
  • ./x test --stage 0 compiler/rustc builds the compiler but runs no tests: it's running cargo test -p rustc, but cargo doesn't understand Rust's tests. You shouldn't need to use this, use test instead (without arguments).
  • ./x build --stage 0 compiler/rustc builds the compiler, but does not build libstd or even libcore. Most of the time, you'll want ./x build library instead, which allows compiling programs without needing to define lang items.

Building vs. running

Note that build --stage N compiler/rustc does not build the stage N compiler: instead it builds the stage N+1 compiler using the stage N compiler.

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

In each stage, two major steps are performed:

  1. std is compiled by the stage N compiler.
  2. That std is linked to programs built by the stage N compiler, including the stage N artifacts (stage N+1 compiler).

This is somewhat intuitive if one thinks of the stage N artifacts as "just" another program we are building with the stage N compiler: build --stage N compiler/rustc is linking the stage N artifacts to the std built by the stage N compiler.

Stages and std

Note that there are two std libraries in play here:

  1. The library linked to stageN/rustc, which was built by stage N-1 (stage N-1 std)
  2. The library used to compile programs with stageN/rustc, which was built by stage N (stage N std).

Stage N std is pretty much necessary for any useful work with the stage N compiler. Without it, you can only compile programs with #![no_core] -- not terribly useful!

The reason these need to be different is because they aren't necessarily ABI-compatible: there could be new layout optimizations, changes to MIR, or other changes to Rust metadata on nightly that aren't present in beta.

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 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.

Cross-compiling rustc

Cross-compiling is the process of compiling code that will run on another architecture. For instance, you might want to build an ARM version of rustc using an x86 machine. Building stage2 std is different when you are cross-compiling.

This is because x uses a trick: if HOST and TARGET are the same, it will reuse stage1 std for stage2! This is sound because stage1 std was compiled with the stage1 compiler, i.e. a compiler using the source code you currently have checked out. So it should be identical (and therefore ABI-compatible) to the std that stage2/rustc would compile.

However, when cross-compiling, stage1 std will only run on the host. So the stage2 compiler has to recompile std for the target.

(See in the table how stage2 only builds non-host std targets).

Why does only libstd use cfg(bootstrap)?

NOTE: for docs on cfg(bootstrap) itself, see Complications of Bootstrapping.

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 features added to std immediately after their addition, without need for them to get into the downloaded beta.

Note this is different from any other Rust program: stage1 rustc is built by the beta compiler, but using the master version of libstd!

The only time rustc uses cfg(bootstrap) is when it adds internal lints that use diagnostic items, or when it uses unstable library features that were recently changed.

What is a 'sysroot'?

When you build a project with cargo, the build artifacts for dependencies are normally stored in target/debug/deps. This only contains dependencies cargo knows about; in particular, it doesn't have the standard library. Where do std or proc_macro come from? It comes from the sysroot, the root of a number of directories where the compiler loads build artifacts at runtime. The sysroot doesn't just store the standard library, though - it includes anything that needs to be loaded at runtime. That includes (but is not limited to):

  • libstd/libtest/libproc_macro
  • The compiler crates themselves, when using rustc_private. In-tree these are always present; out of tree, you need to install rustc-dev with rustup.
  • libLLVM.so, the shared object file for the LLVM project. In-tree this is either built from source or downloaded from CI; out-of-tree, you need to install llvm-tools-preview with rustup.

All the artifacts listed so far are compiler runtime dependencies. You can see them with rustc --print sysroot:

$ ls $(rustc --print sysroot)/lib
libchalk_derive-0685d79833dc9b2b.so  libstd-25c6acf8063a3802.so
libLLVM-11-rust-1.50.0-nightly.so    libtest-57470d2aa8f7aa83.so
librustc_driver-4f0cc9f50e53f0ba.so  libtracing_attributes-e4be92c35ab2a33b.so
librustc_macros-5f0ec4a119c6ac86.so  rustlib

There are also runtime dependencies for the standard library! These are in lib/rustlib, not lib/ directly.

$ ls $(rustc --print sysroot)/lib/rustlib/x86_64-unknown-linux-gnu/lib | head -n 5

rustlib includes libraries like hashbrown and cfg_if, which are not part of the public API of the standard library, but are used to implement it. rustlib is part of the search path for linkers, but lib will never be part of the search path.

-Z force-unstable-if-unmarked

Since rustlib is part of the search path, it means we have to be careful about which crates are included in it. In particular, all crates except for the standard library are built with the flag -Z force-unstable-if-unmarked, which means that you have to use #![feature(rustc_private)] in order to load it (as opposed to the standard library, which is always available).

The -Z force-unstable-if-unmarked flag has a variety of purposes to help enforce that the correct crates are marked as unstable. It was introduced primarily to allow rustc and the standard library to link to arbitrary crates on crates.io which do not themselves use staged_api. rustc also relies on this flag to mark all of its crates as unstable with the rustc_private feature so that each crate does not need to be carefully marked with unstable.

This flag is automatically applied to all of rustc and the standard library by the bootstrap scripts. This is needed because the compiler and all of its dependencies are shipped in the sysroot to all users.

This flag has the following effects:

  • Marks the crate as "unstable" with the rustc_private feature if it is not itself marked as stable or unstable.
  • Allows these crates to access other forced-unstable crates without any need for attributes. Normally a crate would need a #![feature(rustc_private)] attribute to use other unstable crates. However, that would make it impossible for a crate from crates.io to access its own dependencies since that crate won't have a feature(rustc_private) attribute, but everything is compiled with -Z force-unstable-if-unmarked.

Code which does not use -Z force-unstable-if-unmarked should include the #![feature(rustc_private)] crate attribute to access these force-unstable crates. This is needed for things that link rustc, such as miri or clippy.

You can find more discussion about sysroots in:

Passing flags to commands invoked by bootstrap

x allows you to pass stage-specific flags to rustc and cargo when bootstrapping. The RUSTFLAGS_BOOTSTRAP environment variable is passed as RUSTFLAGS to the bootstrap stage (stage0), and RUSTFLAGS_NOT_BOOTSTRAP is passed when building artifacts for later stages. RUSTFLAGS will work, but also affects the build of bootstrap itself, so it will be rare to want to use it. Finally, MAGIC_EXTRA_RUSTFLAGS bypasses the cargo cache to pass flags to rustc without recompiling all dependencies.


CARGOFLAGS will pass arguments to cargo itself (e.g. --timings). CARGOFLAGS_BOOTSTRAP and CARGOFLAGS_NOT_BOOTSTRAP work analogously to RUSTFLAGS_BOOTSTRAP.

--test-args will pass arguments through to the test runner. For tests/ui, this is compiletest; for unit tests and doctests this is the libtest runner. Most test runner accept --help, which you can use to find out the options accepted by the runner.

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/result.rs:1165:5

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 tell the bootstrap shim to print all env variables by adding -vvv to your x command.

Finally, bootstrap makes use of the cc-rs crate which has its own method of configuring C compilers and C flags via environment variables.

Clarification of build command's stdout

In this part, we will investigate the build command's stdout in an action (similar, but more detailed and complete documentation compare to topic above). When you execute x build --dry-run command, the build output will be something like the following:

Building stage0 library artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
Copying stage0 library from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage0 compiler artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
Copying stage0 rustc from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Assembling stage1 compiler (x86_64-unknown-linux-gnu)
Building stage1 library artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
Copying stage1 library from stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Building stage1 tool rust-analyzer-proc-macro-srv (x86_64-unknown-linux-gnu)
Building rustdoc for stage1 (x86_64-unknown-linux-gnu)

Building stage0 {std,compiler} artifacts

These steps use the provided (downloaded, usually) compiler to compile the local Rust source into libraries we can use.

Copying stage0 {std,rustc}

This copies the library and compiler artifacts from Cargo into stage0-sysroot/lib/rustlib/{target-triple}/lib

Assembling stage1 compiler

This copies the libraries we built in "building stage0 ... artifacts" into the stage1 compiler's lib directory. These are the host libraries that the compiler itself uses to run. These aren't actually used by artifacts the new compiler generates. This step also copies the rustc and rustdoc binaries we generated into build/$HOST/stage/bin.

The stage1/bin/rustc is a fully functional compiler, but it doesn't yet have any libraries to link built binaries or libraries to. The next 3 steps will provide those libraries for it; they are mostly equivalent to constructing the stage1/bin compiler so we don't go through them individually.