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Use a nightly Rust version from the beginning

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Philipp Oppermann
2018-10-28 13:00:42 +01:00
parent 92c27c6545
commit 768f81aedc
2 changed files with 18 additions and 23 deletions
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blog/content/second-edition/posts/02-minimal-rust-kernel/index.md
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@@ -249,37 +249,21 @@ The command depends on the rust source code, which we can install with `rustup c
Now we can rerun the above command with `xbuild` instead of `build`:
```
> cargo xbuild --target x86_64-blog_os.json
```
Depending on your version of the Rust compiler you might get the following error:
```
error: The sysroot can't be built for the Stable channel. Switch to nightly.
```
To understand this error, you need to know that the Rust compiler has three release channels: _stable_, _beta_, and _nightly_. The Rust Book explains the difference between these channels really well, so take a minute and [check it out](https://doc.rust-lang.org/book/second-edition/appendix-07-nightly-rust.html#choo-choo-release-channels-and-riding-the-trains).
Some experimental features are only available on the nightly channel. Since Rust uses many of these features for the internal implementation of `core` and other built-in libraries, we need to use a nightly compiler when invoking `cargo xbuild` (since it rebuilds these libraries).
To manage Rust installations I highly recommend [rustup]. It allows you to install nightly, beta, and stable compilers side-by-side and makes it easy to update them. With rustup you can use a nightly compiler for the current directory by running `rustup override add nightly`. Alternatively, you can add a file called `rust-toolchain` with the content `nightly` to the project's root directory.
[rustup]: https://www.rustup.rs/
With a nightly compiler the build finally succeeds:
```
> cargo xbuild --target x86_64-blog_os.json
Compiling core v0.0.0 (file:///…/rust/src/libcore)
Finished release [optimized] target(s) in 52.75 secs
Compiling compiler_builtins v0.1.0 (file:///…/rust/src/libcompiler_builtins)
Finished release [optimized] target(s) in 3.92 secs
Compiling alloc v0.0.0 (/tmp/xargo.9I97eR3uQ3Cq)
Finished release [optimized] target(s) in 7.61s
Compiling blog_os v0.1.0 (file:///…/blog_os)
Finished dev [unoptimized + debuginfo] target(s) in 0.29 secs
```
We see that `cargo xbuild` cross-compiled the `core`, `compiler_builtin`, and `alloc` libraries for our new custom target and then continued to compile our `blog_os` crate.
We see that `cargo xbuild` cross-compiles the `core`, `compiler_builtin`, and `alloc` libraries for our new custom target. Since these libraries use a lot of unstable features internally, this only works with a [nightly Rust compiler]. Afterwards, `cargo xbuild` successfully compiles our `blog_os` crate.
[nightly Rust compiler]: ./second-edition/posts/01-freestanding-rust-binary/index.md#installing-rust-nightly
Now we are able to build our kernel for a bare metal target. However, our `_start` entry point, which will be called by the boot loader, is still empty. So let's output something to screen from it.