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

["David Rheinsberg" <david@readahead.eu>]

Hi Alice,

On Thu, Mar 26, 2026, at 11:33 AM, Alice Ryhl wrote:
> 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.

Thanks, didn't know that!

`std` calls `shrink_to_fit()`. Should I add it as well? How about `shrink_to()`? I don't particularly need them, but I can expose them as well, given that I would have to implement those.

Thanks
David