src/building/how-to-build-and-run.md - rust-lang/rustc-dev-guide - Git at Google

 # How to build and run the compiler

 <div class="warning">

 For `profile = "library"` users, or users who use `download-rustc = true | "if-unchanged"`, please be advised that
 the `./x test library/std` flow where `download-rustc` is active (i.e. no compiler changes) is currently broken.
 This is tracked in <https://github.com/rust-lang/rust/issues/142505>. Only the `./x test` flow is affected in this
 case, `./x {check,build} library/std` should still work.

 In the short-term, you may need to disable `download-rustc` for `./x test library/std`. This can be done either by:

 1. `./x test library/std --set rust.download-rustc=false`
 2. Or set `rust.download-rustc=false` in `bootstrap.toml`.

 Unfortunately that will require building the stage 1 compiler. The bootstrap team is working on this, but
 implementing a maintainable fix is taking some time.

 </div>


 The compiler is built using a tool called `x.py`. You will need to
 have Python installed to run it.

 ## Quick Start

 For a less in-depth quick-start of getting the compiler running, see [quickstart](./quickstart.md).


 ## Get the source code

 The main repository is [`rust-lang/rust`][repo]. This contains the compiler,
 the standard library (including `core`, `alloc`, `test`, `proc_macro`, etc),
 and a bunch of tools (e.g. `rustdoc`, the bootstrapping infrastructure, etc).

 [repo]: https://github.com/rust-lang/rust

 The very first step to work on `rustc` is to clone the repository:

 ```bash
 git clone https://github.com/rust-lang/rust.git
 cd rust
 ```

 ### Partial clone the repository

 Due to the size of the repository, cloning on a slower internet connection can take a long time,
 and requires disk space to store the full history of every file and directory.
 Instead, it is possible to tell git to perform a _partial clone_, which will only fully retrieve
 the current file contents, but will automatically retrieve further file contents when you, e.g.,
 jump back in the history.
 All git commands will continue to work as usual, at the price of requiring an internet connection
 to visit not-yet-loaded points in history.

 ```bash
 git clone --filter='blob:none' https://github.com/rust-lang/rust.git
 cd rust
 ```

 > **NOTE**: [This link](https://github.blog/open-source/git/get-up-to-speed-with-partial-clone-and-shallow-clone/)
 > describes this type of checkout in more detail, and also compares it to other modes, such as
 > shallow cloning.

 ### Shallow clone the repository

 An older alternative to partial clones is to use shallow clone the repository instead.
 To do so, you can use the `--depth N` option with the `git clone` command.
 This instructs `git` to perform a "shallow clone", cloning the repository but truncating it to
 the last `N` commits.

 Passing `--depth 1` tells `git` to clone the repository but truncate the history to the latest
 commit that is on the `master` branch, which is usually fine for browsing the source code or
 building the compiler.

 ```bash
 git clone --depth 1 https://github.com/rust-lang/rust.git
 cd rust
 ```

 > **NOTE**: A shallow clone limits which `git` commands can be run.
 > If you intend to work on and contribute to the compiler, it is
 > generally recommended to fully clone the repository [as shown above](#get-the-source-code),
 > or to perform a [partial clone](#partial-clone-the-repository) instead.
 >
 > For example, `git bisect` and `git blame` require access to the commit history,
 > so they don't work if the repository was cloned with `--depth 1`.

 ## What is `x.py`?

 `x.py` is the build tool for the `rust` repository. It can build docs, run tests, and compile the
 compiler and standard library.

 This chapter focuses on the basics to be productive, but
 if you want to learn more about `x.py`, [read this chapter][bootstrap].

 [bootstrap]: ./bootstrapping/intro.md

 Also, using `x` rather than `x.py` is recommended as:

 > `./x` is the most likely to work on every system (on Unix it runs the shell script
 > that does python version detection, on Windows it will probably run the
 > powershell script - certainly less likely to break than `./x.py` which often just
 > opens the file in an editor).[^1]

 (You can find the platform related scripts around the `x.py`, like `x.ps1`)

 Notice that this is not absolute. For instance, using Nushell in VSCode on Win10,
 typing `x` or `./x` still opens `x.py` in an editor rather than invoking the program. :)

 In the rest of this guide, we use `x` rather than `x.py` directly. The following
 command:

 ```bash
 ./x check
 ```

 could be replaced by:

 ```bash
 ./x.py check
 ```

 ### Running `x.py`

 The `x.py` command can be run directly on most Unix systems in the following format:

 ```sh
 ./x <subcommand> [flags]
 ```

 This is how the documentation and examples assume you are running `x.py`.
 Some alternative ways are:

 ```sh
 # On a Unix shell if you don't have the necessary `python3` command
 ./x <subcommand> [flags]

 # In Windows Powershell (if powershell is configured to run scripts)
 ./x <subcommand> [flags]
 ./x.ps1 <subcommand> [flags]

 # On the Windows Command Prompt (if .py files are configured to run Python)
 x.py <subcommand> [flags]

 # You can also run Python yourself, e.g.:
 python x.py <subcommand> [flags]
 ```

 On Windows, the Powershell commands may give you an error that looks like this:
 ```
 PS C:\Users\vboxuser\rust> ./x
 ./x : File C:\Users\vboxuser\rust\x.ps1 cannot be loaded because running scripts is disabled on this system. For more
 information, see about_Execution_Policies at https://go.microsoft.com/fwlink/?LinkID=135170.
 At line:1 char:1
 + ./x
 + ~~~
     + CategoryInfo          : SecurityError: (:) [], PSSecurityException
     + FullyQualifiedErrorId : UnauthorizedAccess
 ```

 You can avoid this error by allowing powershell to run local scripts:
 ```
 Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser
 ```

 #### Running `x.py` slightly more conveniently

 There is a binary that wraps `x.py` called `x` in `src/tools/x`. All it does is
 run `x.py`, but it can be installed system-wide and run from any subdirectory
 of a checkout. It also looks up the appropriate version of `python` to use.

 You can install it with `cargo install --path src/tools/x`.

 To clarify that this is another global installed binary util, which is
 similar to the one declared in section [What is `x.py`](#what-is-xpy), but
 it works as an independent process to execute the `x.py` rather than calling the
 shell to run the platform related scripts.

 ## Create a `bootstrap.toml`

 To start, run `./x setup` and select the `compiler` defaults. This will do some initialization
 and create a `bootstrap.toml` for you with reasonable defaults. If you use a different default (which
 you'll likely want to do if you want to contribute to an area of rust other than the compiler, such
 as rustdoc), make sure to read information about that default (located in `src/bootstrap/defaults`)
 as the build process may be different for other defaults.

 Alternatively, you can write `bootstrap.toml` by hand. See `bootstrap.example.toml` for all the available
 settings and explanations of them. See `src/bootstrap/defaults` for common settings to change.

 If you have already built `rustc` and you change settings related to LLVM, then you may have to
 execute `rm -rf build` for subsequent configuration changes to take effect. Note that `./x
 clean` will not cause a rebuild of LLVM.

 ## Common `x` commands

 Here are the basic invocations of the `x` commands most commonly used when
 working on `rustc`, `std`, `rustdoc`, and other tools.

 | Command     | When to use it                                                                                               |
 | ----------- | ------------------------------------------------------------------------------------------------------------ |
 | `./x check` | Quick check to see if most things compile; [rust-analyzer can run this automatically for you][rust-analyzer] |
 | `./x build` | Builds `rustc`, `std`, and `rustdoc`                                                                         |
 | `./x test`  | Runs all tests                                                                                               |
 | `./x fmt`   | Formats all code                                                                                             |

 As written, these commands are reasonable starting points. However, there are
 additional options and arguments for each of them that are worth learning for
 serious development work. In particular, `./x build` and `./x test`
 provide many ways to compile or test a subset of the code, which can save a lot
 of time.

 Also, note that `x` supports all kinds of path suffixes for `compiler`, `library`,
 and `src/tools` directories. So, you can simply run `x test tidy` instead of
 `x test src/tools/tidy`. Or, `x build std` instead of `x build library/std`.

 [rust-analyzer]: suggested.html#configuring-rust-analyzer-for-rustc

 See the chapters on
 [testing](../tests/running.md) and [rustdoc](../rustdoc.md) for more details.

 ### Building the compiler

 Note that building will require a relatively large amount of storage space.
 You may want to have upwards of 10 or 15 gigabytes available to build the compiler.

 Once you've created a `bootstrap.toml`, you are now ready to run
 `x`. There are a lot of options here, but let's start with what is
 probably the best "go to" command for building a local compiler:

 ```bash
 ./x build library
 ```

 This may *look* like it only builds the standard library, but that is not the case.
 What this command does is the following:

 - Build `rustc` using the stage0 compiler
   - This produces the stage1 compiler
 - Build `std` using the stage1 compiler

 This final product (stage1 compiler + libs built using that compiler)
 is what you need to build other Rust programs (unless you use `#![no_std]` or
 `#![no_core]`).

 You will probably find that building the stage1 `std` is a bottleneck for you,
 but fear not, there is a (hacky) workaround...
 see [the section on avoiding rebuilds for std][keep-stage].

 [keep-stage]: ./suggested.md#faster-builds-with---keep-stage

 Sometimes you don't need a full build. When doing some kind of
 "type-based refactoring", like renaming a method, or changing the
 signature of some function, you can use `./x check` instead for a much faster build.

 Note that this whole command just gives you a subset of the full `rustc`
 build. The **full** `rustc` build (what you get with `./x build
 --stage 2 compiler/rustc`) has quite a few more steps:

 - Build `rustc` with the stage1 compiler.
   - The resulting compiler here is called the "stage2" compiler, which uses stage1 std from the previous command.
 - Build `librustdoc` and a bunch of other things with the stage2 compiler.

 You almost never need to do this.

 ### Build specific components

 If you are working on the standard library, you probably don't need to build
 every other default component. Instead, you can build a specific component by
 providing its name, like this:

 ```bash
 ./x build --stage 1 library
 ```

 If you choose the `library` profile when running `x setup`, you can omit `--stage 1` (it's the
 default).

 ## Creating a rustup toolchain

 Once you have successfully built `rustc`, you will have created a bunch
 of files in your `build` directory. In order to actually run the
 resulting `rustc`, we recommend creating rustup toolchains. The first
 one will run the stage1 compiler (which we built above). The second
 will execute the stage2 compiler (which we did not build, but which
 you will likely need to build at some point; for example, if you want
 to run the entire test suite).

 ```bash
 rustup toolchain link stage1 build/host/stage1
 rustup toolchain link stage2 build/host/stage2
 ```

 Now you can run the `rustc` you built with. If you run with `-vV`, you
 should see a version number ending in `-dev`, indicating a build from
 your local environment:

 ```bash
 $ rustc +stage1 -vV
 rustc 1.48.0-dev
 binary: rustc
 commit-hash: unknown
 commit-date: unknown
 host: x86_64-unknown-linux-gnu
 release: 1.48.0-dev
 LLVM version: 11.0
 ```

 The rustup toolchain points to the specified toolchain compiled in your `build` directory,
 so the rustup toolchain will be updated whenever `x build` or `x test` are run for
 that toolchain/stage.

 **Note:** the toolchain we've built does not include `cargo`.  In this case, `rustup` will
 fall back to using `cargo` from the installed `nightly`, `beta`, or `stable` toolchain
 (in that order).  If you need to use unstable `cargo` flags, be sure to run
 `rustup install nightly` if you haven't already.  See the
 [rustup documentation on custom toolchains](https://rust-lang.github.io/rustup/concepts/toolchains.html#custom-toolchains).

 **Note:** rust-analyzer and IntelliJ Rust plugin use a component called
 `rust-analyzer-proc-macro-srv` to work with proc macros. If you intend to use a
 custom toolchain for a project (e.g. via `rustup override set stage1`) you may
 want to build this component:

 ```bash
 ./x build proc-macro-srv-cli
 ```

 ## Building targets for cross-compilation

 To produce a compiler that can cross-compile for other targets,
 pass any number of `target` flags to `x build`.
 For example, if your host platform is `x86_64-unknown-linux-gnu`
 and your cross-compilation target is `wasm32-wasip1`, you can build with:

 ```bash
 ./x build --target x86_64-unknown-linux-gnu,wasm32-wasip1
 ```

 Note that if you want the resulting compiler to be able to build crates that
 involve proc macros or build scripts, you must be sure to explicitly build target support for the
 host platform (in this case, `x86_64-unknown-linux-gnu`).

 If you want to always build for other targets without needing to pass flags to `x build`,
 you can configure this in the `[build]` section of your `bootstrap.toml` like so:

 ```toml
 [build]
 target = ["x86_64-unknown-linux-gnu", "wasm32-wasip1"]
 ```

 Note that building for some targets requires having external dependencies installed
 (e.g. building musl targets requires a local copy of musl).
 Any target-specific configuration (e.g. the path to a local copy of musl)
 will need to be provided by your `bootstrap.toml`.
 Please see `bootstrap.example.toml` for information on target-specific configuration keys.

 For examples of the complete configuration necessary to build a target, please visit
 [the rustc book](https://doc.rust-lang.org/rustc/platform-support.html),
 select any target under the "Platform Support" heading on the left,
 and see the section related to building a compiler for that target.
 For targets without a corresponding page in the rustc book,
 it may be useful to [inspect the Dockerfiles](../tests/docker.md)
 that the Rust infrastructure itself uses to set up and configure cross-compilation.

 If you have followed the directions from the prior section on creating a rustup toolchain,
 then once you have built your compiler you will be able to use it to cross-compile like so:

 ```bash
 cargo +stage1 build --target wasm32-wasip1
 ```

 ## Other `x` commands

 Here are a few other useful `x` commands. We'll cover some of them in detail
 in other sections:

 - Building things:
   - `./x build` – builds everything using the stage 1 compiler,
     not just up to `std`
   - `./x build --stage 2` – builds everything with the stage 2 compiler including
     `rustdoc`
 - Running tests (see the [section on running tests](../tests/running.html) for
   more details):
   - `./x test library/std` – runs the unit tests and integration tests from `std`
   - `./x test tests/ui` – runs the `ui` test suite
   - `./x test tests/ui/const-generics` - runs all the tests in
     the `const-generics/` subdirectory of the `ui` test suite
   - `./x test tests/ui/const-generics/const-types.rs` - runs
     the single test `const-types.rs` from the `ui` test suite

 ### Cleaning out build directories

 Sometimes you need to start fresh, but this is normally not the case.
 If you need to run this then bootstrap is most likely not acting right and
 you should file a bug as to what is going wrong. If you do need to clean
 everything up then you only need to run one command!

 ```bash
 ./x clean
 ```

 `rm -rf build` works too, but then you have to rebuild LLVM, which can take
 a long time even on fast computers.

 ## Remarks on disk space

 Building the compiler (especially if beyond stage 1) can require significant amounts of free disk
 space, possibly around 100GB. This is compounded if you have a separate build directory for
 rust-analyzer (e.g. `build-rust-analyzer`). This is easy to hit with dev-desktops which have a [set
 disk
 quota](https://github.com/rust-lang/simpleinfra/blob/8a59e4faeb75a09b072671c74a7cb70160ebef50/ansible/roles/dev-desktop/defaults/main.yml#L7)
 for each user, but this also applies to local development as well. Occasionally, you may need to:

 - Remove `build/` directory.
 - Remove `build-rust-analyzer/` directory (if you have a separate rust-analyzer build directory).
 - Uninstall unnecessary toolchains if you use `cargo-bisect-rustc`. You can check which toolchains
   are installed with `rustup toolchain list`.

 [^1]: issue[#1707](https://github.com/rust-lang/rustc-dev-guide/issues/1707)
	# How to build and run the compiler

	<div class="warning">

	For `profile = "library"` users, or users who use `download-rustc = true \| "if-unchanged"`, please be advised that
	the `./x test library/std` flow where `download-rustc` is active (i.e. no compiler changes) is currently broken.
	This is tracked in <https://github.com/rust-lang/rust/issues/142505>. Only the `./x test` flow is affected in this
	case, `./x {check,build} library/std` should still work.

	In the short-term, you may need to disable `download-rustc` for `./x test library/std`. This can be done either by:

	1. `./x test library/std --set rust.download-rustc=false`
	2. Or set `rust.download-rustc=false` in `bootstrap.toml`.

	Unfortunately that will require building the stage 1 compiler. The bootstrap team is working on this, but
	implementing a maintainable fix is taking some time.

	</div>


	The compiler is built using a tool called `x.py`. You will need to
	have Python installed to run it.

	## Quick Start

	For a less in-depth quick-start of getting the compiler running, see [quickstart](./quickstart.md).


	## Get the source code

	The main repository is [`rust-lang/rust`][repo]. This contains the compiler,
	the standard library (including `core`, `alloc`, `test`, `proc_macro`, etc),
	and a bunch of tools (e.g. `rustdoc`, the bootstrapping infrastructure, etc).

	[repo]: https://github.com/rust-lang/rust

	The very first step to work on `rustc` is to clone the repository:

	```bash
	git clone https://github.com/rust-lang/rust.git
	cd rust
	```

	### Partial clone the repository

	Due to the size of the repository, cloning on a slower internet connection can take a long time,
	and requires disk space to store the full history of every file and directory.
	Instead, it is possible to tell git to perform a _partial clone_, which will only fully retrieve
	the current file contents, but will automatically retrieve further file contents when you, e.g.,
	jump back in the history.
	All git commands will continue to work as usual, at the price of requiring an internet connection
	to visit not-yet-loaded points in history.

	```bash
	git clone --filter='blob:none' https://github.com/rust-lang/rust.git
	cd rust
	```

	> NOTE: [This link](https://github.blog/open-source/git/get-up-to-speed-with-partial-clone-and-shallow-clone/)
	> describes this type of checkout in more detail, and also compares it to other modes, such as
	> shallow cloning.

	### Shallow clone the repository

	An older alternative to partial clones is to use shallow clone the repository instead.
	To do so, you can use the `--depth N` option with the `git clone` command.
	This instructs `git` to perform a "shallow clone", cloning the repository but truncating it to
	the last `N` commits.

	Passing `--depth 1` tells `git` to clone the repository but truncate the history to the latest
	commit that is on the `master` branch, which is usually fine for browsing the source code or
	building the compiler.

	```bash
	git clone --depth 1 https://github.com/rust-lang/rust.git
	cd rust
	```

	> NOTE: A shallow clone limits which `git` commands can be run.
	> If you intend to work on and contribute to the compiler, it is
	> generally recommended to fully clone the repository [as shown above](#get-the-source-code),
	> or to perform a [partial clone](#partial-clone-the-repository) instead.
	>
	> For example, `git bisect` and `git blame` require access to the commit history,
	> so they don't work if the repository was cloned with `--depth 1`.

	## What is `x.py`?

	`x.py` is the build tool for the `rust` repository. It can build docs, run tests, and compile the
	compiler and standard library.

	This chapter focuses on the basics to be productive, but
	if you want to learn more about `x.py`, [read this chapter][bootstrap].

	[bootstrap]: ./bootstrapping/intro.md

	Also, using `x` rather than `x.py` is recommended as:

	> `./x` is the most likely to work on every system (on Unix it runs the shell script
	> that does python version detection, on Windows it will probably run the
	> powershell script - certainly less likely to break than `./x.py` which often just
	> opens the file in an editor).[^1]

	(You can find the platform related scripts around the `x.py`, like `x.ps1`)

	Notice that this is not absolute. For instance, using Nushell in VSCode on Win10,
	typing `x` or `./x` still opens `x.py` in an editor rather than invoking the program. :)

	In the rest of this guide, we use `x` rather than `x.py` directly. The following
	command:

	```bash
	./x check
	```

	could be replaced by:

	```bash
	./x.py check
	```

	### Running `x.py`

	The `x.py` command can be run directly on most Unix systems in the following format:

	```sh
	./x <subcommand> [flags]
	```

	This is how the documentation and examples assume you are running `x.py`.
	Some alternative ways are:

	```sh
	# On a Unix shell if you don't have the necessary `python3` command
	./x <subcommand> [flags]

	# In Windows Powershell (if powershell is configured to run scripts)
	./x <subcommand> [flags]
	./x.ps1 <subcommand> [flags]

	# On the Windows Command Prompt (if .py files are configured to run Python)
	x.py <subcommand> [flags]

	# You can also run Python yourself, e.g.:
	python x.py <subcommand> [flags]
	```

	On Windows, the Powershell commands may give you an error that looks like this:
	```
	PS C:\Users\vboxuser\rust> ./x
	./x : File C:\Users\vboxuser\rust\x.ps1 cannot be loaded because running scripts is disabled on this system. For more
	information, see about_Execution_Policies at https://go.microsoft.com/fwlink/?LinkID=135170.
	At line:1 char:1
	+ ./x
	+ ~~~
	+ CategoryInfo : SecurityError: (:) [], PSSecurityException
	+ FullyQualifiedErrorId : UnauthorizedAccess
	```

	You can avoid this error by allowing powershell to run local scripts:
	```
	Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser
	```

	#### Running `x.py` slightly more conveniently

	There is a binary that wraps `x.py` called `x` in `src/tools/x`. All it does is
	run `x.py`, but it can be installed system-wide and run from any subdirectory
	of a checkout. It also looks up the appropriate version of `python` to use.

	You can install it with `cargo install --path src/tools/x`.

	To clarify that this is another global installed binary util, which is
	similar to the one declared in section [What is `x.py`](#what-is-xpy), but
	it works as an independent process to execute the `x.py` rather than calling the
	shell to run the platform related scripts.

	## Create a `bootstrap.toml`

	To start, run `./x setup` and select the `compiler` defaults. This will do some initialization
	and create a `bootstrap.toml` for you with reasonable defaults. If you use a different default (which
	you'll likely want to do if you want to contribute to an area of rust other than the compiler, such
	as rustdoc), make sure to read information about that default (located in `src/bootstrap/defaults`)
	as the build process may be different for other defaults.

	Alternatively, you can write `bootstrap.toml` by hand. See `bootstrap.example.toml` for all the available
	settings and explanations of them. See `src/bootstrap/defaults` for common settings to change.

	If you have already built `rustc` and you change settings related to LLVM, then you may have to
	execute `rm -rf build` for subsequent configuration changes to take effect. Note that `./x
	clean` will not cause a rebuild of LLVM.

	## Common `x` commands

	Here are the basic invocations of the `x` commands most commonly used when
	working on `rustc`, `std`, `rustdoc`, and other tools.

	\| Command \| When to use it \|
	\| ----------- \| ------------------------------------------------------------------------------------------------------------ \|
	\| `./x check` \| Quick check to see if most things compile; [rust-analyzer can run this automatically for you][rust-analyzer] \|
	\| `./x build` \| Builds `rustc`, `std`, and `rustdoc` \|
	\| `./x test` \| Runs all tests \|
	\| `./x fmt` \| Formats all code \|

	As written, these commands are reasonable starting points. However, there are
	additional options and arguments for each of them that are worth learning for
	serious development work. In particular, `./x build` and `./x test`
	provide many ways to compile or test a subset of the code, which can save a lot
	of time.

	Also, note that `x` supports all kinds of path suffixes for `compiler`, `library`,
	and `src/tools` directories. So, you can simply run `x test tidy` instead of
	`x test src/tools/tidy`. Or, `x build std` instead of `x build library/std`.

	[rust-analyzer]: suggested.html#configuring-rust-analyzer-for-rustc

	See the chapters on
	[testing](../tests/running.md) and [rustdoc](../rustdoc.md) for more details.

	### Building the compiler

	Note that building will require a relatively large amount of storage space.
	You may want to have upwards of 10 or 15 gigabytes available to build the compiler.

	Once you've created a `bootstrap.toml`, you are now ready to run
	`x`. There are a lot of options here, but let's start with what is
	probably the best "go to" command for building a local compiler:

	```bash
	./x build library
	```

	This may look like it only builds the standard library, but that is not the case.
	What this command does is the following:

	- Build `rustc` using the stage0 compiler
	- This produces the stage1 compiler
	- Build `std` using the stage1 compiler

	This final product (stage1 compiler + libs built using that compiler)
	is what you need to build other Rust programs (unless you use `#![no_std]` or
	`#![no_core]`).

	You will probably find that building the stage1 `std` is a bottleneck for you,
	but fear not, there is a (hacky) workaround...
	see [the section on avoiding rebuilds for std][keep-stage].

	[keep-stage]: ./suggested.md#faster-builds-with---keep-stage

	Sometimes you don't need a full build. When doing some kind of
	"type-based refactoring", like renaming a method, or changing the
	signature of some function, you can use `./x check` instead for a much faster build.

	Note that this whole command just gives you a subset of the full `rustc`
	build. The full `rustc` build (what you get with `./x build
	--stage 2 compiler/rustc`) has quite a few more steps:

	- Build `rustc` with the stage1 compiler.
	- The resulting compiler here is called the "stage2" compiler, which uses stage1 std from the previous command.
	- Build `librustdoc` and a bunch of other things with the stage2 compiler.

	You almost never need to do this.

	### Build specific components

	If you are working on the standard library, you probably don't need to build
	every other default component. Instead, you can build a specific component by
	providing its name, like this:

	```bash
	./x build --stage 1 library
	```

	If you choose the `library` profile when running `x setup`, you can omit `--stage 1` (it's the
	default).

	## Creating a rustup toolchain

	Once you have successfully built `rustc`, you will have created a bunch
	of files in your `build` directory. In order to actually run the
	resulting `rustc`, we recommend creating rustup toolchains. The first
	one will run the stage1 compiler (which we built above). The second
	will execute the stage2 compiler (which we did not build, but which
	you will likely need to build at some point; for example, if you want
	to run the entire test suite).

	```bash
	rustup toolchain link stage1 build/host/stage1
	rustup toolchain link stage2 build/host/stage2
	```

	Now you can run the `rustc` you built with. If you run with `-vV`, you
	should see a version number ending in `-dev`, indicating a build from
	your local environment:

	```bash
	$ rustc +stage1 -vV
	rustc 1.48.0-dev
	binary: rustc
	commit-hash: unknown
	commit-date: unknown
	host: x86_64-unknown-linux-gnu
	release: 1.48.0-dev
	LLVM version: 11.0
	```

	The rustup toolchain points to the specified toolchain compiled in your `build` directory,
	so the rustup toolchain will be updated whenever `x build` or `x test` are run for
	that toolchain/stage.

	Note: the toolchain we've built does not include `cargo`. In this case, `rustup` will
	fall back to using `cargo` from the installed `nightly`, `beta`, or `stable` toolchain
	(in that order). If you need to use unstable `cargo` flags, be sure to run
	`rustup install nightly` if you haven't already. See the
	[rustup documentation on custom toolchains](https://rust-lang.github.io/rustup/concepts/toolchains.html#custom-toolchains).

	Note: rust-analyzer and IntelliJ Rust plugin use a component called
	`rust-analyzer-proc-macro-srv` to work with proc macros. If you intend to use a
	custom toolchain for a project (e.g. via `rustup override set stage1`) you may
	want to build this component:

	```bash
	./x build proc-macro-srv-cli
	```

	## Building targets for cross-compilation

	To produce a compiler that can cross-compile for other targets,
	pass any number of `target` flags to `x build`.
	For example, if your host platform is `x86_64-unknown-linux-gnu`
	and your cross-compilation target is `wasm32-wasip1`, you can build with:

	```bash
	./x build --target x86_64-unknown-linux-gnu,wasm32-wasip1
	```

	Note that if you want the resulting compiler to be able to build crates that
	involve proc macros or build scripts, you must be sure to explicitly build target support for the
	host platform (in this case, `x86_64-unknown-linux-gnu`).

	If you want to always build for other targets without needing to pass flags to `x build`,
	you can configure this in the `[build]` section of your `bootstrap.toml` like so:

	```toml
	[build]
	target = ["x86_64-unknown-linux-gnu", "wasm32-wasip1"]
	```

	Note that building for some targets requires having external dependencies installed
	(e.g. building musl targets requires a local copy of musl).
	Any target-specific configuration (e.g. the path to a local copy of musl)
	will need to be provided by your `bootstrap.toml`.
	Please see `bootstrap.example.toml` for information on target-specific configuration keys.

	For examples of the complete configuration necessary to build a target, please visit
	[the rustc book](https://doc.rust-lang.org/rustc/platform-support.html),
	select any target under the "Platform Support" heading on the left,
	and see the section related to building a compiler for that target.
	For targets without a corresponding page in the rustc book,
	it may be useful to [inspect the Dockerfiles](../tests/docker.md)
	that the Rust infrastructure itself uses to set up and configure cross-compilation.

	If you have followed the directions from the prior section on creating a rustup toolchain,
	then once you have built your compiler you will be able to use it to cross-compile like so:

	```bash
	cargo +stage1 build --target wasm32-wasip1
	```

	## Other `x` commands

	Here are a few other useful `x` commands. We'll cover some of them in detail
	in other sections:

	- Building things:
	- `./x build` – builds everything using the stage 1 compiler,
	not just up to `std`
	- `./x build --stage 2` – builds everything with the stage 2 compiler including
	`rustdoc`
	- Running tests (see the [section on running tests](../tests/running.html) for
	more details):
	- `./x test library/std` – runs the unit tests and integration tests from `std`
	- `./x test tests/ui` – runs the `ui` test suite
	- `./x test tests/ui/const-generics` - runs all the tests in
	the `const-generics/` subdirectory of the `ui` test suite
	- `./x test tests/ui/const-generics/const-types.rs` - runs
	the single test `const-types.rs` from the `ui` test suite

	### Cleaning out build directories

	Sometimes you need to start fresh, but this is normally not the case.
	If you need to run this then bootstrap is most likely not acting right and
	you should file a bug as to what is going wrong. If you do need to clean
	everything up then you only need to run one command!

	```bash
	./x clean
	```

	`rm -rf build` works too, but then you have to rebuild LLVM, which can take
	a long time even on fast computers.

	## Remarks on disk space

	Building the compiler (especially if beyond stage 1) can require significant amounts of free disk
	space, possibly around 100GB. This is compounded if you have a separate build directory for
	rust-analyzer (e.g. `build-rust-analyzer`). This is easy to hit with dev-desktops which have a [set
	disk
	quota](https://github.com/rust-lang/simpleinfra/blob/8a59e4faeb75a09b072671c74a7cb70160ebef50/ansible/roles/dev-desktop/defaults/main.yml#L7)
	for each user, but this also applies to local development as well. Occasionally, you may need to:

	- Remove `build/` directory.
	- Remove `build-rust-analyzer/` directory (if you have a separate rust-analyzer build directory).
	- Uninstall unnecessary toolchains if you use `cargo-bisect-rustc`. You can check which toolchains
	are installed with `rustup toolchain list`.

	[^1]: issue[#1707](https://github.com/rust-lang/rustc-dev-guide/issues/1707)