linux-kernel - Re: [PATCH v6 15/26] rust: alloc: implement `collect` for `IntoIter`

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Message-ID: <ZuF_76DHQRukLEMb@pollux>
Date: Wed, 11 Sep 2024 13:33:03 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com,
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
	a.hindborg@...sung.com, aliceryhl@...gle.com,
	akpm@...ux-foundation.org, daniel.almeida@...labora.com,
	faith.ekstrand@...labora.com, boris.brezillon@...labora.com,
	lina@...hilina.net, mcanal@...lia.com, zhiw@...dia.com,
	cjia@...dia.com, jhubbard@...dia.com, airlied@...hat.com,
	ajanulgu@...hat.com, lyude@...hat.com, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v6 15/26] rust: alloc: implement `collect` for `IntoIter`

On Wed, Sep 11, 2024 at 08:53:24AM +0000, Benno Lossin wrote:
> On 11.09.24 02:22, Danilo Krummrich wrote:
> > On Tue, Sep 10, 2024 at 08:12:24PM +0000, Benno Lossin wrote:
> >> On 16.08.24 02:10, Danilo Krummrich wrote:
> >>> +    /// Same as `Iterator::collect` but specialized for `Vec`'s `IntoIter`.
> >>> +    ///
> >>> +    /// Currently, we can't implement `FromIterator`. There are a couple of issues with this trait
> >>> +    /// in the kernel, namely:
> >>> +    ///
> >>> +    /// - Rust's specialization feature is unstable. This prevents us to optimze for the special
> >>> +    ///   case where `I::IntoIter` equals `Vec`'s `IntoIter` type.
> >>> +    /// - We also can't use `I::IntoIter`'s type ID either to work around this, since `FromIterator`
> >>> +    ///   doesn't require this type to be `'static`.
> >>> +    /// - `FromIterator::from_iter` does return `Self` instead of `Result<Self, AllocError>`, hence
> >>> +    ///   we can't properly handle allocation failures.
> >>> +    /// - Neither `Iterator::collect` nor `FromIterator::from_iter` can handle additional allocation
> >>> +    ///   flags.
> >>> +    ///
> >>> +    /// Instead, provide `IntoIter::collect`, such that we can at least convert a `IntoIter` into a
> >>> +    /// `Vec` again.
> >>
> >> I think it's great that you include this in the code, but I don't think
> >> that it should be visible in the documentation,
> > 
> > Why not? I think this information is valuable for users of this API.
> 
> If you want to keep it, then I don't mind, but I would still move it
> underneath `Examples` and add a section header `# Implementation
> Details` or similar.

Sure, we can do that.

> 
> ---
> Cheers,
> Benno
> 
> >> can you move it under
> >> the `Examples` section and turn it into normal comments?
> >>
> >>> +    ///
> >>> +    /// Note that `IntoIter::collect` doesn't require `Flags`, since it re-uses the existing backing
> >>> +    /// buffer. However, this backing buffer may be shrunk to the actual count of elements.
> >>> +    ///
> >>> +    /// # Examples
> >>> +    ///
> >>> +    /// ```
> >>> +    /// let v = kernel::kvec![1, 2, 3]?;
> >>> +    /// let mut it = v.into_iter();
> >>> +    ///
> >>> +    /// assert_eq!(it.next(), Some(1));
> >>> +    ///
> >>> +    /// let v = it.collect(GFP_KERNEL);
> >>> +    /// assert_eq!(v, [2, 3]);
> >>> +    ///
> >>> +    /// # Ok::<(), Error>(())
> >>> +    /// ```
> >>> +    pub fn collect(self, flags: Flags) -> Vec<T, A> {
>