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`.


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].


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