[PATCH v6 4/4] rust: Add warn_on macro

[Alice Ryhl]

On Fri, May 02, 2025 at 06:45:36PM +0900, FUJITA Tomonori wrote:
> 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 at gmail.com>

Reviewed-by: Alice Ryhl <aliceryhl at google.com>