Re: [PATCH v1 12/28] rust: alloc: add `RawVec::try_with_capacity_in()` constructor

Gary Guo
2022-11-14T14:34:14Z

On Thu, 10 Nov 2022 17:41:24 +0100
Miguel Ojeda <ojeda@kernel.org> wrote:

> Add the `RawVec::try_with_capacity_in()` constructor as the fallible
> version of `RawVec::with_capacity_in()`.
>
> The implementation follows the original.
>
> The infallible constructor is implemented in terms of the private
> `RawVec::allocate_in()` constructor, thus also add the private
> `RawVec::try_allocate_in()` constructor following the other.
>
> It will be used to implement `Vec::try_with_capacity{,_in}()` in
> the next patch.
>
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
> rust/alloc/raw_vec.rs | 34 +++++++++++++++++++++++++++++++++-
> 1 file changed, 33 insertions(+), 1 deletion(-)
>
> diff --git a/rust/alloc/raw_vec.rs b/rust/alloc/raw_vec.rs
> index daf5f2da7168..c342f3843972 100644
> --- a/rust/alloc/raw_vec.rs
> +++ b/rust/alloc/raw_vec.rs
> @@ -20,11 +20,11 @@ use crate::collections::TryReserveErrorKind::*;
> #[cfg(test)]
> mod tests;
>
> -#[cfg(not(no_global_oom_handling))]
> enum AllocInit {
> /// The contents of the new memory are uninitialized.
> Uninitialized,
> /// The new memory is guaranteed to be zeroed.
> + #[allow(dead_code)]
> Zeroed,
> }
>
> @@ -133,6 +133,14 @@ impl<T, A: Allocator> RawVec<T, A> {
> Self::allocate_in(capacity, AllocInit::Uninitialized, alloc)
> }
>
> + /// Like `try_with_capacity`, but parameterized over the choice of
> + /// allocator for the returned `RawVec`.
> + #[allow(dead_code)]
> + #[inline]
> + pub fn try_with_capacity_in(capacity: usize, alloc: A) -> Result<Self, TryReserveError> {
> + Self::try_allocate_in(capacity, AllocInit::Uninitialized, alloc)
> + }
> +
> /// Like `with_capacity_zeroed`, but parameterized over the choice
> /// of allocator for the returned `RawVec`.
> #[cfg(not(no_global_oom_handling))]
> @@ -203,6 +211,30 @@ impl<T, A: Allocator> RawVec<T, A> {
> }
> }
>
> + fn try_allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Result<Self, TryReserveError> {
> + // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
> + if mem::size_of::<T>() == 0 || capacity == 0 {
> + return Ok(Self::new_in(alloc));
> + }
> +
> + let layout = Layout::array::<T>(capacity).map_err(|_| CapacityOverflow)?;
> + alloc_guard(layout.size())?;
> + let result = match init {
> + AllocInit::Uninitialized => alloc.allocate(layout),
> + AllocInit::Zeroed => alloc.allocate_zeroed(layout),
> + };
> + let ptr = result.map_err(|_| AllocError { layout, non_exhaustive: () })?;
> +
> + // Allocators currently return a `NonNull<[u8]>` whose length
> + // matches the size requested. If that ever changes, the capacity
> + // here should change to `ptr.len() / mem::size_of::<T>()`.
> + Ok(Self {
> + ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
> + cap: capacity,
> + alloc,
> + })
> + }
> +
> /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
> ///
> /// # Safety