rust: kbuild: provide RUSTC_HAS_DO_NOT_RECOMMEND symbol

Rust 1.85.0 (current stable version) stabilized [1]
`#[diagnostic::do_not_recommend]` [2].

In order to use it across all supported Rust versions, introduce a new
Kconfig symbol for it.

This allows to perform conditional compilation based on it, e.g. on the
use site to enable the attribute:

    #[cfg_attr(RUSTC_HAS_DO_NOT_RECOMMEND, diagnostic::do_not_recommend)]
    impl A for i32 {}

An alternative would have been to `allow` the following warning:

    #![allow(unknown_or_malformed_diagnostic_attributes)]

However, that would lose the checking for typos across all versions,
which we do not want to lose.

One can also use the Kconfig symbol to allow the warning in older
compilers instead, to avoid repeating the `cfg_attr` line above in all
use sites:

    #![cfg_attr(
        not(RUSTC_HAS_DO_NOT_RECOMMEND),
        expect(unknown_or_malformed_diagnostic_attributes)
    )]

That still loses the checking for typos in older versions, but we still
keep it in newer ones, thus we should still catch mistakes eventually.

In this case we can promote it to `expect` as shown above, so that we do
not forget to remove these lines if we stop using the attribute somewhere.

Link: rust-lang/rust#132056 [1]
Link: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-diagnosticdo_not_recommend-attribute [2]
Link: https://lore.kernel.org/rust-for-linux/CANiq72mYfhuRWkjomb1vOMMPOaxvdS6qjfVLAwxUw6ecdqyh2A@mail.gmail.com/
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Oliver Mangold <oliver.mangold@pm.me>

Author: ojeda

init/Kconfig

@@ -147,6 +147,9 @@ config CC_HAS_COUNTED_BY
 config RUSTC_HAS_COERCE_POINTEE
 	def_bool RUSTC_VERSION >= 108400
 
+config RUSTC_HAS_DO_NOT_RECOMMEND
+	def_bool RUSTC_VERSION >= 108500
+
 config PAHOLE_VERSION
 	int
 	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))