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

[Benno Lossin <benno.lossin@proton.me>]

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