doc/rust: Nightly is not needed any more

(cherry picked from commit 7583626)

Author: chrysn

doc/doxygen/src/using-rust.md

@@ -106,26 +106,21 @@ Toolchain {#toolchain}
 
 To install the necessary Rust components, it is easiest use [**rustup**, installed as described on its website].
 
-Using Rust on RIOT needs the latest stable or nightly version of Rust,
-depending on the precise example used.
-(Currently, it's mainly the CoAP parts that use nightly features, and some native builds;
-until stable is universally available, only tests are run on stable by default).
+Using Rust on RIOT needs the latest stable version of Rust.
 
-Make sure you have both the nightly and stable **toolchains**
+Make sure you have the stable **toolchain**
 and the core library for the CPU (**target**) of your choice available:
 
 ```
-$ rustup toolchain add nightly
 $ rustup toolchain add stable
-$ rustup target add thumbv7m-none-eabi --toolchain nightly
 $ rustup target add thumbv7m-none-eabi --toolchain stable
 ```
 
 Substitute thumbv7m-none-eabi with the value of `RUST_TARGET`
-in the output of `make info-build` of an application that has your current board selected,
-or just add it later whenever the Rust compiler complains about not finding the core library for a given target).
-Installing only nightly will work just as well,
-but you may need to remove the `CARGO_CHANNEL = stable` line to run tests.
+in the output of `make info-build` of an application that has your current board selected
+(or just add it later whenever the Rust compiler complains about not finding the core library for a given target).
+Using a beta or nightly will work just as well,
+but you may need to set `CARGO_CHANNEL=nightly` on your shell or in your Makefiles.
 
 
 While Rust comes with its own [cargo] dependency tracker for any Rust code,