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

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: Björn Roy Baron <bjorn3_gh@protonmail.com>
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: rust-lang/rust#140872 [1]
Link: rust-lang/rust#93798 [2]
Link: rust-lang/rust#91504 [3]
Link: https://godbolt.org/z/sxzWTMfzW [4]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/Makefile

@@ -194,6 +194,7 @@ quiet_cmd_rustdoc_test = RUSTDOC T $<
 	RUST_MODFILE=test.rs \
 	OBJTREE=$(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/firmware.rs

@@ -202,7 +202,7 @@ macro_rules! module_firmware {
             };
 
             #[link_section = ".modinfo"]
-            #[used]
+            #[used(compiler)]
             static __MODULE_FIRMWARE: [u8; $($builder)*::create(__MODULE_FIRMWARE_PREFIX)
                 .build_length()] = $($builder)*::create(__MODULE_FIRMWARE_PREFIX).build();
         };

rust/kernel/kunit.rs

@@ -302,7 +302,7 @@ macro_rules! kunit_unsafe_test_suite {
                     is_init: false,
                 };
 
-            #[used]
+            #[used(compiler)]
             #[allow(unused_unsafe)]
             #[cfg_attr(not(target_os = "macos"), link_section = ".kunit_test_suites")]
             static mut KUNIT_TEST_SUITE_ENTRY: *const ::kernel::bindings::kunit_suite =

rust/kernel/lib.rs

@@ -34,6 +34,9 @@
 // Expected to become stable.
 #![feature(arbitrary_self_types)]
 //
+// To be determined.
+#![feature(used_with_arg)]
+//
 // `feature(derive_coerce_pointee)` is expected to become stable. Before Rust
 // 1.84.0, it did not exist, so enable the predecessor features.
 #![cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, feature(derive_coerce_pointee))]

rust/macros/module.rs

@@ -57,7 +57,7 @@ impl<'a> ModInfoBuilder<'a> {
                 {cfg}
                 #[doc(hidden)]
                 #[cfg_attr(not(target_os = \"macos\"), link_section = \".modinfo\")]
-                #[used]
+                #[used(compiler)]
                 pub static __{module}_{counter}: [u8; {length}] = *{string};
             ",
             cfg = if builtin {
@@ -249,7 +249,7 @@ 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: ::core::mem::MaybeUninit<{type_}> =
@@ -273,7 +273,7 @@ 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;
 
@@ -293,7 +293,7 @@ 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;
 
@@ -303,7 +303,7 @@ 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 __{ident}_initcall: extern \"C\" fn() ->
                         ::kernel::ffi::c_int = __{ident}_init;
 

scripts/Makefile.build

@@ -312,10 +312,11 @@ $(obj)/%.lst: $(obj)/%.c FORCE
 #   - Stable since Rust 1.82.0: `feature(asm_const)`, `feature(raw_ref_op)`.
 #   - Stable since Rust 1.87.0: `feature(asm_goto)`.
 #   - Expected to become stable: `feature(arbitrary_self_types)`.
+#   - To be determined: `feature(used_with_arg)`.
 #
 # Please see https://github.com/Rust-for-Linux/linux/issues/2 for details on
 # the unstable features in use.
-rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,raw_ref_op
+rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,raw_ref_op,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