[PATCH] rust: clarify the language unstable features in use

[PATCH] rust: clarify the language unstable features in use

[Miguel Ojeda]

We track the details of which Rust features we use at our usual "live
list" [1] (and its sub-lists), but in light of a discussion in the LWN
article [2], it would help to clarify it in the source code.

In particular, we are very close to rely only on stable Rust language-wise
-- essentially only two language features remain (including the `kernel`
crate).

Thus add some details in both the feature list of the `kernel` crate as
well as the list of allowed features.

This does not over every single feature, and there are quite a few
non-language features that we use too. To have the full picture, please
refer to [1].

Link: https://github.com/Rust-for-Linux/linux/issues/2 [1]
Link: https://lwn.net/Articles/1015409/ [2]
Suggested-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/lib.rs     | 27 ++++++++++++++++++++-------
 scripts/Makefile.build |  8 ++++++++
 2 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index ba0f3b0297b2..660a3ab6f9d4 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -12,21 +12,34 @@
 //! do so first instead of bypassing this crate.
 
 #![no_std]
-#![feature(arbitrary_self_types)]
-#![cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, feature(derive_coerce_pointee))]
-#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(coerce_unsized))]
-#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(dispatch_from_dyn))]
-#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(unsize))]
+
+// Please see https://github.com/Rust-for-Linux/linux/issues/2 for details on
+// the unstable features in use.
+
+// Stable since Rust 1.79.0.
 #![feature(inline_const)]
+
+// Stable since Rust 1.81.0.
 #![feature(lint_reasons)]
-// Stable in Rust 1.82
+
+// Stable since Rust 1.82.0.
 #![feature(raw_ref_op)]
-// Stable in Rust 1.83
+
+// Stable since Rust 1.83.0.
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_mut_refs)]
 #![feature(const_ptr_write)]
 #![feature(const_refs_to_cell)]
 
+// Expected to become stable.
+#![feature(arbitrary_self_types)]
+
+// `feature(derive_coerce_pointee)` is the one expected to become stable.
+#![cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, feature(derive_coerce_pointee))]
+#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(coerce_unsized))]
+#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(dispatch_from_dyn))]
+#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(unsize))]
+
 // Ensure conditional compilation based on the kernel configuration works;
 // otherwise we may silently break things like initcall handling.
 #[cfg(not(CONFIG_RUST))]
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 56be83024851..41a640990cfa 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -226,6 +226,14 @@ $(obj)/%.lst: $(obj)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
+# The features in this list are the ones allowed for non-`rust/` code.
+#
+#   - Stable since Rust 1.81.0: `feature(lint_reasons)`.
+#   - Stable since Rust 1.82.0: `feature(asm_const)`, `feature(raw_ref_op)`.
+#   - Stable since Rust 1.87.0: `feature(asm_goto)`.
+#
+# 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
 
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the

base-commit: e6ea10d5dbe082c54add289b44f08c9fcfe658af
-- 
2.49.0

Re: [PATCH] rust: clarify the language unstable features in use

[Benno Lossin]

On Thu Mar 27, 2025 at 10:13 PM CET, Miguel Ojeda wrote:
> We track the details of which Rust features we use at our usual "live
> list" [1] (and its sub-lists), but in light of a discussion in the LWN
> article [2], it would help to clarify it in the source code.
>
> In particular, we are very close to rely only on stable Rust language-wise
> -- essentially only two language features remain (including the `kernel`
> crate).
>
> Thus add some details in both the feature list of the `kernel` crate as
> well as the list of allowed features.
>
> This does not over every single feature, and there are quite a few
> non-language features that we use too. To have the full picture, please
> refer to [1].
>
> Link: https://github.com/Rust-for-Linux/linux/issues/2 [1]
> Link: https://lwn.net/Articles/1015409/ [2]
> Suggested-by: Andreas Hindborg <a.hindborg@kernel.org>
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

LGTM, so:

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

And left two suggestions below.

> ---
>  rust/kernel/lib.rs     | 27 ++++++++++++++++++++-------
>  scripts/Makefile.build |  8 ++++++++
>  2 files changed, 28 insertions(+), 7 deletions(-)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index ba0f3b0297b2..660a3ab6f9d4 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -12,21 +12,34 @@
>  //! do so first instead of bypassing this crate.
>  
>  #![no_std]
> -#![feature(arbitrary_self_types)]
> -#![cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, feature(derive_coerce_pointee))]
> -#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(coerce_unsized))]
> -#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(dispatch_from_dyn))]
> -#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(unsize))]
> +
> +// Please see https://github.com/Rust-for-Linux/linux/issues/2 for details on
> +// the unstable features in use.
> +
> +// Stable since Rust 1.79.0.
>  #![feature(inline_const)]
> +
> +// Stable since Rust 1.81.0.
>  #![feature(lint_reasons)]
> -// Stable in Rust 1.82
> +
> +// Stable since Rust 1.82.0.
>  #![feature(raw_ref_op)]
> -// Stable in Rust 1.83
> +
> +// Stable since Rust 1.83.0.
>  #![feature(const_maybe_uninit_as_mut_ptr)]
>  #![feature(const_mut_refs)]
>  #![feature(const_ptr_write)]
>  #![feature(const_refs_to_cell)]
>  
> +// Expected to become stable.
> +#![feature(arbitrary_self_types)]
> +
> +// `feature(derive_coerce_pointee)` is the one expected to become stable.
> +#![cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, feature(derive_coerce_pointee))]
> +#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(coerce_unsized))]
> +#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(dispatch_from_dyn))]
> +#![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(unsize))]

Does it make sense to add another comment in front of these three along
the lines of:

    // Before Rust 1.82.0 `feature(derive_coerce_pointee)` doesn't exist, so enable the predecessor
    // features.

> +
>  // Ensure conditional compilation based on the kernel configuration works;
>  // otherwise we may silently break things like initcall handling.
>  #[cfg(not(CONFIG_RUST))]
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 56be83024851..41a640990cfa 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -226,6 +226,14 @@ $(obj)/%.lst: $(obj)/%.c FORCE
>  # Compile Rust sources (.rs)
>  # ---------------------------------------------------------------------------
>  
> +# The features in this list are the ones allowed for non-`rust/` code.
> +#
> +#   - Stable since Rust 1.81.0: `feature(lint_reasons)`.
> +#   - Stable since Rust 1.82.0: `feature(asm_const)`, `feature(raw_ref_op)`.
> +#   - Stable since Rust 1.87.0: `feature(asm_goto)`.

Probably a good idea to add:

    #   - Expected to become stable: `arbitrary_self_types`.

---
Cheers,
Benno

> +#
> +# 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
>  
>  # `--out-dir` is required to avoid temporaries being created by `rustc` in the
>
> base-commit: e6ea10d5dbe082c54add289b44f08c9fcfe658af

Re: [PATCH] rust: clarify the language unstable features in use

[kernel test robot]

Hi Miguel,

kernel test robot noticed the following build errors:

[auto build test ERROR on e6ea10d5dbe082c54add289b44f08c9fcfe658af]

url:    https://github.com/intel-lab-lkp/linux/commits/Miguel-Ojeda/rust-clarify-the-language-unstable-features-in-use/20250328-051650
base:   e6ea10d5dbe082c54add289b44f08c9fcfe658af
patch link:    https://lore.kernel.org/r/20250327211302.286313-1-ojeda%40kernel.org
patch subject: [PATCH] rust: clarify the language unstable features in use
config: x86_64-rhel-9.4-rust (https://download.01.org/0day-ci/archive/20250328/202503280805.Oq84ECdB-lkp@intel.com/config)
compiler: clang version 18.1.8 (https://github.com/llvm/llvm-project 3b5b5c1ec4a3095ab096dd780e84d7ab81f3d7ff)
rustc: rustc 1.78.0 (9b00956e5 2024-04-29)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250328/202503280805.Oq84ECdB-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202503280805.Oq84ECdB-lkp@intel.com/

All errors (new ones prefixed by >>):

   PATH=/opt/cross/clang-18/bin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
   INFO PATH=/opt/cross/rustc-1.78.0-bindgen-0.65.1/cargo/bin:/opt/cross/clang-18/bin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
   /usr/bin/timeout -k 100 12h /usr/bin/make W=1 --keep-going LLVM=1 -j32 -C source O=/kbuild/obj/consumer/x86_64-rhel-9.4-rust ARCH=x86_64 SHELL=/bin/bash rustfmtcheck
   make: Entering directory '/kbuild/src/consumer'
   make[1]: Entering directory '/kbuild/obj/consumer/x86_64-rhel-9.4-rust'
>> Diff in rust/kernel/lib.rs at line 12:
    //! do so first instead of bypassing this crate.
    
    #![no_std]
   -
    // Please see https://github.com/Rust-for-Linux/linux/issues/2 for details on
    // the unstable features in use.
    
   Diff in rust/kernel/lib.rs at line 19:
    // Stable since Rust 1.79.0.
    #![feature(inline_const)]
   -
    // Stable since Rust 1.81.0.
    #![feature(lint_reasons)]
   -
    // Stable since Rust 1.82.0.
    #![feature(raw_ref_op)]
   -
    // Stable since Rust 1.83.0.
    #![feature(const_maybe_uninit_as_mut_ptr)]
    #![feature(const_mut_refs)]
   Diff in rust/kernel/lib.rs at line 31:
    #![feature(const_ptr_write)]
    #![feature(const_refs_to_cell)]
   -
    // Expected to become stable.
    #![feature(arbitrary_self_types)]
   -
    // `feature(derive_coerce_pointee)` is the one expected to become stable.
    #![cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, feature(derive_coerce_pointee))]
    #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(coerce_unsized))]
   make[2]: *** [Makefile:1811: rustfmt] Error 123
   make[2]: Target 'rustfmtcheck' not remade because of errors.
   make[1]: Leaving directory '/kbuild/obj/consumer/x86_64-rhel-9.4-rust'
   make[1]: *** [Makefile:251: __sub-make] Error 2
   make[1]: Target 'rustfmtcheck' not remade because of errors.
   make: *** [Makefile:251: __sub-make] Error 2
   make: Target 'rustfmtcheck' not remade because of errors.
   make: Leaving directory '/kbuild/src/consumer'

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH] rust: clarify the language unstable features in use

[Miguel Ojeda]

On Fri, Mar 28, 2025 at 1:10 AM kernel test robot <lkp@intel.com> wrote:
>
>    rustfmtcheck

I will replace the newlines with an empty `//`; otherwise it reads worse.

Most of these lines will go away in some months, so I don't think it
is worth diverging from the default even if `rustfmt` had a config for
this.

Cheers,
Miguel

Re: [PATCH] rust: clarify the language unstable features in use

[Miguel Ojeda]

On Thu, Mar 27, 2025 at 10:13 PM Miguel Ojeda <ojeda@kernel.org> wrote:
>
> We track the details of which Rust features we use at our usual "live
> list" [1] (and its sub-lists), but in light of a discussion in the LWN
> article [2], it would help to clarify it in the source code.
>
> In particular, we are very close to rely only on stable Rust language-wise
> -- essentially only two language features remain (including the `kernel`
> crate).
>
> Thus add some details in both the feature list of the `kernel` crate as
> well as the list of allowed features.
>
> This does not over every single feature, and there are quite a few
> non-language features that we use too. To have the full picture, please
> refer to [1].
>
> Link: https://github.com/Rust-for-Linux/linux/issues/2 [1]
> Link: https://lwn.net/Articles/1015409/ [2]
> Suggested-by: Andreas Hindborg <a.hindborg@kernel.org>
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Applied to `rust-next` -- thanks everyone!

    [ Improved comments with suggestions from the list. - Miguel ]

Cheers,
Miguel