Re: [PATCH v2] rust: enable slice_flatten feature and provide it through an extension trait

["Alexandre Courbot" <acourbot@nvidia.com>]

On Tue Nov 4, 2025 at 11:10 PM JST, Alice Ryhl wrote:
> On Tue, Nov 04, 2025 at 12:53:18PM +0900, Alexandre Courbot wrote:
>> In Rust 1.80, the previously unstable `slice::flatten` family of methods
>> have been stabilized and renamed to `slice::as_flattened`.
>> 
>> This creates an issue as we want to use `as_flattened`, but need to
>> support the MSRV (which at the moment is Rust 1.78) where it is named
>> `flatten`.
>> 
>> Solve this by enabling the `slice_flatten` feature, and providing an
>> `as_flattened` implementation through an extension trait for compiler
>> versions where it is not available.
>> 
>> This lets code use `as_flattened` portably by just adding
>> 
>>     #[cfg(not(CONFIG_RUSTC_HAS_SLICE_AS_FLATTENED))]
>>     use kernel::slice::AsFlattened;
>> 
>> This extension trait can be removed once the MSRV passes 1.80.
>> 
>> Suggested-by: Miguel Ojeda <ojeda@kernel.org>
>> Link: https://lore.kernel.org/all/CANiq72kK4pG=O35NwxPNoTO17oRcg1yfGcvr3==Fi4edr+sfmw@mail.gmail.com/
>> Acked-by: Danilo Krummrich <dakr@kernel.org>
>> Acked-by: Miguel Ojeda <ojeda@kernel.org>
>> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
>> ---
>> This patch was part of the Nova GSP boot series [1], but since it
>> requires attention from the core Rust team (and possibly the build
>> maintainers?) and is otherwise buried under Nova patches, I am taking
>> the freedom to send it separately for visibility.
>> 
>> For v2, the methods are aligned with the final names of the standard
>> library, and the extension trait is only visible when needed. This
>> simplifies both the patch, and the extra labor for user code.
>> 
>> [1] https://lore.kernel.org/all/20251029-gsp_boot-v7-0-34227afad347@nvidia.com/
>> [2] https://lore.kernel.org/all/CANiq72kK4pG=O35NwxPNoTO17oRcg1yfGcvr3==Fi4edr+sfmw@mail.gmail.com/
>
> With the below concern verified, you may add:
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>

Thanks! You may want to check the v3 that I just posted [1] which also
addressed your comment about having the extensions trait in the prelude.

[1] https://lore.kernel.org/rust-for-linux/20251104-b4-as-flattened-v3-1-6cb9c26b45cd@nvidia.com/T/#u

>
>> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
>> index d0ee33a487be..a84b9e3171a3 100644
>> --- a/scripts/Makefile.build
>> +++ b/scripts/Makefile.build
>> @@ -308,6 +308,7 @@ $(obj)/%.lst: $(obj)/%.c FORCE
>>  
>>  # The features in this list are the ones allowed for non-`rust/` code.
>>  #
>> +#   - Stable since Rust 1.80.0: `feature(slice_flatten)`.
>>  #   - Stable since Rust 1.81.0: `feature(lint_reasons)`.
>>  #   - Stable since Rust 1.82.0: `feature(asm_const)`,
>>  #     `feature(offset_of_nested)`, `feature(raw_ref_op)`.
>> @@ -317,7 +318,7 @@ $(obj)/%.lst: $(obj)/%.c FORCE
>>  #
>>  # 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,offset_of_nested,raw_ref_op,used_with_arg
>> +rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,offset_of_nested,raw_ref_op,slice_flatten,used_with_arg
>
> You should double-check, but I don't think you need to list it here
> because all uses of the unstable method are under the `rust/` directory.

I will check that, and indeed the comment above says as much.