# What Bootstrapping does

<!-- toc -->

[*Bootstrapping*][boot] 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More accurately, it means using an older compiler to compile a newer version of
the same compiler.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][ocaml-compiler]. 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.

[`./x.py`]: https://github.com/rust-lang/rust/blob/master/x.py

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

[bootstrap-internals]: https://github.com/rust-lang/rust/blob/master/src/bootstrap/README.md

## Stages of bootstrapping

### Overview

- 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][rustconf22-talk] at RustConf 2022, with
detailed explanations below.

The `A`, `B`, `C`, and `D` show the ordering of the stages of bootstrapping.
<span style="background-color: lightblue; color: black">Blue</span> nodes are
downloaded, <span style="background-color: yellow; color: black">yellow</span>
nodes are built with the `stage0` compiler, and <span style="background-color:
lightgreen; color: black">green</span> nodes are built with the `stage1`
compiler.

[rustconf22-talk]: https://www.youtube.com/watch?v=oUIjG-y4zaA

```mermaid
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`],
[`library/std`], and [`compiler/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.

[`compiler/rustc`]: https://github.com/rust-lang/rust/tree/master/compiler/rustc
[`library/std`]: https://github.com/rust-lang/rust/tree/master/library/std
[`src/bootstrap`]: https://github.com/rust-lang/rust/tree/master/src/bootstrap

### 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 `stage1`.

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](../how-to-build-and-run.html#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

The script [`./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](#understanding-stages-of-bootstrap).

[`./x`]: https://github.com/rust-lang/rust/blob/master/x

## Complications of bootstrapping

Since the build system uses the current beta compiler to build a `stage1`
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][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`.
   Setting `--cfg bootstrap` in this way is used for 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: allowing use of `#![feature(...)]`
   with a compiler that's not `nightly`. _Setting `RUSTC_BOOTSTRAP=1` should
   never be used except when bootstrapping the compiler._

[boot]: https://en.wikipedia.org/wiki/Bootstrapping_(compilers)
[intrinsics]: ../../appendix/glossary.md#intrinsic
[ocaml-compiler]: https://github.com/rust-lang/rust/tree/ef75860a0a72f79f97216f8aaa5b388d98da6480/src/boot

## Understanding stages of bootstrap

### Overview

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`

[rlib]: ../../serialization.md

#### Examples

- `./x test tests/ui` means to build the `stage1` compiler and run `compiletest`
  on it. If you're working on the compiler, this is normally the test command
  you want.
- `./x test --stage 0 library/std` means to run tests on the standard library
  without building `rustc` from source ('build with `stage0`, then test the
  artifacts'). If you're working on the standard library, this is normally the
  test command you want.
- `./x build --stage 0` means to build with the beta `rustc`.
- `./x doc --stage 0` means to document using the beta `rustdoc`.

#### 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 `stage1` 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 `stage1`, 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`. The flag `--keep-stage` simply instructs the build script to 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 the following logic: 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)`?

For docs on `cfg(bootstrap)` itself, see [Complications of
Bootstrapping](#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` compiler.

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? They come 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):

- Libraries `libstd`/`libtest`/`libproc_macro`.
- Compiler crates themselves, when using `rustc_private`. In-tree these are
  always present; out of tree, you need to install `rustc-dev` with `rustup`.
- Shared object file `libLLVM.so` 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
libaddr2line-6c8e02b8fedc1e5f.rlib
libadler-9ef2480568df55af.rlib
liballoc-9c4002b5f79ba0e1.rlib
libcfg_if-512eb53291f6de7e.rlib
libcompiler_builtins-ef2408da76957905.rlib
```

Directory `lib/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. Also `lib/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 `lib/rustlib/` is part of the search path 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 `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 forced-unstable
crates. This is needed for things which link `rustc` its self, such as `MIRI` or
`clippy`.

You can find more discussion about sysroots in:
- The [rustdoc PR] explaining why it uses `extern crate` for dependencies loaded
  from `sysroot`
- [Discussions about sysroot on
  Zulip](https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/deps.20in.20sysroot/)
- [Discussions about building rustdoc out of
  tree](https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/How.20to.20create.20an.20executable.20accessing.20.60rustc_private.60.3F)

[rustdoc PR]: https://github.com/rust-lang/rust/pull/76728

## Passing flags to commands invoked by `bootstrap`

Conveniently `./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.

- `RUSTDOCFLAGS`, `RUSTDOCFLAGS_BOOTSTRAP` and `RUSTDOCFLAGS_NOT_BOOTSTRAP` are
  analogous to `RUSTFLAGS`, but for `rustdoc`.
- `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 doc tests 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:

```text
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 -- such as you're trying to compile `rustc` or
`std` or something which 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][env-vars] of configuring `C` compilers and `C` flags via environment
variables.

[cc-rs crate]: https://github.com/rust-lang/cc-rs
[env-vars]: https://docs.rs/cc/latest/cc/#external-configuration-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:

```text
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 here.
