RISC-V: disallow gcc + rust builds

During the discussion before supporting rust on riscv, it was decided
not to support gcc yet, due to differences in extension handling
compared to llvm (only the version of libclang matching the c compiler
is supported). Recently Jason Montleon reported [1] that building with
gcc caused build issues, due to unsupported arguments being passed to
libclang. After some discussion between myself and Miguel, it is better
to disable gcc + rust builds to match the original intent, and
subsequently support it when an appropriate set of extensions can be
deduced from the version of libclang.

Closes: https://lore.kernel.org/all/[email protected]/ [1]
Link: https://lore.kernel.org/all/20240926-battering-revolt-6c6a7827413e@spud/ [2]
Fixes: 70a57b2 ("RISC-V: enable building 64-bit kernels with rust support")
Cc: [email protected]
Reported-by: Jason Montleon <[email protected]>
Signed-off-by: Conor Dooley <[email protected]>
Acked-by: Miguel Ojeda <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Link: https://lore.kernel.org/r/20241001-playlist-deceiving-16ece2f440f5@spud
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: ConchuOD

Documentation/rust/arch-support.rst

@@ -17,7 +17,7 @@ Architecture   Level of support  Constraints
 =============  ================  ==============================================
 ``arm64``      Maintained        Little Endian only.
 ``loongarch``  Maintained        \-
-``riscv``      Maintained        ``riscv64`` only.
+``riscv``      Maintained        ``riscv64`` and LLVM/Clang only.
 ``um``         Maintained        \-
 ``x86``        Maintained        ``x86_64`` only.
 =============  ================  ==============================================

arch/riscv/Kconfig

@@ -177,7 +177,7 @@ config RISCV
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RETHOOK if !XIP_KERNEL
 	select HAVE_RSEQ
-	select HAVE_RUST if RUSTC_SUPPORTS_RISCV
+	select HAVE_RUST if RUSTC_SUPPORTS_RISCV && CC_IS_CLANG
 	select HAVE_SAMPLE_FTRACE_DIRECT
 	select HAVE_SAMPLE_FTRACE_DIRECT_MULTI
 	select HAVE_STACKPROTECTOR