rust: Don't be so picky about toolchains

doc/doxygen/src/using-rust.md

@@ -106,26 +106,23 @@ 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).
-
-Make sure you have both the nightly and stable **toolchains**
-and the core library for the CPU (**target**) of your choice available:
+While the compiler installed through rustup can generate code for all supported platforms,
+you will still need to download the core libraries for your **target**:
 
 ```
-$ rustup toolchain add nightly
-$ rustup toolchain add stable
-$ rustup target add thumbv7m-none-eabi --toolchain nightly
-$ rustup target add thumbv7m-none-eabi --toolchain stable
+$ rustup target add thumbv7m-none-eabi
 ```
 
 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).
+
+If you prefer to use other **targets** than the default "stable"
+(popular choices are "beta", "nightly" or a particular nightly such as "nightly-2022-10-14"),
+you'll need to make sure that the core library is available for that target
+by running the `target add` line with a parameter of `--toolchain nightly` or similar.
+Note that some tests force the toolchain to "stable",
+so you'll need to either have that available or override the `CARGO_CHANNEL` make variable.
 
 
 While Rust comes with its own [cargo] dependency tracker for any Rust code,

examples/rust-gcoap/Makefile

@@ -41,9 +41,6 @@ BASELIBS += $(APPLICATION_RUST_MODULE).module
 
 FEATURES_REQUIRED += rust_target
 
-# This example requires a nightly version because of the CoAP dependencies
-CARGO_CHANNEL ?= ${CARGO_CHANNEL_NIGHTLY}
-
 # Currently unknown, something related to the LED_PORT definition that doesn't
 # pass C2Rust's transpilation
 BOARD_BLACKLIST := ek-lm4f120xl

examples/rust-hello-world/Makefile

@@ -21,11 +21,6 @@ BASELIBS += $(APPLICATION_RUST_MODULE).module
 
 FEATURES_REQUIRED += rust_target
 
-# While this example can be built with stable Rust on many platforms, the
-# nightly channel is selected to smoothly transition from running this example
-# to experimenting with it (which might easily lead into nightly territory).
-CARGO_CHANNEL ?= $(CARGO_CHANNEL_NIGHTLY)
-
 # Currently unknown, something related to the LED_PORT definition that doesn't
 # pass C2Rust's transpilation
 BOARD_BLACKLIST := ek-lm4f120xl