rust: use #[used(compiler)] to fix build and modpost with Rust >= 1.89.0

commit 7498159226 upstream.

Starting with Rust 1.89.0 (expected 2025-08-07), the Rust compiler fails
to build the `rusttest` target due to undefined references such as:

    kernel...-cgu.0:(.text....+0x116): undefined reference to
    `rust_helper_kunit_get_current_test'

Moreover, tooling like `modpost` gets confused:

    WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/gpu/drm/nova/nova.o
    ERROR: modpost: missing MODULE_LICENSE() in drivers/gpu/nova-core/nova_core.o

The reason behind both issues is that the Rust compiler will now [1]
treat `#[used]` as `#[used(linker)]` instead of `#[used(compiler)]`
for our targets. This means that the retain section flag (`R`,
`SHF_GNU_RETAIN`) will be used and that they will be marked as `unique`
too, with different IDs. In turn, that means we end up with undefined
references that did not get discarded in `rusttest` and that multiple
`.modinfo` sections are generated, which confuse tooling like `modpost`
because they only expect one.

Thus start using `#[used(compiler)]` to keep the previous behavior
and to be explicit about what we want. Sadly, it is an unstable feature
(`used_with_arg`) [2] -- we will talk to upstream Rust about it. The good
news is that it has been available for a long time (Rust >= 1.60) [3].

The changes should also be fine for previous Rust versions, since they
behave the same way as before [4].

Alternatively, we could use `#[no_mangle]` or `#[export_name = ...]`
since those still behave like `#[used(compiler)]`, but of course it is
not really what we want to express, and it requires other changes to
avoid symbol conflicts.

Cc: David Wood <david@davidtw.co>
Cc: Wesley Wiser <wwiser@gmail.com>
Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Link: https://github.com/rust-lang/rust/pull/140872 [1]
Link: https://github.com/rust-lang/rust/issues/93798 [2]
Link: https://github.com/rust-lang/rust/pull/91504 [3]
Link: https://godbolt.org/z/sxzWTMfzW [4]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
Link: https://lore.kernel.org/r/20250712160103.1244945-3-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

rust/Makefile

@@ -157,6 +157,7 @@ quiet_cmd_rustdoc_test = RUSTDOC T $<
       cmd_rustdoc_test = \
 	OBJTREE=$(abspath $(objtree)) \
 	$(RUSTDOC) --test $(rust_common_flags) \
+		-Zcrate-attr='feature(used_with_arg)' \
 		@$(objtree)/include/generated/rustc_cfg \
 		$(rustc_target_flags) $(rustdoc_test_target_flags) \
 		$(rustdoc_test_quiet) \

rust/kernel/lib.rs

@@ -18,6 +18,7 @@
 #![feature(inline_const)]
 #![feature(lint_reasons)]
 #![feature(unsize)]
+#![feature(used_with_arg)]
 
 // Ensure conditional compilation based on the kernel configuration works;
 // otherwise we may silently break things like initcall handling.

rust/macros/module.rs

@@ -57,7 +57,7 @@ impl<'a> ModInfoBuilder<'a> {
                 {cfg}
                 #[doc(hidden)]
                 #[link_section = \".modinfo\"]
-                #[used]
+                #[used(compiler)]
                 pub static __{module}_{counter}: [u8; {length}] = *{string};
             ",
             cfg = if builtin {
@@ -230,7 +230,7 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
                     // key or a new section. For the moment, keep it simple.
                     #[cfg(MODULE)]
                     #[doc(hidden)]
-                    #[used]
+                    #[used(compiler)]
                     static __IS_RUST_MODULE: () = ();
 
                     static mut __MOD: Option<{type_}> = None;
@@ -253,7 +253,7 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
 
                     #[cfg(MODULE)]
                     #[doc(hidden)]
-                    #[used]
+                    #[used(compiler)]
                     #[link_section = \".init.data\"]
                     static __UNIQUE_ID___addressable_init_module: unsafe extern \"C\" fn() -> i32 = init_module;
 
@@ -273,7 +273,7 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
 
                     #[cfg(MODULE)]
                     #[doc(hidden)]
-                    #[used]
+                    #[used(compiler)]
                     #[link_section = \".exit.data\"]
                     static __UNIQUE_ID___addressable_cleanup_module: extern \"C\" fn() = cleanup_module;
 
@@ -283,7 +283,7 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
                     #[cfg(not(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS))]
                     #[doc(hidden)]
                     #[link_section = \"{initcall_section}\"]
-                    #[used]
+                    #[used(compiler)]
                     pub static __{name}_initcall: extern \"C\" fn() -> kernel::ffi::c_int = __{name}_init;
 
                     #[cfg(not(MODULE))]

scripts/Makefile.build

@@ -248,7 +248,7 @@ $(obj)/%.lst: $(obj)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
-rust_allowed_features := arbitrary_self_types,lint_reasons
+rust_allowed_features := arbitrary_self_types,lint_reasons,used_with_arg
 
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the
 # current working directory, which may be not accessible in the out-of-tree