Introduce rust-toolchain

Make use of the new format, introduced in
https://blog.rust-lang.org/2020/11/27/Rustup-1.23.0.html#new-format-for-rust-toolchain

README.CN.md

@@ -52,26 +52,19 @@ _带上我最诚挚的问候,<br>Andre ([@andre-richter])_
 3. 安装正确的`Rust`工具链:
    1. 如果你已经安装了一个版本的Rust:
       ```bash
-      rustup toolchain add nightly-2020-11-20
-      rustup default nightly-2020-11-20
-      rustup component add llvm-tools-preview
-      rustup target add aarch64-unknown-none-softfloat
       cargo install cargo-binutils rustfilt
       ```
 
-   2. 如果你想要全新安装:
+   1. 如果你想要全新安装:
       ```bash
-      curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- \
-          --default-toolchain nightly-2020-11-20                           \
-          --component llvm-tools-preview
+      curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
 
       source $HOME/.cargo/env
-      rustup target add aarch64-unknown-none-softfloat
       cargo install cargo-binutils rustfilt
       ```
 
-4. 如果你使用 `Visual Studio Code`，我强烈推荐你安装[Rust Analyzer 扩展]。
-5. 如果你使用的**不是**Linux，那么你还需要安装一些`Ruby` gems。
+1. 如果你使用 `Visual Studio Code`，我强烈推荐你安装[Rust Analyzer 扩展]。
+1. 如果你使用的**不是**Linux，那么你还需要安装一些`Ruby` gems。
 
 ```bash
 sudo gem install bundler

README.md

@@ -65,29 +65,23 @@ other Unix flavors such as **macOS**, but this is only _experimental_.
 
 1. [Install Docker][install_docker].
 1. Ensure your user account is in the [docker group].
-1. Install a suitable `Rust` toolchain:
+1. Prepare the `Rust` toolchain. Most of it will be handled on first use through the
+   [rust-toolchain](rust-toolchain) file. What's left for us to do is:
    1. If you already have a version of Rust installed:
       ```bash
-      rustup toolchain add nightly-2020-11-20
-      rustup default nightly-2020-11-20
-      rustup component add llvm-tools-preview
-      rustup target add aarch64-unknown-none-softfloat
       cargo install cargo-binutils rustfilt
       ```
 
-   2. If you need a fresh install:
+   1. If you need to install Rust from scratch:
       ```bash
-      curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- \
-          --default-toolchain nightly-2020-11-20                           \
-          --component llvm-tools-preview
+      curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
 
       source $HOME/.cargo/env
-      rustup target add aarch64-unknown-none-softfloat
       cargo install cargo-binutils rustfilt
       ```
 
-3. In case you use `Visual Studio Code`, I strongly recommend installing the [Rust Analyzer extension].
-4. If you are **NOT** running Linux, some `Ruby` gems are needed as well:
+1. In case you use `Visual Studio Code`, I strongly recommend installing the [Rust Analyzer extension].
+1. If you are **NOT** running Linux, some `Ruby` gems are needed as well:
 
    ```bash
    sudo gem install bundler

rust-toolchain

@@ -0,0 +1,4 @@
+[toolchain]
+channel = "nightly-2020-12-08"
+components = ["llvm-tools-preview"]
+targets = ["aarch64-unknown-none-softfloat"]