makefiles/rust: Remove CARGO_CHANNEL special casing

Back when specific control of the Rust version used with RIOT was
needed, CARGO_CHANNEL was added to explicitly set the Rust version with
consideration for CI special cases.

Rust's mechansims of selecting a toolchain can be used instead now.

doc/doxygen/src/using-rust.md

@@ -108,19 +108,17 @@ To install the necessary Rust components, it is easiest use [**rustup**, install
 
 Using Rust on RIOT needs the latest stable version of Rust.
 
-Make sure you have the stable **toolchain**
+Make sure you have the core library for the CPU (**target**) of your choice available:
-and the core library for the CPU (**target**) of your choice available:
 
 ```
-$ rustup toolchain add stable
+$ rustup target add thumbv7m-none-eabi
-$ 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).
-Using a beta or nightly will work just as well,
+Using the beta or nightly toolchains will work just as well
-but you may need to set `CARGO_CHANNEL=nightly` on your shell or in your Makefiles.
+if they are selected through rustup's override mechanism.
 
 
 While Rust comes with its own [cargo] dependency tracker for any Rust code,

examples/rust-gcoap/Makefile

@@ -41,8 +41,6 @@ BASELIBS += $(APPLICATION_RUST_MODULE).module
 
 FEATURES_REQUIRED += rust_target
 
-CARGO_CHANNEL ?= stable
-
 # 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
 
-# All Rust components RIOT uses work on stable Rust. If any extra libraries
-# were to require a more recent version, switch to `CARGO_CHANNEL =
-# $(CARGO_CHANNEL_NIGHTLY)` to use whichever nightly version is available.
-CARGO_CHANNEL ?= stable
-
 # Currently unknown, something related to the LED_PORT definition that doesn't
 # pass C2Rust's transpilation
 BOARD_BLACKLIST := ek-lm4f120xl

makefiles/cargo-settings.inc.mk

@@ -4,28 +4,6 @@
 # binary is derived from this.
 CARGO_PROFILE ?= release
 
-# Value for CARGO_CHANNEL when using nightly
-#
-# As different environments have different versions of nightly installed, but
-# rustup / cargo does not take "the latest installed nightly" for a toolchain,
-# a good value is determined dynamically. Typical values this takes are
-# `nightly` (on regular installations) and `nightly-2022-03-08` (or whichever
-# date it is currently pinned to) in riotbuild.
-#
-# Workaround-For: https://github.com/rust-lang/rustup/issues/3015
-#
-# This does not get evaluated unless actually used; if rustup is not installed,
-# the default value will likely not be usable but at least set the user on the
-# right track.
-CARGO_CHANNEL_NIGHTLY = $(shell rustup toolchain list | sed 's/ .*//' |grep nightly | tail -n1 || echo nightly)
-
-# The Rust version to use.
-#
-# Examples should set this to either `stable` or `$(CARGO_CHANNEL_NIGHTLY)`.
-# The default is empty, which is suitable for applications that select their
-# version through a `rust-toolchain.yaml` file.
-CARGO_CHANNEL ?=
-
 # Note that if we did not set this explicitly, CARGO_LIB would have to
 # understand which value cargo uses in absence of CARGO_TARGET_DIR, which would
 # be $(APPDIR)/target.

makefiles/cargo-targets.inc.mk

@@ -55,15 +55,15 @@ $(CARGO_LIB): $(RIOTBUILD_CONFIG_HEADER_C) $(BUILDDEPS) $(CARGO_COMPILE_COMMANDS
 	$(Q)# If distribution installed cargos ever grow the capacity to build RIOT, this absence of `rustup` might be OK. But that'd need them to both have cross tools around and cross core libs, none of which is currently the case.
 	$(Q)# Ad grepping for "std": We're not *actually* checking for std but more for core -- but rust-stc-$TARGET is the name of any standard libraries that'd be available for that target.
 	$(Q)[ x"$(findstring build-std,$(CARGO_OPTIONS))" != x"" ] || \
-		(rustup component list $(patsubst %,--toolchain %,$(CARGO_CHANNEL)) --installed | grep 'rust-std-$(RUST_TARGET)$$' -q) || \
+		(rustup component list --installed | grep 'rust-std-$(RUST_TARGET)$$' -q) || \
 		($(COLOR_ECHO) \
-		'$(COLOR_RED)Error: No Rust libraries are installed for the board'"'"'s CPU.$(COLOR_RESET) Run\n    $(COLOR_GREEN)$$$(COLOR_RESET) rustup target add $(RUST_TARGET) $(patsubst %,--toolchain %,$(CARGO_CHANNEL))\nor set `CARGO_OPTIONS=-Zbuild-std=core`.'; \
+		'$(COLOR_RED)Error: No Rust libraries are installed for the board'"'"'s CPU.$(COLOR_RESET) Run\n    $(COLOR_GREEN)$$$(COLOR_RESET) rustup target add $(RUST_TARGET)\nor set `CARGO_OPTIONS=-Zbuild-std=core`.'; \
 		exit 1)
 	$(Q)# finally call out to cargo. mind the "+" to pass down make's jobserver.
 	$(Q)+ CC= CFLAGS= CPPFLAGS= CXXFLAGS= \
 		RIOT_COMPILE_COMMANDS_JSON="$(CARGO_COMPILE_COMMANDS)" \
 		RIOT_USEMODULE="$(USEMODULE)" \
-		cargo $(patsubst +,,+${CARGO_CHANNEL}) \
+		cargo \
 			build \
 			--target $(RUST_TARGET) \
 			--profile $(CARGO_PROFILE) \

makefiles/info.inc.mk

@@ -90,7 +90,6 @@ info-build:
 	@echo -e 'CXXEXFLAGS:$(patsubst %, \n\t%, $(CXXEXFLAGS))'
 	@echo ''
 	@echo 'RUST_TARGET: $(RUST_TARGET)'
-	@echo 'CARGO_CHANNEL: $(CARGO_CHANNEL)'
 	@echo 'CARGO_PROFILE: $(CARGO_PROFILE)'
 	@echo 'CARGO_OPTIONS: $(CARGO_OPTIONS)'
 	@echo ''
@@ -250,5 +249,5 @@ info-programmers-supported:
 	@echo $(sort $(PROGRAMMERS_SUPPORTED))
 
 info-rust:
-	cargo $(patsubst +,,+${CARGO_CHANNEL}) version
+	cargo version
 	c2rust --version

tests/rust_libs/Makefile

@@ -5,10 +5,6 @@ USEMODULE += shell_democommands
 
 FEATURES_REQUIRED += rust_target
 
-# Testing on stable to ensure that no nightly features are needed when Rust is
-# pulled in through modules.
-CARGO_CHANNEL = stable
-
 # Currently unknown, something related to the LED_PORT definition that doesn't
 # pass C2Rust's transpilation
 BOARD_BLACKLIST := ek-lm4f120xl

tests/rust_minimal/Makefile

@@ -5,10 +5,6 @@ BASELIBS += $(APPLICATION_RUST_MODULE).module
 
 FEATURES_REQUIRED += rust_target
 
-# Testing on stable to ensure that no nightly features are needed for basic
-# Rust usage.
-CARGO_CHANNEL = stable
-
 # Currently unknown, something related to the LED_PORT definition that doesn't
 # pass C2Rust's transpilation
 BOARD_BLACKLIST := ek-lm4f120xl