Re: [PATCH v3 3/7] rust: alloc: add Vec::push_within_capacity

Alice Ryhl
2025-04-24T11:47:47Z

On Wed, Apr 23, 2025 at 11:38:28AM -0400, Tamir Duberstein wrote:
> On Tue, Apr 22, 2025 at 5:53 AM Alice Ryhl <aliceryhl@google.com> wrote:
> >
> > This introduces a new method called `push_within_capacity` for appending
> > to a vector without attempting to allocate if the capacity is full. Rust
> > Binder will use this in various places to safely push to a vector while
> > holding a spinlock.
> >
> > The implementation is moved to a push_within_capacity_unchecked method.
> > This is preferred over having push() call push_within_capacity()
> > followed by an unwrap_unchecked() for simpler unsafe.
> >
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > + /// Appends an element to the back of the [`Vec`] instance without reallocating.
> > + ///
> > + /// # Safety
> > + ///
> > + /// The length must be less than the capacity.
> > + pub unsafe fn push_within_capacity_unchecked(&mut self, v: T) {
>
> Did you intend for this to be pub? The commit message doesn't
> obviously indicate it.

Well, I don't think it hurts.

> > let spare = self.spare_capacity_mut();
> >
> > // SAFETY: The call to `reserve` was successful so the spare capacity is at least 1.
>
> What call to reserve?

I have to update this comment, thanks.

> > unsafe { spare.get_unchecked_mut(0) }.write(v);
> >
> > // SAFETY: We just initialised the first spare entry, so it is safe to increase the length
> > - // by 1. We also know that the new length is <= capacity because of the previous call to
> > - // `reserve` above.
> > + // by 1. We also know that the new length is <= capacity because the caller guarantees that
> > + // the length is less than the capacity at the beginning of this function.
> > unsafe { self.inc_len(1) };
> > - Ok(())
> > }

Alice