Re: [PATCH 6.15.y] rust: use `#[used(compiler)]` to fix build and `modpost` with Rust >= 1.89.0

[Sasha Levin <sashal@kernel.org>]

[ Sasha's backport helper bot ]

Hi,

✅ All tests passed successfully. No issues detected.
No action required from the submitter.

The upstream commit SHA1 provided is correct: 7498159226772d66f150dd406be462d75964a366

Note: The patch differs from the upstream commit:
---
1:  749815922677 ! 1:  a3a4555a1d32 rust: use `#[used(compiler)]` to fix build and `modpost` with Rust >= 1.89.0
    @@ Commit message
         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>
    +    (cherry picked from commit 7498159226772d66f150dd406be462d75964a366)
    +    Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
     
      ## rust/Makefile ##
     @@ rust/Makefile: quiet_cmd_rustdoc_test = RUSTDOC T $<
    @@ rust/kernel/kunit.rs: macro_rules! kunit_unsafe_test_suite {
     
      ## rust/kernel/lib.rs ##
     @@
    - // Expected to become stable.
    - #![feature(arbitrary_self_types)]
    - //
    + #![feature(const_mut_refs)]
    + #![feature(const_ptr_write)]
    + #![feature(const_refs_to_cell)]
     +// 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))]
    + 
    + // Ensure conditional compilation based on the kernel configuration works;
    + // otherwise we may silently break things like initcall handling.
     
      ## rust/macros/module.rs ##
     @@ rust/macros/module.rs: fn emit_base(&mut self, field: &str, content: &str, builtin: bool) {
    @@ rust/macros/module.rs: mod __module_init {{
     +                    #[used(compiler)]
                          static __IS_RUST_MODULE: () = ();
      
    -                     static mut __MOD: ::core::mem::MaybeUninit<{type_}> =
    +                     static mut __MOD: core::mem::MaybeUninit<{type_}> =
     @@ rust/macros/module.rs: mod __module_init {{
      
                          #[cfg(MODULE)]
    @@ rust/macros/module.rs: mod __module_init {{
                          #[link_section = \"{initcall_section}\"]
     -                    #[used]
     +                    #[used(compiler)]
    -                     pub static __{ident}_initcall: extern \"C\" fn() ->
    -                         ::kernel::ffi::c_int = __{ident}_init;
    +                     pub static __{name}_initcall: extern \"C\" fn() -> kernel::ffi::c_int = __{name}_init;
      
    +                     #[cfg(not(MODULE))]
     
      ## scripts/Makefile.build ##
     @@ scripts/Makefile.build: $(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.
    + # Compile Rust sources (.rs)
    + # ---------------------------------------------------------------------------
    + 
     -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
      

---

Results of testing on various branches:

| Branch                    | Patch Apply | Build Test |
|---------------------------|-------------|------------|
| origin/linux-6.15.y       | Success     | Success    |