Re: [PATCH] rust/alloc: add Vec::into_boxed_slice()

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Mar 26, 2026 at 10:56:20AM +0100, David Rheinsberg wrote:
> Add `Vec::into_boxed_slice()` similar to
> `std::vec::Vec::into_boxed_slice()` [1].
> 
> There is currently no way to easily consume the allocation of a vector.
> However, it is very convenient to use `Vec` to initialize a dynamically
> sized array and then "seal" it, so it can be passed along as a Box:
> 
>     fn create_from(src: &[T]) -> Result<KBox<[U]>, AllocError> {
>         let v = Vec::with_capacity(n, GFP_KERNEL)?;
> 
>         for i in src {
>             v.push(foo(i)?, GFP_KERNEL)?;
>         }
> 
>         Ok(v.into_boxed_slice())
>     }
> 
> A valid alternative is to use `Box::new_uninit()` rather than
> `Vec::with_capacity()`, and eventually convert the box via
> `Box::assume_init()`. This works but needlessly requires unsafe code,
> awkward drop handling, etc. Using `Vec` is the much simpler solution.
> 
> [1] https://doc.rust-lang.org/std/vec/struct.Vec.html#method.into_boxed_slice
> 
> Signed-off-by: David Rheinsberg <david@readahead.eu>

> +    /// Excess capacity is retained in the allocation, but lost until the box
> +    /// is dropped.

We currently say that you should provide the right length when freeing
an allocation, but this is going to violate that. You should probably
invoke realloc here, like stdlib does.

Alice