makefiles: Define RUST_TARGET for use with Cargo / Rust

For RISC-V and Cortex-M-not-3, triples are known and have worked in some
configuration, but do not work at the moment and stay disabled until the
reference platforms (native, M3) have been established well.

cpu/cortexm_common/Kconfig

@@ -80,6 +80,7 @@ config CPU_CORE_CORTEX_M3
     bool
     select CPU_ARCH_ARMV7M
     select CPU_CORE_CORTEX_M
+    select HAS_RUST_TARGET
 
 config CPU_CORE_CORTEX_M33
     bool
@@ -90,12 +91,14 @@ config CPU_CORE_CORTEX_M4
     bool
     select CPU_ARCH_ARMV7M
     select CPU_CORE_CORTEX_M
+    select HAS_RUST_TARGET
 
 config CPU_CORE_CORTEX_M4F
     bool
     select CPU_ARCH_ARMV7M
     select CPU_CORE_CORTEX_M
     select HAS_CORTEXM_FPU
+    select HAS_RUST_TARGET
 
 config CPU_CORE_CORTEX_M7
     bool

cpu/cortexm_common/Makefile.features

@@ -18,26 +18,38 @@ ifneq (,$(filter $(CPU_CORE),cortex-m4f cortex-m7))
 endif
 
 # Set CPU_ARCH depending on the CPU_CORE
+#
+# RUST_TARGET is only used when building Rust code; any users need to require
+# the `rust_target` feature to esnure things are checked properly.
 ifeq ($(CPU_CORE),cortex-m0)
   CPU_ARCH := armv6m
+  #RUST_TARGET = thumbv6m-none-eabi
 else ifeq ($(CPU_CORE),cortex-m0plus)
   CPU_ARCH := armv6m
+  #RUST_TARGET = thumbv6m-none-eabi
 else ifeq ($(CPU_CORE),cortex-m23)
   CPU_ARCH := armv8m
 else ifeq ($(CPU_CORE),cortex-m3)
   CPU_ARCH := armv7m
+  RUST_TARGET = thumbv7m-none-eabi
 else ifeq ($(CPU_CORE),cortex-m33)
   CPU_ARCH := armv8m
 else ifeq ($(CPU_CORE),cortex-m4)
   CPU_ARCH := armv7m
+  RUST_TARGET = thumbv7em-none-eabi
 else ifeq ($(CPU_CORE),cortex-m4f)
   CPU_ARCH := armv7m
+  RUST_TARGET = thumbv7em-none-eabihf
 else ifeq ($(CPU_CORE),cortex-m7)
   CPU_ARCH := armv7m
 else
   $(error Unkwnown cortexm core: $(CPU_CORE))
 endif
 
+ifneq (,$(RUST_TARGET))
+  FEATURES_PROVIDED += rust_target
+endif
+
 FEATURES_PROVIDED += no_idle_thread
 
 # This configuration enables modules that are only available when using Kconfig

cpu/native/Kconfig

@@ -62,6 +62,7 @@ config NATIVE_OS_LINUX
     select HAS_PERIPH_GPIO
     select HAS_PERIPH_GPIO_IRQ
     select HAS_PERIPH_SPI
+    select HAS_RUST_TARGET if "$(OS_ARCH)" = "x86_64"
 
 config NATIVE_OS_FREEBSD
     bool

cpu/native/Makefile.features

@@ -18,6 +18,9 @@ FEATURES_PROVIDED += periph_hwrng
 FEATURES_PROVIDED += periph_pm
 FEATURES_PROVIDED += periph_pwm
 FEATURES_PROVIDED += periph_timer_periodic
+ifeq ($(OS) $(OS_ARCH),Linux x86_64)
+  FEATURES_PROVIDED += rust_target
+endif
 FEATURES_PROVIDED += ssp
 
 ifeq ($(OS),Linux)

cpu/native/Makefile.include

@@ -10,3 +10,8 @@ ifneq (,$(filter periph_can,$(USEMODULE)))
 endif
 
 TOOLCHAINS_SUPPORTED = gnu llvm afl
+
+# Platform triple as used by Rust
+ifeq ($(OS) $(OS_ARCH),Linux x86_64)
+  RUST_TARGET = i686-unknown-linux-gnu
+endif

cpu/riscv_common/Kconfig

@@ -12,6 +12,7 @@ config CPU_ARCH_RISCV
     select HAS_NEWLIB
     select HAS_PERIPH_CORETIMER
     select HAS_PICOLIBC if '$(RIOT_CI_BUILD)' != '1'
+    #select HAS_RUST_TARGET
     select HAS_SSP
 
     select MODULE_MALLOC_THREAD_SAFE if TEST_KCONFIG

cpu/riscv_common/Makefile.features

@@ -8,6 +8,7 @@ FEATURES_PROVIDED += cpp
 FEATURES_PROVIDED += libstdcpp
 FEATURES_PROVIDED += newlib
 FEATURES_PROVIDED += periph_coretimer
+#FEATURES_PROVIDED += rust_target
 FEATURES_PROVIDED += ssp
 
 # RISC-V toolchain on CI does not work properly with picolibc yet

kconfigs/Kconfig.features

@@ -42,6 +42,11 @@ config HAS_BACKUP_RAM
     help
         Indicates that Backup RAM is supported.
 
+config HAS_RUST_TARGET
+    bool
+    help
+        Indicates that a Rust target definition ("triple") is known.
+
 config HAS_CPP
     bool
     help

makefiles/arch/riscv.inc.mk

@@ -54,3 +54,8 @@ CFLAGS += $(CFLAGS_CPU) $(CFLAGS_DBG) $(CFLAGS_OPT) $(CFLAGS_LINK)
 ASFLAGS += $(CFLAGS_CPU) $(CFLAGS_DBG)
 # export linker flags
 LINKFLAGS += $(CFLAGS_CPU) $(CFLAGS_LINK) $(CFLAGS_DBG) $(CFLAGS_OPT) -nostartfiles -Wl,--gc-sections -static -lgcc
+
+# Platform triple as used by Rust
+RUST_TARGET = riscv32imac-unknown-none-elf
+# Workaround for https://github.com/rust-lang/rust-bindgen/issues/1555
+CARGO_EXTRACFLAGS += --target=riscv32

makefiles/info-global.inc.mk

@@ -45,6 +45,7 @@ define board_unsatisfied_features
   undefine CPU_ARCH
   undefine CPU_CORE
   undefine CPU_FAM
+  undefine RUST_TARGET
 
   include $(RIOTBASE)/Makefile.features
   # always select provided architecture features

makefiles/vars.inc.mk

@@ -6,6 +6,7 @@ export QQ                    # as Q, but be more quiet
 export QUIET                 # The parameter to use whether to show verbose makefile commands or not.
 
 export OS                    # The operating system of the build host
+export OS_ARCH               # The build host's hardware architecture
 
 export APPLICATION           # The application, set in the Makefile which is run by the user.
 export APPLICATION_MODULE    # The application module name.