Re: [PATCH 1/3] rust: alloc: add Vec::truncate method

[Benno Lossin <benno.lossin@proton.me>]

On Sat Mar 15, 2025 at 12:15 PM CET, Andrew Ballance wrote:
> On Sat, Mar 15, 2025 at 10:09:26AM +0000, Benno Lossin wrote:
>> On Sat Mar 15, 2025 at 3:42 AM CET, Andrew Ballance wrote:
>> > implements the equivalent to the std's Vec::truncate
>> > on the kernel's Vec type.
>> >
>> > Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
>> > ---
>> >  rust/kernel/alloc/kvec.rs | 36 ++++++++++++++++++++++++++++++++++++
>> >  1 file changed, 36 insertions(+)
>> >
>> > diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
>> > index ae9d072741ce..75e9feebb81f 100644
>> > --- a/rust/kernel/alloc/kvec.rs
>> > +++ b/rust/kernel/alloc/kvec.rs
>> > @@ -452,6 +452,42 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
>> >  
>> >          Ok(())
>> >      }
>> > +
>> > +    /// 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.
>> > +    /// # Examples
>> > +    /// ```
>> > +    /// let mut v = kernel::kvec![1, 2, 3]?;
>> > +    /// v.truncate(1);
>> > +    /// assert_eq!(v.len(), 1);
>> > +    /// assert_eq!(&v, &[1]);
>> > +    ///
>> > +    /// # Ok::<(), Error>(())
>> > +    /// ```
>> > +    pub fn truncate(&mut self, len: usize) {
>> > +        if len >= self.len() {
>> > +            return;
>> > +        }
>> > +
>> > +        // [new_len, len) is guaranteed to be valid because [0, len) is guaranteed to be valid
>> > +        let drop_range = len..self.len();
>> > +
>> > +        // SAFETY:
>> > +        // we can safely ignore the bounds check because we already did our own check
>> > +        let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
>> 
>> What's this `get_unchecked_mut` method, I don't see it in `rust-next` or
>> `alloc-next`.
>
> Vec derefs into a slice which implements get_uncheked_mut
> https://rust.docs.kernel.org/next/kernel/alloc/kvec/struct.Vec.html#method.get_unchecked_mut

Ah, I forgot about that... Can you change the safety comment to:

    // SAFETY: `drop_range` is a subrange of `[0, len)` by the bounds check above.

---
Cheers,
Benno