rust: Add warn_on macro

Add warn_on macro, uses the BUG/WARN feature (lib/bug.c) via assembly
for x86_64/arm64/riscv.

The current Rust code simply wraps BUG() macro but doesn't provide the
proper debug information. The BUG/WARN feature can only be used from
assembly.

This uses the assembly code exported by the C side via ARCH_WARN_ASM
macro. To avoid duplicating the assembly code, this approach follows
the same strategy as the static branch code: it generates the assembly
code for Rust using the C preprocessor at compile time.

Similarly, ARCH_WARN_REACHABLE is also used at compile time to
generate the assembly code; objtool's reachable annotation code. It's
used for only architectures that use objtool.

For now, Loongarch and arm just use a wrapper for WARN macro.

UML doesn't use the assembly BUG/WARN feature; just wrapping generic
BUG/WARN functions implemented in C works.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250502094537.231725-5-fujita.tomonori@gmail.com
[ Avoid evaluating the condition twice (a good idea in general,
  but it also matches the C side). Simplify with `as_char_ptr()`
  to avoid a cast. Cast to `ffi` integer types for
  `warn_slowpath_fmt`. Avoid cast for `null()`. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/rust/helpers/bug.c b/rust/helpers/bug.c
index e2d13babc737..a62c96f507d1 100644
--- a/rust/helpers/bug.c
+++ b/rust/helpers/bug.c
@@ -6,3 +6,8 @@ __noreturn void rust_helper_BUG(void)
 {
 	BUG();
 }
+
+bool rust_helper_WARN_ON(bool cond)
+{
+	return WARN_ON(cond);
+}