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5th chunk of `src/building/how-to-build-and-run.md`
7240465c7e5296968df1eaccb20bd1fa4098928cf37d2b0300000001000008bb
[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,
Title: Cross-Compilation Targets, `rust-analyzer`, and Configuring Build Targets in `bootstrap.toml`
Summary
This section provides instructions for building the `rust-analyzer-proc-macro-srv` component when using a custom toolchain. It details how to build targets for cross-compilation using the `--target` flag with the `x build` command, including the necessity of building host platform support for crates with proc macros or build scripts. It also explains how to configure default build targets in the `bootstrap.toml` file and emphasizes the need for external dependencies and target-specific configurations. It references the `rustc` book and Dockerfiles for configuration examples. Finally, it connects back to the previous section on creating a rustup toolchain.