rust: build_error: remove RUST_BUILD_ASSERT_WARN support

There are several problems with `RUST_BUILD_ASSERT_WARN`:

  - ld.lld behaves differently than ld.bfd and emits warnings even
    when all calls are correct and optimized away in `DENY` mode.

    This could be solved by restricting the `WARN` mode so that
    it can only be used when the GNU linker is used.

  - Since we need to export the `rust_build_error` for loadable
    kernel modules, even ld.bfd considers that as a use.

  - For loadable modules, we do not get warnings since the section
    is in `build_error`, not the module.

    These last two issues may be possible to solve all by creating
    a symbol and section per loadable kernel module.

While all the above may be possible to solve, supporting the `WARN`
mode gets more complex and it is not worth to support. It was
introduced back when we had configurable optimization levels, but
nowadays optimizations are always enabled. Therefore, `DENY` is
probably good enough for all use cases, with `ALLOW` being
the escape hatch.

Thus remove the `WARN` support.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

lib/Kconfig.debug

@@ -2820,12 +2820,6 @@ config RUST_BUILD_ASSERT_ALLOW
 	  Unoptimized calls to `build_error!` will be converted to `panic!`
 	  and checked at runtime.
 
-config RUST_BUILD_ASSERT_WARN
-	bool "Warn"
-	help
-	  Unoptimized calls to `build_error!` will be converted to `panic!`
-	  and checked at runtime, but warnings will be generated when building.
-
 config RUST_BUILD_ASSERT_DENY
 	bool "Deny"
 	help

rust/build_error.rs

@@ -22,8 +22,3 @@
 pub const fn build_error(msg: &'static str) -> ! {
     panic!("{}", msg);
 }
-
-#[cfg(CONFIG_RUST_BUILD_ASSERT_WARN)]
-#[link_section = ".gnu.warning.rust_build_error"]
-#[used]
-static BUILD_ERROR_WARNING: [u8; 45] = *b"call to build_error present after compilation";