rustfmt. To make things like `cargo fmt` work correctly in your repo, run

```console
cd <path to rustc repo>
rustup override set nightly
```

after [installing a nightly toolchain] with `rustup`. Don't forget to do this
for all directories you have [setup a worktree for]. You may need to use the
pinned nightly version from `src/stage0`, but often the normal `nightly` channel
will work.

**Note** see [the section on vscode] for how to configure it with this real
rustfmt `x` uses, and [the section on rustup] for how to setup `rustup`
toolchain for your bootstrapped compiler

**Note** This does _not_ allow you to build `rustc` with cargo directly. You
still have to use `x` to work on the compiler or standard library, this just
lets you use `cargo fmt`.


## Faster Builds with CI-rustc  

If you are not working on the compiler, you often don't need to build the compiler tree.
For example, you can skip building the compiler and only build the `library` tree or the
tools under `src/tools`. To achieve that, you have to enable this by setting the `download-rustc`
option in your configuration. This tells bootstrap to use the latest nightly compiler for `stage > 0`
steps, meaning it will have two precompiled compilers: stage0 compiler and `download-rustc` compiler
for `stage > 0` steps. This way, it will never need to build the in-tree compiler. As a result, your
build time will be significantly reduced by not building the in-tree compiler.

## Using incremental compilation

You can further enable the `--incremental` flag to save additional time in
subsequent rebuilds:

```bash
./x test tests/ui --incremental --test-args issue-1234
```

If you don't want to include the flag with every command, you can enable it in
the `bootstrap.toml`:

```toml
[rust]
incremental = true
```

Note that incremental compilation will use more disk space than usual. If disk
space is a concern for you, you might want to check the size of the `build`
directory from time to time.

## Fine-tuning optimizations

Setting `optimize = false` makes the compiler too slow for tests. However, to
improve the test cycle, you can disable optimizations selectively only for the
crates you'll have to rebuild
([source](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/incremental.20compilation.20question/near/202712165)).
For example, when working on `rustc_mir_build`, the `rustc_mir_build` and
`rustc_driver` crates take the most time to incrementally rebuild. You could
therefore set the following in the root `Cargo.toml`:

```toml
[profile.release.package.rustc_mir_build]
opt-level = 0
[profile.release.package.rustc_driver]
opt-level = 0
```

## Working on multiple branches at the same time