different `Cargo.toml` in `library/{std,alloc,core}/Cargo.toml`. Here is an
example for adding `NEW_TARGET_ARCH` as `target_arch`:

*`library/std/Cargo.toml`*:
```diff
  [lints.rust.unexpected_cfgs]
  level = "warn"
  check-cfg = [
      'cfg(bootstrap)',
-      'cfg(target_arch, values("xtensa"))',
+      # #[cfg(bootstrap)] NEW_TARGET_ARCH
+      'cfg(target_arch, values("xtensa", "NEW_TARGET_ARCH"))',
```

To use this target in bootstrap, we need to explicitly add the target triple to the `STAGE0_MISSING_TARGETS`
list in `src/bootstrap/src/core/sanity.rs`. This is necessary because the default compiler bootstrap uses does
not recognize the new target we just added. Therefore, it should be added to `STAGE0_MISSING_TARGETS` so that the
bootstrap is aware that this target is not yet supported by the stage0 compiler.

```diff
const STAGE0_MISSING_TARGETS: &[&str] = &[
+   "NEW_TARGET_TRIPLE"
];
```

## Patching crates

You may need to make changes to crates that the compiler depends on,
such as [`libc`][] or [`cc`][]. If so, you can use Cargo's
[`[patch]`][patch] ability. For example, if you want to use an
unreleased version of `libc`, you can add it to the top-level
`Cargo.toml` file:

```diff
diff --git a/Cargo.toml b/Cargo.toml
index 1e83f05e0ca..4d0172071c1 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -113,6 +113,8 @@ cargo-util = { path = "src/tools/cargo/crates/cargo-util" }
 [patch.crates-io]
+libc = { git = "https://github.com/rust-lang/libc", rev = "0bf7ce340699dcbacabdf5f16a242d2219a49ee0" }

 # See comments in `src/tools/rustc-workspace-hack/README.md` for what's going on
 # here
 rustc-workspace-hack = { path = 'src/tools/rustc-workspace-hack' }
```

After this, run `cargo update -p libc` to update the lockfiles.

Beware that if you patch to a local `path` dependency, this will enable
warnings for that dependency. Some dependencies are not warning-free, and due
to the `deny-warnings` setting in `bootstrap.toml`, the build may suddenly start
to fail.
To work around warnings, you may want to:
- Modify the dependency to remove the warnings
- Or for local development purposes, suppress the warnings by setting deny-warnings = false in bootstrap.toml.

```toml
# bootstrap.toml
[rust]
deny-warnings = false
```


## Cross-compiling

Once you have a target specification in JSON and in the code, you can
cross-compile `rustc`:

```
DESTDIR=/path/to/install/in \
./x install -i --stage 1 --host aarch64-apple-darwin.json --target aarch64-apple-darwin \
compiler/rustc library/std
```

If your target specification is already available in the bootstrap
compiler, you can use it instead of the JSON file for both arguments.

## Promoting a target from tier 2 (target) to tier 2 (host)

There are two levels of tier 2 targets:
  a) Targets that are only cross-compiled (`rustup target add`)
  b) Targets that [have a native toolchain][tier2-native] (`rustup toolchain install`)


For an example of promoting a target from cross-compiled to native,
see [#75914](https://github.com/rust-lang/rust/pull/75914).