Re: [PATCH 4/5] rust: alloc: add Vec::drain_all

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Mar 20, 2025 at 06:12:50PM -0400, Tamir Duberstein wrote:
> On Thu, Mar 20, 2025 at 9:56 AM Alice Ryhl <aliceryhl@google.com> wrote:
> >
> > This is like the stdlib method drain, except that it's hard-coded to use
> > the entire vector's range. Rust Binder uses it in the range allocator to
> > take ownership of everything in a vector in a case where reusing the
> > vector is desirable.
> >
> > Implementing `DrainAll` in terms of `slice::IterMut` lets us reuse some
> > nice optimizations in core for the case where T is a ZST.
> >
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > ---
> >  rust/kernel/alloc/kvec.rs | 57 +++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 57 insertions(+)
> >
> > diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> > index df930ff0d0b85b8b03c9b7932a2b31dfb62612ed..303198509885f5e24b74da5a92382b518de3e1c0 100644
> > --- a/rust/kernel/alloc/kvec.rs
> > +++ b/rust/kernel/alloc/kvec.rs
> > @@ -564,6 +564,30 @@ pub fn truncate(&mut self, len: usize) {
> >          //   len, therefore we have exclusive access to [`new_len`, `old_len`)
> >          unsafe { ptr::drop_in_place(ptr) };
> >      }
> > +
> > +    /// Takes ownership of all items in this vector without consuming the allocation.
> > +    ///
> > +    /// # Examples
> > +    ///
> > +    /// ```
> > +    /// let mut v = kernel::kvec![0, 1, 2, 3]?;
> > +    ///
> > +    /// for (i, j) in v.drain_all().enumerate() {
> > +    ///     assert_eq!(i, j);
> > +    /// }
> > +    ///
> > +    /// assert!(v.capacity() >= 4);
> > +    /// ```
> > +    pub fn drain_all(&mut self) -> DrainAll<'_, T> {
> > +        let len = self.len();
> > +        // INVARIANT: The first 0 elements are valid.
> > +        self.len = 0;
> 
> Could you use `self.dec_len(self.len)` here? Then you'd have a &mut
> [T] rather than `MaybeUninit`. Provided you agree `dec_len` is sound,
> of course.

I think that `&mut MaybeUninit<T>` is better in this case. Calling
assume_init_read on a `&mut MaybeUninit<T>` does not leave the
MaybeUninit in an invalid state in the same way that calling `ptr::read`
on an `&mut T` does.

Alice