Add page about libcore for cross compiling

_posts/2015-09-02-setup-rust.md

@@ -73,7 +73,10 @@ We can now build it using `cargo build`. To make sure, we are building it for th
 ```bash
 cargo build --target=x86_64-unknown-linux-gnu
 ```
-It creates a static library at `target/x86_64-unknown-linux-gnu/debug/libblog_os.a`, which can be linked with our assembly kernel. To build and link the rust library on `make`, we extend our `Makefile`([full file][github makefile]):
+It creates a static library at `target/x86_64-unknown-linux-gnu/debug/libblog_os.a`, which can be linked with our assembly kernel. If you're getting an error about a missing `core` crate, [look here][cross compile libcore].
+[cross compile libcore]: {{ site.url }}/rust-os/cross-compile-libcore.html
+
+To build and link the rust library on `make`, we extend our `Makefile`([full file][github makefile]):
 
 ```make
 # ...

rust-os/cross-compile-libcore.md

@@ -0,0 +1,48 @@
+---
+layout: page
+title: "Cross Compiling: libcore"
+category: "rust-os"
+---
+So you're getting an ``error: can't find crate for `core` [E0463]`` when using `--target x86_64-unknown-linux-gnu`. That means that you're not running Linux or not using using a x86_64 processor.
+
+**If you have an x86_64 processor and want a quick fix**, try it with `x86_64-pc-windows-gnu` or `x86_64-apple-darwin` (or simply omit the explicit `--target`).
+
+The idiomatic alternative and the only option for non x86_64 CPUs is described below. Note that you need to [cross compile binutils], too.
+[cross compile binutils]: {{ site.url }}/rust-os/cross-compile-binutils.html
+
+## Libcore
+The core library is a dependency-free library that is added implicitly when using `#![no_std]`. It provides basic standard library features like Option or Iterator. The core library is installed together with the rust compiler (just like the std library). But the installed libcore is specific to your architecture. If you aren't working on x86_64 Linux and pass `‑‑target x86_64‑unknown‑linux‑gnu` to cargo, it can't find a x86_64 libcore. To fix this, you can either download it or build it using cargo.
+
+## Download it
+You need to download the 64-bit Linux Rust build corresponding to your installed nightly. You can either just update to the current nightly and download the current nightly source [here][Rust downloads]. Or you retrieve your installed version through `rustc --version` and search the corresponding subfolder [here](http://static.rust-lang.org/dist/).
+[Rust downloads]: https://www.rust-lang.org/downloads.html
+
+After extracting it and you need to copy the `x86_64-unknown-linux-gnu` folder in `rust-std-x86_64-unknown-linux-gnu/lib/rustlib` to your local Rust installation. For multirust, the right target folder is `~/.multirust/toolchains/nightly/lib/rustlib`. That's it!
+
+## Build it using cargo
+The alternative is to use cargo to build libcore. But this variant has one big disadvantage: You have to modify each crate you depend on because it needs to use the same libcore. So you can't just add a crates.io dependency anymore, you need to fork and modify it first.
+
+If you want to build libcore anyway, you need its source code. You can either clone the [rust repository] \(makes updates easy) or manually [download the Rust source][Rust downloads] \(faster and less memory).
+[rust repository]: https://github.com/rust-lang/rust
+
+Now we create a new cargo project named `core`, but delete its `src` folder:
+
+```bash
+cargo new core
+rm -r core/src
+```
+
+Then we create a symbolic link named `src` to the `rust/src/libcore` of the Rust source code:
+
+```bash
+ln -s ../rust/src/libcore core/src
+```
+
+To use our new libcore crate (instead of the one installed together with rust) in our OS, we need to add it as a local dependency in the `Cargo.toml`:
+
+```toml
+...
+[dependencies.core]
+path = "core"
+```
+Now cargo compiles libcore for all Rust targets automatically.