Re: [PATCH v2 1/2] rust: add basic Pin<Vec<T, A>> abstractions

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Oct 09, 2025 at 06:12:33PM +0000, Markus Probst wrote:
> Implement core Pin<Vec<T, A>> abstractions, including
>  * `Vec::pin` and `Vec::into_pin` for constructing a `Pin<Vec<T, A>>`.
>    If T does not implement `Unpin`, its values will never be moved.
>  * an extension for `Pin<Vec<T, A>>` allowing PinInit to be initialied on a
>    Pin<Vec>, as well as truncating and popping values from the Vec
> 
> Signed-off-by: Markus Probst <markus.probst@posteo.de>
> ---
>  rust/kernel/alloc.rs      |  1 +
>  rust/kernel/alloc/kvec.rs | 86 +++++++++++++++++++++++++++++++++++++++
>  rust/kernel/prelude.rs    |  2 +-
>  3 files changed, 88 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> index a2c49e5494d3..9c129eaf0625 100644
> --- a/rust/kernel/alloc.rs
> +++ b/rust/kernel/alloc.rs
> @@ -24,6 +24,7 @@
>  pub use self::kvec::KVec;
>  pub use self::kvec::VVec;
>  pub use self::kvec::Vec;
> +pub use self::kvec::PinnedVecExt;
>  
>  /// Indicates an allocation error.
>  #[derive(Copy, Clone, PartialEq, Eq, Debug)]
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index 3c72e0bdddb8..d5582a7f17e9 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -16,11 +16,13 @@
>      ops::DerefMut,
>      ops::Index,
>      ops::IndexMut,
> +    pin::Pin,
>      ptr,
>      ptr::NonNull,
>      slice,
>      slice::SliceIndex,
>  };
> +use pin_init::PinInit;
>  
>  mod errors;
>  pub use self::errors::{InsertError, PushError, RemoveError};
> @@ -109,6 +111,21 @@ pub struct Vec<T, A: Allocator> {
>      _p: PhantomData<A>,
>  }
>  
> +/// Extension for Pin<Vec<T, A>>
> +pub trait PinnedVecExt<T> {
> +    /// Pin-initializes P and appends it to the back of the [`Vec`] instance without reallocating.
> +    fn push_pin_init<E: From<PushError<P>>, P: PinInit<T, E>>(&mut self, init: P) -> Result<(), E>;
> +
> +    /// Shortens the vector, setting the length to `len` and drops the removed values.
> +    /// If `len` is greater than or equal to the current length, this does nothing.
> +    ///
> +    /// This has no effect on the capacity and will not allocate.
> +    fn truncate(&mut self, len: usize);
> +
> +    /// Removes the last element from a vector and drops it returning true, or false if it is empty.
> +    fn pop(&mut self) -> bool;
> +}

Please no extension traits just for this. Just provide `self: Pin<&mut Self>`
methods on Vec.

Alice