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

[Markus Probst <markus.probst@posteo.de>]

I might be wrong here, but as far as I know, it is not possible to get
a `Pin<&mut Vec<T, A>>` from a `&mut Pin<Vec<T, A>>`, only a `&mut
Pin<&mut [T, A]>` which is not usable by any of these methods.

Thanks
- Markus Probst

On Fri, 2025-10-10 at 11:33 +0000, Alice Ryhl wrote:
> 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