- Stages of bootstrapping
- Complications of bootstrapping
- Contributing to bootstrap
- Understanding stages of bootstrap
- Passing flags to commands invoked by
- Environment Variables
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 More accurately, it means using an older compiler to compile a newer version of the same compiler.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
x.py works: it downloads the current beta release of
rustc, then uses it to compile the new compiler.
rustc is done in stages. Here's a diagram, adapted from Joshua Nelson's
talk on bootstrapping at RustConf 2022, with detailed explanations below.
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
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;
The stage0 compiler is usually the current beta
and its associated dynamic libraries,
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
rustc, the stage0 compiler uses the freshly compiled
There are two concepts at play here:
a compiler (with its set of dependencies)
and its 'target' or 'object' libraries (
Both are staged, but in a staggered manner.
The rustc source code is then compiled with the stage0 compiler to produce the stage1 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
Note that the
proc_macro crate avoids this issue with a C FFI layer called
allowing it to be used with stage 1.
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
which you can build with
./x.py build library.
See Building the Compiler.
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.
x.py tries to be helpful and pick the stage you most likely meant for each subcommand.
These defaults are as follows:
You can always override the stage by passing
--stage N explicitly.
For more information about stages, see below.
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
There are two methods used:
- The build system sets
--cfg bootstrapwhen 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
- 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.
When you use the bootstrap system, you'll call it through
However, most of the code lives in
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
bootstrap.py is what gets run by
x.py. It takes care of downloading the
stage0 compiler, which will then build the bootstrap binary written in
Because there are two separate codebases behind
x.py, they need to
be kept in sync. In particular, both
bootstrap.py and the bootstrap binary
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.
This section is a work in progress. In the meantime, you can see an example contribution here.
This is a detailed look into the separate bootstrap stages.
x.py uses is that:
--stage Nflag means to run the stage N compiler (
- 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.
Anything you can build with
x.py is a build artifact.
Build artifacts include, but are not limited to:
- binaries, like
- shared objects, like
- rlib files, like
- HTML files generated by rustdoc, like
./x.py build --stage 0means to build with the beta
./x.py doc --stage 0means to document using the beta
./x.py test --stage 0 library/stdmeans to run tests on the standard library without building
rustcfrom 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.py test src/test/uimeans to build the stage 1 compiler and run
compileteston it. If you're working on the compiler, this is normally the test command you want.
./x.py test --stage 0 src/test/uiis not useful: it runs tests on the beta compiler and doesn't build
rustcfrom source. Use
test src/test/uiinstead, which builds stage 1 from source.
./x.py test --stage 0 compiler/rustcbuilds 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
testinstead (without arguments).
./x.py build --stage 0 compiler/rustcbuilds the compiler, but does not build libstd or even libcore. Most of the time, you'll want
./x.py build libraryinstead, which allows compiling programs without needing to define lang items.
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:
stdis compiled by the stage N compiler.
stdis 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
built by the stage N compiler.
Note that there are two
std libraries in play here:
- The library linked to
stageN/rustc, which was built by stage N-1 (stage N-1
- The library used to compile programs with
stageN/rustc, which was built by stage N (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
--keep-stage simply assumes the previous compile is fine and copies those
artifacts into the appropriate place, skipping the cargo invocation.
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.
std is different when you are cross-compiling.
This is because
x.py uses a trick: if
TARGET are the same,
it will reuse stage1
std for stage2! This is sound because stage1
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)
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
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,
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
is built by the beta compiler, but using the master version of libstd!
The only time
cfg(bootstrap) is when it adds internal lints
that use diagnostic items. This happens very rarely.
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
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
- The compiler crates themselves, when using
rustc_private. In-tree these are always present; out of tree, you need to install
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
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
$ ls $(rustc --print sysroot)/lib/rustlib/x86_64-unknown-linux-gnu/lib | head -n 5 libaddr2line-6c8e02b8fedc1e5f.rlib libadler-9ef2480568df55af.rlib liballoc-9c4002b5f79ba0e1.rlib libcfg_if-512eb53291f6de7e.rlib libcompiler_builtins-ef2408da76957905.rlib
rustlib includes libraries like
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.
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
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).
-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
rustc also relies on
this flag to mark all of its crates as unstable with the
feature so that each crate does not need to be carefully marked with
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_privatefeature 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
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
You can find more discussion about sysroots in:
- The rustdoc PR explaining why it uses
extern cratefor dependencies loaded from sysroot
- Discussions about sysroot on Zulip
- Discussions about building rustdoc out of tree
x.py allows you to pass stage-specific flags to
cargo when bootstrapping.
RUSTFLAGS_BOOTSTRAP environment variable is passed as
RUSTFLAGS to the bootstrap stage
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.
MAGIC_EXTRA_RUSTFLAGS bypasses the
cargo cache to pass flags to rustc without
recompiling all dependencies.
RUSTDOCFLAGS_NOT_BOOTSTRAP are anologous to
RUSTFLAGS, but for rustdoc.
CARGOFLAGS will pass arguments to cargo itself (e.g.
CARGOFLAGS_NOT_BOOTSTRAP work analogously to
--test-args will pass arguments through to the test runner. For
src/test/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.
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
./stageN/bin/rustc gives an error about environment variables, that
usually means something is quite wrong -- or you're trying to compile e.g.
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
This is an incomplete reference for the outputs generated by bootstrap:
|Stage 0 Action||Output|
--stage=0 stops here.
|Stage 1 Action||Output|
|copy (uplift) |
|copy (uplift) |
|copy (uplift) |
--stage=1 stops here.
|Stage 2 Action||Output|
|copy (uplift) |
|copy (uplift) |
--stage=2 stops here.