linux-kernel - Re: [PATCH v2 1/3] rust: alloc: add Vec::truncate method

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Message-ID: <Z9ckWiW7X0hX5Wvt@pollux>
Date: Sun, 16 Mar 2025 20:19:54 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Andrew Ballance <andrewjballance@...il.com>
Cc: airlied@...il.com, simona@...ll.ch, maarten.lankhorst@...ux.intel.com,
	mripard@...nel.org, tzimmermann@...e.de, corbet@....net,
	ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com,
	gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
	a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu,
	acourbot@...dia.com, nouveau@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 1/3] rust: alloc: add Vec::truncate method

On Sun, Mar 16, 2025 at 06:16:42AM -0500, Andrew Ballance wrote:
> implement the equivalent to the std's Vec::truncate
> on the kernel's Vec type.
> 
> Signed-off-by: Andrew Ballance <andrewjballance@...il.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..18bcc59f0b38 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;
> +        }
> +
> +        let drop_range = len..self.len();
> +
> +        // SAFETY: `drop_range` is a subrange of `[0, len)` by the bounds check above.
> +        let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
> +
> +        // SAFETY:
> +        // - this will always shrink the vector because of the above bounds check
> +        // - [`new_len`, `self.len`) will be dropped through the call to `drop_in_place` below

We've just figured out that this part is not needed after all, sorry for the
inconvenience. No need to resend for this though, I can remove this line when
applying the patch.

> +        unsafe { self.set_len(len) };
> +
> +        // SAFETY:
> +        // - the dropped values are valid `T`s by the type invariant
> +        // - we are allowed to invalidate [`new_len`, `old_len`) because we just changed the
> +        //   len, therefore we have exclusive access to [`new_len`, `old_len`)
> +        unsafe { ptr::drop_in_place(ptr) };
> +    }
>  }
>  
>  impl<T: Clone, A: Allocator> Vec<T, A> {
> -- 
> 2.48.1
>