Re: [PATCH v1 13/28] rust: alloc: add `Vec::try_with_capacity{,_in}()` constructors

[Gary Guo <gary@garyguo.net>]

On Thu, 10 Nov 2022 17:41:25 +0100
Miguel Ojeda <ojeda@kernel.org> wrote:

> Add `Vec::try_with_capacity()` and `Vec::try_with_capacity_in()` as
> the fallible versions of `Vec::with_capacity()` and
> `Vec::with_capacity_in()`, respectively.
> 
> The implementations follow the originals and use the previously
> added `RawVec::try_with_capacity_in()`.
> 
> In turn, `Vec::try_with_capacity()` will be used to implement
> the `CString` type (which wraps a `Vec<u8>`) in a later patch.
> 
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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

> ---
>  rust/alloc/raw_vec.rs |  1 -
>  rust/alloc/vec/mod.rs | 89 +++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 89 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/alloc/raw_vec.rs b/rust/alloc/raw_vec.rs
> index c342f3843972..eb77db5def55 100644
> --- a/rust/alloc/raw_vec.rs
> +++ b/rust/alloc/raw_vec.rs
> @@ -135,7 +135,6 @@ impl<T, A: Allocator> RawVec<T, A> {
>  
>      /// 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)
> diff --git a/rust/alloc/vec/mod.rs b/rust/alloc/vec/mod.rs
> index 540787804cc2..8ac6c1e3b2a8 100644
> --- a/rust/alloc/vec/mod.rs
> +++ b/rust/alloc/vec/mod.rs
> @@ -472,6 +472,48 @@ impl<T> Vec<T> {
>          Self::with_capacity_in(capacity, Global)
>      }
>  
> +    /// Tries to construct a new, empty `Vec<T>` with the specified capacity.
> +    ///
> +    /// The vector will be able to hold exactly `capacity` elements without
> +    /// reallocating. If `capacity` is 0, the vector will not allocate.
> +    ///
> +    /// It is important to note that although the returned vector has the
> +    /// *capacity* specified, the vector will have a zero *length*. For an
> +    /// explanation of the difference between length and capacity, see
> +    /// *[Capacity and reallocation]*.
> +    ///
> +    /// [Capacity and reallocation]: #capacity-and-reallocation
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// let mut vec = Vec::try_with_capacity(10).unwrap();
> +    ///
> +    /// // The vector contains no items, even though it has capacity for more
> +    /// assert_eq!(vec.len(), 0);
> +    /// assert_eq!(vec.capacity(), 10);
> +    ///
> +    /// // These are all done without reallocating...
> +    /// for i in 0..10 {
> +    ///     vec.push(i);
> +    /// }
> +    /// assert_eq!(vec.len(), 10);
> +    /// assert_eq!(vec.capacity(), 10);
> +    ///
> +    /// // ...but this may make the vector reallocate
> +    /// vec.push(11);
> +    /// assert_eq!(vec.len(), 11);
> +    /// assert!(vec.capacity() >= 11);
> +    ///
> +    /// let mut result = Vec::try_with_capacity(usize::MAX);
> +    /// assert!(result.is_err());
> +    /// ```
> +    #[inline]
> +    #[stable(feature = "kernel", since = "1.0.0")]
> +    pub fn try_with_capacity(capacity: usize) -> Result<Self, TryReserveError> {
> +        Self::try_with_capacity_in(capacity, Global)
> +    }
> +
>      /// Creates a `Vec<T>` directly from the raw components of another vector.
>      ///
>      /// # Safety
> @@ -617,6 +659,53 @@ impl<T, A: Allocator> Vec<T, A> {
>          Vec { buf: RawVec::with_capacity_in(capacity, alloc), len: 0 }
>      }
>  
> +    /// Tries to construct a new, empty `Vec<T, A>` with the specified capacity
> +    /// with the provided allocator.
> +    ///
> +    /// The vector will be able to hold exactly `capacity` elements without
> +    /// reallocating. If `capacity` is 0, the vector will not allocate.
> +    ///
> +    /// It is important to note that although the returned vector has the
> +    /// *capacity* specified, the vector will have a zero *length*. For an
> +    /// explanation of the difference between length and capacity, see
> +    /// *[Capacity and reallocation]*.
> +    ///
> +    /// [Capacity and reallocation]: #capacity-and-reallocation
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// #![feature(allocator_api)]
> +    ///
> +    /// use std::alloc::System;
> +    ///
> +    /// let mut vec = Vec::try_with_capacity_in(10, System).unwrap();
> +    ///
> +    /// // The vector contains no items, even though it has capacity for more
> +    /// assert_eq!(vec.len(), 0);
> +    /// assert_eq!(vec.capacity(), 10);
> +    ///
> +    /// // These are all done without reallocating...
> +    /// for i in 0..10 {
> +    ///     vec.push(i);
> +    /// }
> +    /// assert_eq!(vec.len(), 10);
> +    /// assert_eq!(vec.capacity(), 10);
> +    ///
> +    /// // ...but this may make the vector reallocate
> +    /// vec.push(11);
> +    /// assert_eq!(vec.len(), 11);
> +    /// assert!(vec.capacity() >= 11);
> +    ///
> +    /// let mut result = Vec::try_with_capacity_in(usize::MAX, System);
> +    /// assert!(result.is_err());
> +    /// ```
> +    #[inline]
> +    #[stable(feature = "kernel", since = "1.0.0")]
> +    pub fn try_with_capacity_in(capacity: usize, alloc: A) -> Result<Self, TryReserveError> {
> +        Ok(Vec { buf: RawVec::try_with_capacity_in(capacity, alloc)?, len: 0 })
> +    }
> +
>      /// Creates a `Vec<T, A>` directly from the raw components of another vector.
>      ///
>      /// # Safety